Five structures (two each in the C 7 eq - and β-regions, and one in the β′-region of the φ(NC α)ψ (C αC′) potential energy surface) were determined of the model dipeptide N-acetyl N′-methyl serine amide by ab initio gradient geometry refinement at the 4-21G level. Energy differences connected with conformations transitions of the CH 2OH side group in the same region of φψ space are found to be similar to differences connected with transitions between the C 7 eq -and β-regions. Different CH 2OH torsional arrangements can lead to changes of 12° in the backbone parameter, NC αC′, and of approximately 0.005 Å in the NC α bond 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, C 7 eq -region, the α-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.