Effects of side chains in gas-phase amino acids: Conformational analysis and relative stabilities

B3LYP and MP2 theoretical methods were employed to study the structures and relative stabilities of the Gly, Ala, Val, Aib, F-Gly, I-Gly and dF-Gly conformers. It was found that the relative stabilities of the Gly, Ala, Val and Aib conformers increase in the same orders of V < IV < III < II < I, due to the similar shapes of the corresponding conformers. Two exceptions exist for the above orders: In Gly, conformer IV is slightly more stable than conformer III; in Aib, conformer II is more stable than conformer I. The relative stability of conformer II vs. I is gradually enhanced by the substitutions of the lateral H atoms with the alkyl groups, especially in the case of the double substitutions to form Aib. The geometries and relative stabilities of the F-Gly, I-Gly and dF-Gly conformers are quite different from Gly as well as Ala, Aib and Val with the alkyl side chains. The driving forces in the conformers of the halogenated amino acids are mainly the intramolecular hydrogen bonding interactions, at least four hydrogen bonds formed in each dF-Gly conformer. For F-Gly and I-Gly, three conformers II-IV exist whereas conformers I and V are spontaneously transformed into conformers IV and III, respectively. It was found that the functional rather than alkyl side chains interact strongly with the NH2CHCOOH fragments and accordingly have more remarkable influences on the geometries and relative stabilities of the conformers.