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

A conformational and electronic study on the energetically preferred conformations (γ L ) of N- and C-protected L-cysteine (P-CONH-CH(CH 2 SH)-CONH-Q, where P and Q may be H or Me) was carried out. After restraining the backbone (BB) conformation to its global minimum (γ 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 [Formula: see text] BB- or BB [Formula: see text] SC-type intramolecular hydrogen bonding. Five conformers, however, had distances between a proton and a heteroatom that suggested hydrogen bonding.Key words: L-cysteine diamides, side-chain potential-energy surface, ab initio and DFT geometry optimization, AIM analysis, intramolecular hydrogen bonding.