Ab initio geometry refinement of some selected structures of the model dipeptide N-acetyl N'-methyl serine amide

by Siam, Khamis S.; Klimkowski, Valentine Joseph; Van Alsenoy, Christian; Ewbank, John David; Schäfer, Lothar

Five structures (two each in the C7eq- and beta-regions, and one in the beta'-region of the phi(N-Calpha)-psi(Calpha-C') potential energy surface) were determined of the model dipeptideN-acetylN'-methyl serine amide by ab initio gradient geometry refinement at the 4-21 G level. Energy differences connected with conformational transitions of the CH2OH side group in the same region of phi-psi space are found to be similar to differences connected with transitions between the C7eq- and beta-regions. Different CH2OH torsional arrangements can lead to changes of 1-2deg in the backbone parameter,N-Calpha-C', and of approximately 0.005 Aring in the N-Calphabond length. Similar conformationally dependent changes in bond lengths and angles, caused by the side group, are found for the peptide groups. In the most stable conformation investigated in this study, C7eq-region, the alpha-carbon atom is the bridge head of a tricyclic system consisting of a five-, a six- and a seven-membered ring, each closed by internal hydrogen bonding.

Journal
Journal of Molecular Structure-Theochem
Volume
152
Issue
3-4
Year
1987
Start Page
261
ISBN/ISSN
2210-2728; 0166-1280
DOI
10.1016/0166-1280(87)80066-0