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

The gas-phase basicities of alanine (Ala) and di- and tripeptides of a lanine and glycine were obtained by proton transfer reactions in a Fou rier 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), alanin e, and the four dipeptides (GlyGly, GlyAla, AlaGly, AlaAla). Minimum-e nergy 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 basicit y and proton affinity values were obtained. The relative basicities ca lculated at the highest level, 6-31G//6-31G*, are in good agreement w ith the experimental values. Corrections that can be made to improve t he calculated basicities are discussed in detail. The minimum-energy s tructures of the twelve species show consistent patterns of intramolec ular hydrogen bonding in five-membered cyclic (C-5) forms; the presenc e of the alanyl methyl group has almost no effect on the structures. F or the dipeptides and the tripeptides, the location of this methyl gro up at the N-terminus has the greatest impact on basicity, which shows the intrinsic ability of the methyl substituent near the protonation s ite to stabilize the ion.