Dirk Nolting

Ionization of imidazole in the gas phase, microhydrated environments, and in aqueous solution.

Hydration of neutral and cationic imidazole is studied by means of ab initio and molecular dynamics calculations, and by photoelectron spectroscopy of the neutral species in a liquid microjet. The calculations show the importance of long range solvent polarization and of the difference between the structure of water molecules in the first shell around the neutral vs cationic species for determining vertical and adiabatic ionization potentials. The vertical ionization potential of neutral imidazole of 8.06 eV calculated using a nonequilibrium polarizable continuum model agrees well with the value of 8.26 eV obtained experimentally for an aqueous solution at pH 10.6.

The electronic spectrum of protonated adenine: Theory and experiment

In this work we present the results of a combined experimental and theoretical study concerned with the question how a proton changes the electronic spectrum and dynamics of adenine. In the experimental part, isolated adenine ions have been formed by electro-spray ionisation, stored, mass-selected and cooled in a Paul trap and dissociated by resonant photoexcitation with ns UV laser pulses. The S(0)-S1 spectrum of protonated adenine recorded by fragment ion detection lies in a similar energy range as the first pipi* transition of neutral 9H-adenine. It shows a flat onset with a broad substructure, indicating a large S(0)-S1 geometry shift and an ultra-short lifetime. In the theoretical part, relative energies of the ground and the excited states of the most important tautomers have been calculated by means of a combined density functional theory and multi-reference configuration interaction approach. Protonation at the nitrogen in position 1 of the neutral 9H-adenine tautomer yields the most stable protonated adenine species, 1H-9H-A+. The 3H-7H-A+ and the 3H-9H-A+ tautomers, formed by protonation of 7H- and 9H-adenine in 3-position, are higher in energy by 162 cm(-1) and 688 cm(-1), respectively. Other tautomers lie at considerably higher energies. Calculated vertical absorption spectra are reported for all investigated tautomers whereas geometry optimisations of excited states have been carried out only for the most interesting ones. The S1 state energies and geometries are found to depend on the protonation site. The theoretical data match best with the experimental onset of the spectrum for the 1H-9H-A+ tautomer although we cannot definitely exclude contributions to the experimental spectrum from the 3H-7H-A+ tautomer at higher energies. The vertical S(0)--> S1 excitation energy is similar to the one in neutral 9H-adenine. As for the neutral adenine, we find a conical intersection of the S1 of protonated adenine with the ground state in an out-of-plane coordinate but at lower energies and accessible without barrier.

Protonation effect on the electronic spectrum of tryptophan in the gas phase

In this work we investigate how the presence of a proton changes the electronic spectrum of tryptophan (Trp). For the S0–S1 transition of protonated Trp gas phase results can be compared to theoretical calculations. Trp ions have been formed by electro-spray ionization, stored in a cooled Paul trap, mass-selected and dissociated by resonant photoexcitation by ns UV laser pulses. The S0–S1 spectrum shows a distinct onset, a gap and a broad rise at higher energies. Interestingly the absorption of protonated Trp is in the same energy range as the S0–S1 transition of neutral Trp. For calculation of the relative tautomer and conformer energies for the ground and the excited states a combined density functional theory/multi-reference configuration interaction approach was used. The proton is put into different positions: The 1 (1-H+–Trp) and the 3 positions in indole and the N terminal (N–H+–Trp). Protonation at the N terminal forms the most stable tautomer. Attaching the proton to position 1 of the indole chromophore results in a very high energy. When the proton is approached in position 3 of the indole chromophore a chemical reaction takes place and a bridged isomer is formed (BI–H+). This isomer lies only 400 cm−1 above the N–H+–Trp form, but the activation barrier for the protonation is expected to be high. We calculated the excited states of the most stable neutral conformer, the neutral precursor conformer for the N–H+–Trp tautomer, the 1-H+–Trp and the N–H+–Trp tautomers and the BI–H+ isomer. The comparison of all S0–S1 transition energies shows that the S1 state, here the Lb state, is not sensitive to the protonation, the site of protonation or even the isomerization. According to theory, the experimentally observed protonated Trp is most probably the N–H+–Trp tautomer. For this tautomer the agreement of theoretical and experimental S0–S1 energies is excellent (3.5 nm). Its energetic position of the La state is blue-shifted and strongly structure-sensitive in all protonated species.

Pseudoequivalent nitrogen atoms in aqueous imidazole distinguished by chemical shifts in photoelectron spectroscopy

The photoelectron spectra of aqueous imidazole are presented, and the N 1s and C 1s binding energies are assigned aided by density functional theory calculations. The chemical equivalency of the two nitrogens of the cationic form is directly identified by the occurrence of a single N 1s photoelectron peak, which results from the delocalization of the positive charge over the molecule as a consequence of the Cv symmetry of the system. In contrast to NMR measurements, the photoemission process is faster than the rapid proton exchange in the aqueous environment, making the pseudoequivalent nitrogens of the neutral state clearly distinguishable with a N 1s binding energy shift of 1.7 eV.

Ionization of Aqueous Cations: Photoelectron Spectroscopy and ab Initio Calculations of Protonated Imidazole

Photoelectron spectroscopy and ab initio calculations employing a nonequilibrium polarizable continuum model were employed for determining the vertical ionization potential of aqueous protonated imidazole. The experimental value of 8.96 eV is in in excellent agreement with calculations, which also perform quantitatively for ionization of aqueous alkali cations as benchmark species. The present results show that protonation of imidazole increases its vertical ionization potential up in water by 0.7 eV, which is significantly larger than the resolution of the experiment or the error of the calculation. This combined experimental and computational approach may open the possibility for quantitatively analyzing the protonation state of histidine, of which imidazole is the titratable side chain group, in aqueous peptides and proteins.

Excited state dynamics and fragmentation channels of the protonated dipeptide H2N-Leu-Trp-COOH.

The excited state dynamics of the isolated and protonated peptide H(2)N-Leu-Trp-COOH are analyzed by fs pump-probe spectroscopy. The peptides are brought into the gas phase by electrospray ionization, and fs pump-probe excitation is detected by fragment ion formation. The pump laser addressed the excited pipi* state of the indole chromophore of the amino acid tryptophan. The subsequent excited state dynamics agreed with a biexponential decay with time constants of 500 fs and 10 ps. This is considerably shorter than the lifetime of neutral tryptophan in solution and in proteins, but similar to isolated, protonated tryptophan. Several models are discussed to explain the experimental results but the detailed quenching mechanism remains unresolved.

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

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. The R2PI spectrum contains several vibronic transitions with relatively small spacing. Density functional theory predicts more than 8 conformers. Applying spectral UV/UV hole burning spectroscopy we found that the strongest S0–S1 transitions belong to a single conformer. The transition at 35 058 cm−1 is attributed to the S1 origin and the other transitions to inter-ring modes. 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. To our knowledge these are the first PE spectra of a dipeptide. The spectra are broad with a smooth onset. The lowest ionization onset lies at 7.709 eV, but is assumed to be not the adiabatic IE. 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. This structure has the highest IE of all conformers investigated and shows a strong geometry change upon ionization. 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. 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.