| Abstract
| - Gas-phase H/D exchange experiments with CD3OD and D2O and quantum chemical ab initio G3(MP2)calculations were carried out on protonated histidine and protonated histidine methyl ester in order to elucidatetheir bonding and structure. The H/D exchange experiments show that both ions have three equivalent fasthydrogens and one appreciably slower exchangeable hydrogen assigned to the protonated amino groupparticipating in a strong intramolecular hydrogen bond (IHB) with the nearest N(sp2) nitrogen of the imidazolefragment and to the distal ring NH-group, respectively. It is taken for granted that the proton exchange in theIHB is much faster than the H/D exchange. Unlike in other protonated amino acids (glycine, proline,phenylalanine, tyrosine, and tryptophan) studied earlier, the exchange rate of the carboxyl group in protonatedhistidine is slower than that of the amino group. The most stable conformers and the enthalpies of neutral andprotonated histidine and its methyl ester are calculated at the G3(MP2) level of theory. It is shown that strongintramolecular hydrogen bonding between the amino group and the imidazole ring nitrogen sites is responsiblefor the stability and specific properties of the protonated histidine. It is found that the proton fluctuates betweenthe amino and imidazole groups in the protonated form across an almost vanishing barrier. Proton affinity(PA) of histidine calculated by the G3(MP2) method is 233.2 and 232.4 kcal mol-1 for protonation at theimidazole ring and at the amino group nitrogens, respectively, which is about 3−5 kcal mol-1 lower than thereported experimental value.
|
