The relative stabilities of glycine tautomers involved in the intramolecular proton transfer are investigated computationally by considering glycine-water complexes containing up to five water molecules. The supermolecule results are compared with continuum calculations. Specific solute-solvent interactions and solvent induced changes in the solute wave function are considered using the natural bond orbitals (NBO) method. The stabilization of the zwitterion upon solvation is explained by the changes in the wave functions localized on the forming and breaking bonds as well as by the different interaction energies in the zwitterionic and neutral clusters. Only the neutral species exist in mono-and dihydrated clusters and in the gas phase. In the smaller clusters, zwitterions are mainly stabilized by conformational effects, whereas in larger clusters, in particular when glycine is solvated on both sides of its heavy atom backbone, polarization effects dominate the stability of a given tautomer. Generally, the strength of the solute-solvent interactions is governed by the intermolecular charge transfer interactions. As the solvation progresses, the hypothetical gaseous zwitterion is better solvated than the gaseous neutral, making zwitterion to neutral tautomerization progressively less exothermic for clusters containing up to three water molecules, and endothermic for larger clusters. The neutral isomer does not exist for some solvent arrangements with five water molecules. Only solvent arrangements in which water molecules do not interact with the reactive proton are considered. Hence, the experimentally observed double well potential energy surface may be due to such an interaction or to a different reaction mechanism.

Small peptide ions are studied by time-resolved photodissociation (TRPD). Laser desorption of neutral peptides is combined with laser photoionization in an ion trap followed by thermalization, laser photodissociation, and time-of-flight mass analysis. Ionization and excitation take place through an aromatic chromophore at the C-terminus of the peptide, whereas dissociation produces the immonium ion at the N-terminus. The purpose is to uncover the role of intramolecular vibrational redistribution (IVR) in unimolecular fragmentations of peptide radical cations the excitation of which is site-selective. Whereas previous experiments concentrated on mass spectra, the avenue taken here is the determination of microcanonical rate constants. The rate constants are measured at a fairly well-defined internal energy E for two peptides possessing the same chromophore, undergoing the same fragmentation but having a different number of degrees of freedom. Experimental rate measurements in the range of ∼10 2 -10 5 s -1 will be presented for the peptides leucyl tyrosine (LeuTyr) and leucyl leucyl tyrosine (LeuLeuTyr). One-color (280.5 nm) two-photon ionization, thermalization for 1980 ms, and excitation at 579 nm of LeuTyr and LeuLeuTyr yield (4.8 ( 1.8) × 10 3 and (2.9 ( 1.9) × 10 2 s -1 inverse time constants, that is, rate constants, respectively. The rate constants provide a clear indication that the peptide length (i.e., its number of degrees of freedom) strongly correlates with the dissociation rate. This has been tested further through measurements at different photodissociation energies and through Rice-Ramsperger-Kassel-Marcus/quasi equilibrium theory (RRKM/QET) calculations that are demonstrated to be in good agreement with the experimental observations, indicating that the internal energy, E, is randomized. In other words, these peptides do not circumvent IVR.

The water-assisted tautomerization of glycine has been investigated at the B3LYP/6-31+G** level using supermolecules containing up to six water molecules as well as considering a 1:1 glycine-water complex embedded in a continuum. The conformations of the tautomers in this mechanism do not display an intramolecular H bond, instead the functional groups are bridged by a water molecule. The replacement of the intramolecular H bond by the bridging water reduces the polarity of the N-H bond in the zwitterion and increases that of the O-H bond in the neutral, stabilizing the zwitterion. Both the charge transfer effects and electrostatic interactions stabilize the nonintramolecularly H-bonded zwitterion conformer over the intramolecularly hydrogen bonded one. The nonintramolecularly H-bonded neutral is favored only by charge transfer effects. Although there is no strong evidence whether the intramolecularly hydrogen bonded or non hydrogen bonded structures are favored in the bulk solution represented as a dielectric continuum, it is likely that the latter species are more stable. The free energy of activation of the water-assisted mechanism is higher than the intramolecular proton transfer channel. However, when the presumably higher conformational energy of the zwitterion reacting in the intramolecular mechanism is taken into account, both mechanisms are observed to compete. The various conformers of the neutral glycine may form via multiple proton transfer reactions through several water molecules instead of a conformational rearrangement.

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