Peptide model XXVIII:: An exploratory ab initio and density functional study on the side-chainbackbone interaction in N-acetyl-L-cysteine-N-methylamide and N-formyl-L-cysteinamide in their γL-backbone conformations

A conformational and electronic study on the energetically preferred conformations (gamma(L)) of N- and C-protected L-cysteine (P-CONH-CH(CH2SH)-CONH-Q, where P and Q may be H or Me) was carried out. After restraining the backbone (BB) conformation to its global minimum (gamma(L) or C-7(eq)), all nine possible side-chain (SC) conformations were subjected to geometry optimization at the HF/3-21G and the B3LYP/6-31G(d,p) levels of theory. Seven of the nine side-chain conformers were located on the potential-energy surface. All conformers were subjected to an AIM (atoms in molecules) analysis. This study indicates that three of the seven optimized conformers exhibited either or both SC --> BB- or BB --> SC-type intramolecular hydrogen bonding. Five conformers, however, had distances between a proton and a heteroatom that suggested hydrogen bonding.