EXPERIMENTAL AND AB INITIO STUDIES ON PROTONATIONS OF ALANINE AND SMALL PEPTIDES OF ALANINE AND GLYCINE

摘要

The gas-phase basicities of alanine (Ala) and di- and tripeptides of alanine and glycine were obtained by proton transfer reactions in a Fourier transform ion cyclotron resonance mass spectrometer. In addition, ab initio Hartree-Fock molecular orbital calculations were performed on the neutral and amino N-protonated species of glycine (Gly), alanine, and the four dipeptides (GlyGly, GlyAla, AlaGly, AlaAla). Minimum-energy structures were determined using both the 3-21G and 6-31G* basis sets with full geometry optimizations. Employing zero-point energies and thermal energies at 298.15 K and 1 atm calculated at the 3-21G//3-21G level and electronic energies at the 3-21G//3-21G, 6-31G*//3-21G, and 6-31G*//6-31G* levels, three sets of theoretical gas-phase basicity and proton affinity values were obtained. The relative basicities calculated at the highest level, 6-31G*//6-31G*, are in good agreement with the experimental values. Corrections that can be made to improve the calculated basicities are discussed in detail. The minimum-energy structures of the twelve species show consistent patterns of intramolecular hydrogen bonding in five-membered cyclic (C-5) forms; the presence of the alanyl methyl group has almost no effect on the structures. For the dipeptides and the tripeptides, the location of this methyl group at the N-terminus has the greatest impact on basicity, which shows the intrinsic ability of the methyl substituent near the protonation site to stabilize the ion. [References: 57]