Filename: b316185d

Back | Show only these items: | The dipeptide cyclic(glycyltryptophanyl) in the gas phase: A concerted action of density functional calculations, S0–S1 two-photon ionization, spectral UV/UV hole burning and laser photoelectron spectroscopy

1
The dipeptide cyclic(glycyltryptophanyl) in the gas phase: A concerted action of density functional calculations, S₀–S₁ two-photon ionization, spectral UV/UV hole burning and laser photoelectron spectroscopy

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

The S₀–S₁ spectrum of the dipeptide cyclic(glycyltryptophanyl) (cGW) has been recorded by two-photon ionization (R2PI) spectroscopy of thermally evaporated, jet-cooled molecules.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj2

The R2PI spectrum contains several vibronic transitions with relatively small spacing.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

Density functional theory predicts more than 8 conformers.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

Applying spectral UV/UV hole burning spectroscopy we found that the strongest S₀–S₁ transitions belong to a single conformer.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

The transition at 35 058 cm⁻¹ is attributed to the S₁ origin and the other transitions to inter-ring modes.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

To obtain further information on the conformer structure, resonant two-colour two-photon ionization photoelectron spectroscopy (R(1 + 1′)PI PES) has been performed via the two most intense S₀–S₁ transitions.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa1

To our knowledge these are the first PE spectra of a dipeptide.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

The spectra are broad with a smooth onset.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

The lowest ionization onset lies at 7.709 eV, but is assumed to be not the adiabatic IE.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con3

Theoretical calculations with the B3LYP/6-311++G** theory predict that in the most stable neutral conformer an N–H group of the dipeptide ring binds to the indole π system.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

This structure has the highest IE of all conformers investigated and shows a strong geometry change upon ionization.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

The fact that the IE of the experimentally observed cGW conformer is higher than that of 3-methylindole is taken as a signature of a repulsion between the two rings in the cation: This could be explained by the N–H group of the peptide ring now interacting with the positively charged indole π system.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con4

The conformational assignment by R(1 + 1′)PI PES is tentative, because all of the lowest-energetic neutral conformers are expected to behave similarly in regard to ionization energies.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con5

Introduction

The advantage of gas phase measurements is the applicability of spectroscopy methods such as resonant two-photon ionization (R2PI) spectroscopy combined with mass-selective detection, spectral UV/UV or IR/UV hole burning (UV/UV SHB or IR/UV SHB) spectroscopy and resonant two-colour two-photon ionization photoelectron spectroscopy (R(1 + 1′)PI PES).

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot1

Further advantages of gas phase investigations are usually sharp spectra which reveal detailed structural information.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot1

Most importantly, however, gas phase data can be directly compared to theory.

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot2

For reviews on biological molecules in the gas phase see .^{ref. 1–4}

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

Levy and co-workers were the first to perform spectroscopy of di- and tripeptides in the gas phase by using laser desorption with R2PI and mass-selective detection.^5,6

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

For tryptophan-containing peptides they obtained sharp spectra with distinct lines despite the many vibrational degrees of freedom and the many possible conformers.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

The spectroscopy of laser-desorbed small peptides was later improved by using double resonance methods.^7,8

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

Neutral laser desorption and R2P ionization and laser fragmentation was also introduced as a new species selective inlet system into mass spectrometry.^9–11

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

Thermal evaporation of natural unprotected peptides is impossible.^12,13

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac6

Up to now only for some modified peptides in which the polar functional groups have been protected by methyl groups a thermal evaporation of intact molecules is reported.^14,15

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac6

Cyclic dipeptides have no polar N or C terminal and hence, as we show here, can be vaporized intact by heating.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

To our knowledge up to now no investigation of cyclic peptides in the gas phase has been performed.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

UV/UV or IR/UV double resonance methods are widely used in spectroscopy of floppy molecules and clusters.²

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met1

The strategy to obtain structural information by using zero kinetic energy electron PES (ZEKE¹⁶), mass analyzed threshold ionization (MATI¹⁷) or high resolution conventional laser PES¹⁸ is not new.^18–25

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met2

In these publications model systems, such as extended molecules with a chromophore and a flexible side chain, have been investigated.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac8

Whereas for non-polar side groups sharp PE spectra are observed exclusively^18–20 for polar groups sharp spectra are only expected for conformers where the interaction of the charge site and the polar group are minimized, i.e. unfolded, linear structures.¹⁸

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac8

In folded structures the polar group solvates the charge strongly and structural changes upon ionization become so large that it is impossible to reach the origin of the cation ground state and obtain vibrationally resolved spectra.²²

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac8

Nevertheless, the IE shift together with theory may provide useful information to allow an identification of the neutral conformation.²²

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

Only in one case of an aromatic molecule with a polar side chain, both folded and unfolded conformers showed sharp vibrationally resolved spectra.^24,25

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

In this paper we report on spectroscopy of the cyclic(glycyltryptophanyl) (cGW) in the gas phase.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj2

R2PI spectroscopy with mass selection has been used to record the S₀–S₁ spectrum.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met3

UV/UV SHB was applied to analyze the number of conformers present in the molecular beam.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met3

To obtain further information on the conformer structure, R(1 + 1′)PI PES was performed for the two most intense S₀–S₁ transitions.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met3

Density functional theory (DFT) calculations on the B3LYP/6-311++G** level are used to interpret the number and structure of observed conformers, IEs and geometry shifts upon ionization.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa2

A tentative conformer identification is given on the basis of a comparison between experiment and theory.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac11

Experimental

The cyclic peptide cGW was purchased from Bachem.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

It has been synthesized from pure L-tryptophan (W) and hence cGW has also L chirality.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

The apparatus is described in detail elsewhere.¹⁸

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

Briefly it consists (Fig. 1) of a supersonic jet chamber (a), a field-free time-of-flight PE spectrometer (b) and a pulsed linear time-of-flight mass spectrometer (c).

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

A modified pulsed nozzle (General Valve, orifice diameter 300 μm) was heated to achieve a suitable sample vapor pressure.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met4

The indole sample has been added to a sample container above the nozzle and heated to 325 K. The cGW sample, crystalline at room temperature, is placed directly in the nozzle head and heated to 525 K. Ar was used as carrier gas with a backpressure of 5 bar.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met4

The jet passed the skimmer (1.5 mm orifice) and crossed the laser beam at right angles in the middle of the PE spectrometer.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

The time-of-flight PE spectrometer is 900 mm long, μ-metal shielded and covered inside by graphite.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

Only photoelectrons emitted into the small acceptance angle of the two-stage micro channel plate (MCP) detector were detected and their time measured with high precision in a start–stop logic.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

Note that for photoelectron spectroscopy no ion extraction plates have been used within the PE spectrometer.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met4

After ionization, the ions drift downwards with their supersonic jet velocity into a pulsed linear time-of-flight (linTOF) mass spectrometer (mass resolution m/Δm × 100).

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

Hence R2PI mass spectra, mass-analyzed R2PI spectra and R(1 + 1′)PI PE spectra with simultaneous mass control can be recorded in the same apparatus.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

Only for UV/UV SHB experiments three deflection plates were introduced at the ionization site in the PE spectrometer.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

In this case the ions were formed in an electric field and deflected into the former electron detector.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

The laser system consisted of two Nd:YAG laser-pumped and frequency-doubled tunable dye lasers (5–7 ns pulse width) and an additional excimer-pumped frequency-doubled dye laser which can be arbitrarily shifted in time with respect to the Nd:YAG-pumped dye lasers.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

R2PI spectroscopy

For recording the R2PI spectrum ions were formed in the PE spectrometer and, after drifting down, mass-analyzed in the pulsed linear TOF mass spectrometer.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

The S₀–S₁ spectrum was recorded by tuning the laser wavelength and recording the parent cations with a gated integrator.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

For this purpose we used the Nd:YAG-pumped frequency doubled dye laser (resolution in the UV: < 1 cm⁻¹, 100 μJ pulse⁻¹, 1000 mm lens out of focus).

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

We found that the cooling efficiency of the supersonic jet very much depended on the expansion gas and improved with increasing mass of the carrier gas.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

UV/UV SHB

For the SHB a three-plate ion optics (repeller plate, intermediate plate, grounded exit plate) was mounted in the PE spectrometer which deflected and accelerated the ions to the electron detector of the PE spectrometer.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod3

By changing the MCP voltages suitable for ion detection mass and UV/UV SHB spectra have been measured.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod3

The mass resolution m/Δm of this linTOF is 1500.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac12

This setup is needed to deflect the ions formed by the hole burning laser by using a pulsed electric field scheme.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod3

The hole burning laser performed ground state depletion by R2PI 4 μs before the probe laser ionizes the remaining neutrals.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod3

For ion discrimination coming from different lasers, ions stemming from the hole burning laser were for 4 μs accelerated backwards (+500 V at the second plate, repeller plate grounded) and neutralized on the repeller plate.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod3

A suitable pulsed voltage (750 V) was then applied to the repeller shortly after new ions had been formed by the probe laser.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met5

The SHB spectrum was recorded by scanning the hole burning laser while the probe laser was set to the S₀–S₁ origin transition of cGW.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met5

Each time when the neutral ground state of the cGW conformer was depopulated a dip appeared in the probe laser signal.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met5

Note that in this mode the dip depth directly correlates with the transition moment of the transition.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod3

Also a reversed scheme has been used to test the percentage of depletion for each transition of interest.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met5

(R(1 + 1′)PI) PE spectroscopy

For R(1 + 1′)PI PE spectroscopy ionization was performed by two frequency-doubled dye lasers pumped by the same Nd:YAG laser.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod4

The ionizing laser (laser 2) was time-delayed by 3 ns against laser 1 which performed the S₀–S₁ excitation.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod4

The photon energy of laser 2 was not sufficient for a two-photon ionization.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

The one-photon signal of laser 1 was lower than 5% of the total two-color ion signal.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

Typical laser pulse energies were 10 μJ for laser 1 and 20 μJ for laser 2 (1000 mm lens, out of focus).

Type: Method | Advantage: None | Novelty: New | ConceptID: Met6

To optimize the electron energy resolution, the photon energy of laser 2 was chosen to keep the maximum photon excess energy below 200 meV.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met6

For indole we achieved a PE energy resolution ΔE = 1 meV and an absolute accuracy of ±2 meV.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

The accuracy for cGW was ±3 meV due to the smooth onset at the ionization onset.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

Theoretical results

Our motivation to perform calculations on cGW was (i) to obtain the relative energies of the neutral and cation conformers, (ii) to calculate the vibrational frequencies for the lowest conformers, (iii) to calculate the vertical IEs and (iv) to explore the structural differences between the neutral and cation for the most stable conformers.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa3

For conformer assignment we use the comparison of the experimental to calculated data, such as relative neutral conformer energies, vertical IEs and neutral to cation geometry shifts.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met7

So far no ab initio or DFT calculations of a cyclic peptide are reported.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac13

We used the density functional B3LYP²⁶ because according to our experience¹⁸ it seems to better describe flexible molecules with polar groups than other methods.

Type: Method | Advantage: Yes | Novelty: New | ConceptID: Met8

It is well known that B3LYP systematically underestimates intermolecular electron correlation forces as present in folded molecules.

Type: Method | Advantage: No | Novelty: New | ConceptID: Met8

There could be several explanations.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac14

One would be an accidental error compensation, another that electrostatic forces dominate the binding.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac14

The greatest advantage of DFT methods is that they are fast enough to allow the use of big basis sets even for large molecules.

Type: Method | Advantage: Yes | Novelty: New | ConceptID: Met8

This is very important for folded molecules because it minimizes the basis set superposition error usually favoring folded molecules energetically.

Type: Method | Advantage: Yes | Novelty: New | ConceptID: Met8

DFT calculations have been performed with the Gaussian 98 program.²⁷

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

The basis sets 3-21G, 6-31+G* and 6-311++G** were used.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod5

The large basis set 6-311++G** was applied for neutral cGW to especially account for diffuse electron distributions.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod5

By increasing the basis set from 3-21G to 6-31+G* we observe big changes in the relative conformer energies.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

A saturation effect occurs when increasing the basis set from 6-31+G* to 6-311++G** (Table 1) which might be due to accidental error compensation for the smaller basis set.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

We also calculated vibrational frequencies (B3LYP/6-31+G*) for the neutral conformers 1, 2 and 3 which are needed for calculating the Gibbs free energy and also used for a tentative vibrational assignment in section IV.2.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met8

The conformers of neutral cGW

To find the local minima the conformational landscape of cGW has been explored using the program Tinker.²⁸

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa4

It performs a general conformational search for the entire potential energy surface by using a hopping algorithm²⁹ which is able to overcome barriers.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod6

The program Tinker also minimizes each conformer after its generation.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac15

The chosen MM3 force field³⁰ is known to be quite successful in reproducing molecular IR spectra.

Type: Method | Advantage: Yes | Novelty: New | ConceptID: Met9

In order to make sure we do not miss a conformer, in addition also a potential energy surface scan was performed using the OPLS all atom force field.³¹

Type: Method | Advantage: None | Novelty: New | ConceptID: Met9

The results from the both scans were collected and the conformers processed with B3LYP by stepwise increasing the basis set size.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met9

For discussions of the type of conformers of cGW in Fig. 2a the relevant rotational axes and the resultant structural possibilities are shown.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac16

100

The conformers of cGW can be classified as follows: The peptide ring can be moved into three positions when rotating about the C₁–C₂ axis (Fig. 2a, right side, see also atomic numbering C₀–C₂).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs9

101

For each of these conformers the indole ring then can be switched into six positions by rotation around the C₀–C₁ axis (Fig. 2b, left side).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs9

102

This gives theoretically 18 conformers.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs9

103

The most stable conformers are formed by rotating the C₁–C₂ axis into the plane which is perpendicular to the indole ring (perpendicular conformer class).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

104

The other tilted conformers are sterically unfavoured or unstable.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

105

By our calculations 8 stable conformers have been found for neutral cGW.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs10

106

The calculated relative energies with respect to the lowest conformer are presented in Table 1.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs11

107

A full geometry optimization of the neutral conformers was performed by using the B3LYP/6-311++G** method.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod5

108

The calculations show that the peptide ring (cyclic(glycylglycyl) or 2,5-piperazinedione) is not planar resulting in a double well potential (see Fig. 2b).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs12

109

In the unperturbed cyclic(glycylglycyl) the minima are equal in energy and the barrier is expected to be low because of the high flexibility between the amide groups described in biochemical text books.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs12

110

In cGW this double well is superimposed with a ring-to-ring interaction potential (schematic drawing in Fig. 2b).

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod7

111

Depending on the size and steepness of this ring-to-ring interaction potential, for each of the above-mentioned conformers either two sub-conformers a and b (this is the case for conformers 1 and 3), or only one conformer (a or b for conformers 2, 4, 5, 6, 7, 8) exist.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

112

The peptide-ring sub-conformers are very similar in energy (energy difference 41 cm⁻¹ for conformers 1a and 1b, and 18 cm⁻¹ for conformers 3a and 3b, see Table 1).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs13

113

The calculated isomerization barrier (B3LYP/6-31+G*) from conformer 1b to conformer 1a is 2 cm⁻¹ and already the zero point vibration would be above the barrier.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

114

The same is expected for conformers 3a and 3b for which the energetic difference is even smaller.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs14

115

The accuracy of the reaction barrier calculation is not known.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac17

116

However, even if the isomerization barrier between the a and the b forms would be larger than 2 cm⁻¹ but lower than 300 cm⁻¹, the more unstable form would be converted to the lower conformer in the supersonic expansion.³²

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac18

117

In the following we assume that conformers 1b and 3b are experimentally not observable.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp1

118

It is evident, however, that the resulting broad and anharmonic potential strongly reduces the vibrational frequencies in this coordinate, increases the vibrational density and by this favors these conformers on the Gibbs free energy landscape.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp1

119

In Table 1 the results of the B3LYP density functional calculations of the 8 stable neutral conformers of the dipeptide cGW (see also a and b conformers) using the 3-21G, 6-31+G* and 6-311++G** basis sets are presented.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs15

120

Relative conformer energies are given in cm⁻¹.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac19

121

The four lowest structures are shown in Fig. 3.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs16

122

With the increase of the basis set from 3-21G to 6-31+G* the energetic order of some conformers changes.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs17

123

When increasing the basis set from 6-31+G* to 6-311++G** the conformer order is conserved and the changes in the relative conformer energies become small, which might be due to an error compensation for the smaller basis set.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs17

124

As discussed above, structures 1b and 3b are not stable by zero point vibration.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp1

125

Conformer 2 which is characterized by a C–H⋯π interaction has a relative energy of 194 cm⁻¹.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs18

126

Conformer 3a which is similar to conformer 1a but with the indole ring switched by 180° lies 355 cm⁻¹ above 1a.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs19

127

In this conformer the N–H group seems to have better access to the indole π system.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs20

128

The fact that it is less stable than conformer 1a is attributed to a positive partial charge on the carbon in position 4.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con6

129

Only conformers with relative energies well below 365 cm⁻¹ (∼ kT at 525 K) are expected to have measurable populations in the beam.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp2

130

This also holds for conformer 2 but not for conformer 3.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp2

131

To calculate the exact thermal population beside the energy difference of the electronic energies also the vibrational zero point energy and the vibrational density of states has to be considered.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod8

132

Assuming harmonic potentials all this would be included in the Gibbs free energy.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod8

133

For a temperature of 300 K it shows an increase of the energetic spacing between conformer 1 and 2 from 355 cm⁻¹ to 675 cm⁻¹.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs21

134

This energy difference is expected to further increase when the strong anharmonicity of the inversion coordinate in conformer 1 would be taken into account.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp3

135

Hence from theory we predict that a great entropy effect is forcing the dipeptide population into conformer 1.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con7

136

The high energy of conformer 3 (355 cm⁻¹) above conformer 1 should result in a marginal or vanishing population of conformer 3.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp4

137

Comparing conformer 1 and 3 the entropy argument is not helpful because also conformer 3 has a broad double minimum in the peptide ring inversion coordinate.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac20

138

It is well known that barriers lower than 300 cm⁻¹ can be overcome during the supersonic jet expansion.³²

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac21

139

To compare theoretical data to the experimental data it would be important to calculate the barrier heights between conformers 2 and 1 as well as 3 and 1.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac22

140

Unfortunately, because of the complexity of the coordinates and the size of the system, it is impossible to calculate such isomerization barriers.

Type: Method | Advantage: No | Novelty: New | ConceptID: Met8

The cGW cation

141

We calculated vertical (6-311++G** basis set) and adiabatic (3-21G and 6-31+G* basis sets) IEs for comparison with the PE results in the hope to be able to assign the structure of the observed conformer.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa2

142

The cation geometry optimization was performed with the 3-21G basis set for all 8 conformers, and with the 6-31+G* basis set for seven conformers.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod5

143

The B3LYP/6-31+G* calculation for conformer 7 did not converge presumably because the potential surface is very flat.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con8

144

We checked the dependence of the cation structural changes on the basis set, but found that the 3-21G structures are very similar to the structures found with the 6-31+G* basis set.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs22

145

The calculated data are collected in Table 2 and the comparison of neutral and cation structures is shown in Fig. 4.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs22

146

We also calculated the vertical IEs of the b conformers of structures 1 and 3, although it is clear that they are not populated due to their low isomerization barriers (see above).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac23

147

The differences between the vertical IEs of conformers a and b nevertheless give hints as to how steep the ion ground state potential at the vertical geometry might be.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs23

148

When optimizing the geometry in the cation ground state the sub-conformers a and b formed by the peptide ring inversion converge for conformers 1a and 1b as well as 3a and 3b to a single minimum.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

149

This is explained by a large repulsive peptide ring-to-indole ring interaction in the cation which forces the system into one minimum.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

150

A calculation of the geometrical average of the changes of all atomic coordinates between neutral and cation (termed RMS value in Table 2) shows that, except one conformer, all conformers undergo large structural changes upon ionization.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

151

Fig. 4 shows the neutral-to-cation geometry changes of conformers 1, 2 and 6.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs24

152

For this the neutral and the cation structure are overlaid with the indole ring and conformational changes indicated by arrows and lines.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

153

In all cases the structural changes are so large that it might be difficult to reach the adiabatic IE.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con9

154

For conformers 1, 2 (Fig. 4a,b) and 3 (not shown) the large geometry change is explained by the difference between the attractive (neutral) versus the repulsive (cation) N–H⋯π (conformers 1 and 3) or C–H⋯π (conformer 2) interaction.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

155

In Fig. 4c the neutral and the cationic structures of conformer 6, the most stable cation conformer, are compared.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs25

156

In this conformer the CO group (note the negative partial charge on O) has access to the indole chromophore.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs25

157

Whereas conformer 6 is rather unstable in the neutral due to the repulsive CO⋯π interaction it becomes the most stable conformer when the π system is positively charged.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

Experimental results and discussion

158

In the following we present R2PI mass, R2PI S₀–S₁, UV/UV SHB and R(1 + 1′)PI PE spectra of the dipeptide cGW.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac24

The R2PI mass spectrum

159

For recording the R2PI mass spectrum of cGW (Fig. 5) the second harmonic of a Nd:YAG laser-pumped dye laser was tuned to the S₀–S₁ origin transition.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met3

160

The mass spectrum was recorded by using the PE spectrometer as flight tube for the linTOF.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

161

Mass 234 Da corresponds to the parent radical cation.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs26

162

Two fragments with masses 131 Da and 117 Da are observed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs26

163

These masses correspond to the neutral fragments 3-methylindole (131 Da) and indole (117 Da) which are then ionized by the laser pulse.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

164

In the case of a cationic fragmentation of tryptophan instead of mass 117 Da mass 130 Da would be expected as the most intense fragment.³³

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp5

165

Measurements with different laser intensities and wavelengths confirm that both masses are neutral fragments containing the indole chromophore.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs27

166

The ionization of the neutral fragments is a minor contribution in the mass spectrum at low photon energies but increases when coming closer to the indole S₀–S₁ resonance.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

167

Because of the mass selection there should be no influence of the fragment ionization on the R2PI spectrum of cGW.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp6

168

Nevertheless we find a small peak in the cGW spectrum at exactly the energy of the S₀–S₁ origin transition of indole which does not show the usual a¹ and b¹ progressions of cGW transitions.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs28

169

We believe that this peak is due to an intermolecular charge transfer (CT) effect where after ionization the charge hops from indole to the cGW which has the lower IE.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con10

Mass selective R2PI spectroscopy: the S₀–S₁ transition

170

To record the R2PI spectrum of cGW the second harmonic of a Nd:YAG laser-pumped dye laser was tuned and the intensity of the parent ion monitored as a function of the wavelength.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met3

171

In the R2PI spectrum shown in Fig. 6 an intense first transition is followed by several transitions which have decreasing intensities with increasing energy.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs29

172

The spacing is not regular which may be a hint for the presence of several conformers in the beam.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp7

173

UV/UV SHB spectroscopy proves (see section III.3) that this is not the case: All intense transitions correspond to one single conformer.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

174

In the following we make use of this information and assign transitions a¹, b¹, c¹, d¹ and presumably Y to vibrations of this conformer.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac25

175

A tentative conformer identification has been made in section IV.4.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac26

176

The R2PI spectrum of cGW shown in Fig. 6 covers 450 cm⁻¹.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

177

No transitions were observed below 35 000 cm⁻¹.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs30

178

Above 35 300 cm⁻¹ the indole vibrations and their many combination bands with vibrations a, b, c and d are expected.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp8

179

The justification to confine our investigation to a range of 450 cm⁻¹ is, that the four most stable calculated conformers have an attractive peptide ring-to-indole ring interaction and therefore should have similar S₀–S₁ origin energies.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp8

180

The S₀–S₁ origin transition of cGW is situated at 35 058 cm⁻¹.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs31

181

It lies 180 cm⁻¹ below the S₀–S₁ transition of indole³⁴ and 178 cm⁻¹ above the S₀–S₁ transition of 3-methylindole.³⁵

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac27

182

The S₁ energy in cGW is expected to be influenced by three parameters: (i) The electronic effect caused by the increase of the side chain of indole from 3-methylindole to cGW, (ii) the solvation of indole by the dipeptide ring and (iii) vibrational zero point energy effects.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp9

183

Concerning (i) we expect in analogy to the alkylbenzene series (relative shift with respect to methylbenzene: +110 cm⁻¹ for ethylbenzene, +105 cm⁻¹ for propylbenzene, +99 cm⁻¹ for butylbenzene³⁶) a blue shift in comparison to 3-methylindole when increasing the chain size.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp9

184

Concerning (ii): In a structure where the N–H group can interact with the indole π system (see most stable conformer 1 in Fig. 2a), a red shift of the S₀–S₁ transition is expected when the π system is excited, i.e. enlarged,¹⁸ because the N–H⋯π interaction is stabilized further.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp9

185

However, (iii) this shift may be partially compensated by a change in the zero point vibrational energy if some vibrations increase strongly with the excitation to S₁.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp9

186

Interestingly the origin of S₁ in cGW lies 353 cm⁻¹ above the S₁ of the lowest and 163 cm⁻¹ above the second conformer of linear NH₂–G–W–COOH.^5,6,8

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac28

187

The fact that our red-shift is not as strong as in linear peptides may be interpreted as follows: The peptide ring formation forces the functional groups CO and N–H of the peptide backbone into positions which do not allow ideal solvation of the indole ring.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp10

188

This lengthy discussion shows how difficult it is to interpret the tryptophan S₀–S₁ absorption in peptides with regard to structural changes of the environment.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac29

189

The relatively intense transitions to one quantum of the low-frequency modes a¹ (30 cm⁻¹) and b¹ (52 cm⁻¹) indicate some S₀–S₁ geometry change in the corresponding coordinates.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

190

Interestingly the transition a² is not observed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs32

191

Only the combination band a¹b¹ and tentatively a small transition b² is observed.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs32

192

Because the intense indole vibrations lie above 300 cm⁻¹,³⁴ the vibrations a, b, c (108 cm⁻¹) and d (174 cm⁻¹) are attributed to vibrations involving a relative motion between the two rings.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con2

193

For conformer 1a the frequency calculation of the neutral ground state gives the following vibrations below 90 cm⁻¹: 21 cm⁻¹ (global ring–ring bending), 39 cm⁻¹ (ring–ring torsion along C₁–C₂), 51 cm⁻¹ (boat to inverse boat motion in the peptide ring) and 59 cm⁻¹ (higher order bending of the peptide ring, the angle at C₀–C₁–C₂ and the indole ring).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs33

194

The frequency of 51 cm⁻¹ for the ring inversion mode is an artefact of the harmonic potential approximation.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res17

195

Because of the small energy difference of 41 cm⁻¹ between conformers 1a and 1b, at the energy of the first vibrational quantum the real potential width is much broader as assumed by the harmonic approximation.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res18

196

This should lead to a very low experimental frequency.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res18

197

Excitation of this motion would, however, only be expected if the peptide ring would have a strong coupling to the indole ring, which is not expected.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp11

198

For a tentative assignment of the vibration in S₁ we therefore only take the global ring–ring vibrations into account.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac30

199

The ring–ring interaction in the lowest-energetic conformer 1 is attractive.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac30

200

One can expect that this attraction increases in the S₁ state due to the increase of the size of the π system upon excitation.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp12

201

As a consequence the frequencies of the ring–ring motions would increase.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp12

202

Hence the S₀ vibrations with the energy 21 cm⁻¹ and 39 cm⁻¹ might correspond to the S₁ vibrations a¹ and b¹.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp12

203

The type of these vibrational motions also agrees with the concept that the N–H⋯π interaction changes when exciting from S₀ to S₁.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp13

204

The S₁ vibrations observed in the S₀–S₁ spectra of linear NH₂–G–W–COOH are 24 cm⁻¹ and 38 cm⁻¹ and not too far off our values although the structures are very different.^5,6,8

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac31

205

This indicates that the first two or three low-frequency vibrations of all similar-sized chromophore–tail molecules lie in the range of 20–50 cm⁻¹.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp14

206

For conformer 2 for example the first three calculated S₀ vibrational frequencies are with 26 cm⁻¹, 41 cm⁻¹ and 56 cm⁻¹ very similar to those of conformer 1.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs34

207

Hence the S₀ frequencies provide no arguments for conformer assignment.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res19

208

The S₀–S₁ R2PI spectrum was recorded with 5 bar Ar nozzle backpressure.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

209

Experiments with He and Ne as carrier gas resulted in less perfect vibrationally cooling.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs35

Spectral UV/UV hole burning spectroscopy (UV/UV SHB)

210

To clarify the number of conformers in our molecular beam, we used R2PI UV/UV SHB.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa5

211

As mentioned in the experimental section, for the SHB a three-plate ion optics was mounted and the PE spectrometer used as a linTOF mass spectrometer.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met5

212

This setup was needed in order to remove and neutralize the prompt ions formed by the hole burning laser by applying a pulsed electric field.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac32

213

The spectrum was recorded by scanning the hole burning laser while the probe laser was set to the S₀–S₁ origin transition.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met5

214

Each time when the neutral ground state of the cGW conformer was depopulated by the hole burning laser a dip appeared in the probe laser parent ion signal.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs36

215

Note that with this scheme the transition moments of the individual peaks determine the depth of the SHB dip.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met5

216

In Fig. 7a the UV/UV SHB spectrum of cGW and in (b) for comparison the R2PI spectrum are shown.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs37

217

We confined our investigation on the low-energetic section of the R2PI spectrum of cGW because (i) it contains the most intense transitions and (ii) the lowest-energetic four conformers are expected to lie in this range because of their similar peptide ring-to-indole ring interaction.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp15

218

In Fig. 7a all transitions except E show a strong hole burning effect and hence correspond to one conformer.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res20

219

To investigate the hole burning for transition E in detail, but also to probe accidental overlap of conformer transitions, we carefully checked the degree of hole burning for each transition.

Type: Goal | Advantage: None | Novelty: None | ConceptID: Goa6

220

Fig. 8 shows the degree of depletion of the probe signal by the hole burning laser for the first six transitions of the R2PI spectrum.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs38

221

The conditions have been the same as for Fig. 7a, except that now the hole burning laser is kept fix at transition A and the probe laser is scanned to the individual transitions A-F.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met5

222

With this scheme the depletion percentage should be the same for all transitions.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac33

223

The transition intensities recorded with the probe laser have been measured with the hole burning laser “off” (left trace) and “on” (right trace).

Type: Method | Advantage: None | Novelty: New | ConceptID: Met5

224

For each “off” and “on” sequence of each transition 15 measurements (each averaged over 20 laser shots) are displayed.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met5

225

The hole burning reduces the probe signal by 50% for all transitions.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs39

226

This shows that no origin transition of a minor conformer component is accidentally covered by a vibronic transition of the most stable conformer.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res21

227

Note that theoretically more than 50% of the neutral ground state levels can be depopulated³⁷.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac34

Photoelectron spectroscopy

228

Photoelectron spectroscopy is in principle a double-resonance spectroscopy which is able to provide conformer- and state-selective radical cation spectra.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod9

229

The initial hope was that we could sort out and assign the different conformers of cGW by PE spectroscopy alone.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp16

230

To achieve a resolution of 1 meV it is necessary to keep the electron kinetic energy below 200 meV.³⁸

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac35

231

To fulfil this condition a second frequency-doubled dye laser which is pumped by the second harmonic of the same Nd:YAG laser was used for ionization of excited S₁ cGW.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod10

232

The R2PI PE spectra of cGW ionized via the vibrationless origin transition of S₁ and the transition S₁ a¹ are shown in Fig. 9a and b.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs40

233

Both spectra show a broad ionization onset with no vibrational structure.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs40

234

According to the calculations of the neutral and cation structures which show a great geometry change upon ionization (Fig. 4a) we believe that we do not observe the origin transition to the cation ground state, but excite to a dense higher excited vibrational manifold.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con11

235

Due to the smooth onset the IE can only be determined with an accuracy of ±3 meV.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac36

236

The fact that the ionization onset via the S₁a¹ state is 6 cm⁻¹ lower than when ionized via the S₁ 000 state (7.715 eV ±3 meV) is attributed to a better Franck–Condon factor to lower vibrational quantum numbers in the cation ground state.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con12

237

The PE spectrum of indole (Fig. 9c) taken in the same machine shows that we could resolve vibrational structures if their spacing were larger than 10 cm⁻¹.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res22

238

The vibrations of the indole cation ground state have been recently investigated in detail by ZEKE spectroscopy³⁹ and are not discussed here.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac37

239

The IE value of indole determined by ZEKE spectroscopy allows us to calibrate our indole and cGW PE spectra with an accuracy of ±2 meV.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met10

240

Unfortunately there is a background in the peptide PE spectra which stems from ionization of neutral fragments, preferentially indole, which is wavelength-dependent.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs41

241

It is small for the wavelength range of the first two S₁ transitions of cGW, but increases as the closer the wavelength comes to the indole S₀–S₁ origin.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs41

242

Without coincidence techniques it is in principle impossible to distinguish between fragment and peptide photoelectrons.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp17

243

We therefore confined our PE measurements to the first two S₀–S₁ transitions.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met10

244

In Fig. 9 the IE of 3-methylindole (7.514 eV⁴⁰) is also marked.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs42

245

The IEs of substituted indoles should monotonically decrease with chain length (alkylbenzene⁴¹).

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac38

246

The fact that with respect to 3-methylindole the IE of cGW is blue-shifted and the smooth onset indicates a relatively strong repulsive interaction between the two rings in the cation, as predicted by theory.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res23

247

Unfortunately the calculation of absolute IEs is relatively inaccurate and the measured IE corresponds neither directly to the calculated adiabatic IE nor the vertical IE.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res24

248

The fact that conformer 1 has the highest IE of all conformers agrees well with the observed blue shift with respect to 3-methylindole.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con13

Conclusion and tentative conformer assignment

249

DFT predicts that one conformer (conformer 1) is predominantly stable and further favoured by entropy due to a low-energy peptide ring inversion mode.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res25

250

This is qualitatively in agreement with our R2PI and R2PI SHB spectra in which we attribute the most intense transition in the spectra to one conformer.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res25

251

In conformer 1 the N–H group of the dipeptide ring interacts with the π system.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

252

This interaction is attractive and is expected to increase with S₁ excitation.

Type: Hypothesis | Advantage: None | Novelty: None | ConceptID: Hyp18

253

This is qualitatively confirmed by comparison to the calculated low-frequency S₀ vibrations.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res26

254

The fact that the experimentally observed conformer has an IE which is 195 meV higher than 3-methylindole shows that the solvation of the charge in the indole by the peptide ring is repulsive.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con14

255

The most stable neutral conformer has the highest vertical IE of all 8 conformers and shows a very strong geometry change upon ionization.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con15

256

Hence conformer 1 fulfils all experimental findings.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res27

257

We admit that also conformer 2 and 3 follow the pattern of a attractive ring–ring interaction in the neutral cGW and a repulsive ring–ring interaction in the cationic cGW and that our arguments for the conformer assignment are very weak.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac39

258

For a confident conformer assignment it would be helpful to have several conformers in the molecular beam and be able to compare their IEs.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac40

259

Because the relative energies of calculated IEs are more accurate this would allow a more confident assignment.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac40

260

In conclusion, we propose that conformer 1 is the cGW conformer we observe in our molecular beam.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

261

Our work shows that R2PI and the R(1 + 1′)PI PE spectra predominantly provide information about the structural changes in the nearest environment of the chromophore.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res28

262

In cGW, however, the structural changes upon ionization are so great that with conventional high resolution PES a resolution of vibrational structures in the PE spectrum is impossible.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con16

263

In general a similar neutral-to-cation Franck–Condon problem might exist for peptides or amino acids which have polar groups near the chromophore.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con17

264

The IR/UV double resonance technique may provide additional clues for conformer identification and especially allow us to distinguish between conformers 1 (N–H⋯π interaction) and 2 (C–H⋯π interaction).

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con18

Introduction

Experimental

R2PI spectroscopy

UV/UV SHB

(R(1 + 1′)PI) PE spectroscopy

Theoretical results

The conformers of neutral cGW

The cGW cation

Experimental results and discussion

The R2PI mass spectrum

Mass selective R2PI spectroscopy: the S0–S1 transition

Spectral UV/UV hole burning spectroscopy (UV/UV SHB)

Photoelectron spectroscopy

Conclusion and tentative conformer assignment

Mass selective R2PI spectroscopy: the S₀–S₁ transition