An exhaustive conformational analysis of N-acetyl-L-cysteine-N-methylamide.: Identification of the complete set of interconversion pathways on the ab initio and DFT potential energy hypersurface

The full conformational space of N-acetyl-L-cysteine-N-methylamide was explored by ab initio (RHF/6-31G(d)) and DFT (B3LYP/6-31G(d)) computations. Multidimensional conformational analysis predicts 81 structures in N-acetyl-L-cysteine-N-methylamide, but only 47 relaxed structures were previously determined at the RHF/3-21G level of theory. These structures were now optimized using RHF/6-31G(d) and B3LYP/6-31G(d) approaches. Seven conformational migrations were observed when recalculated at higher level of theory. Besides these major changes, only smaller conformational shifts were operative for the remaining stationary points. The exploration of the whole conformational space of N-acetyl-L-cysteine-N-methylamide, including the transition-state structures allowing the conformational interconversion among the low-energy forms, was analyzed in this study. Our results offer new insights into the influence of polar side chains on the conformational preferences of peptide structures.