Conformational transitions and geometry differences between low‐energy conformers of N‐acetyl‐N′‐methyl alanineamide: An ab initio study at the 4‐21G level with gradient relaxed geometries

Energy pathways between the α R , β′, C , and β‐regions of the conformational energy surface of N ‐acetyl‐ N ′‐methylalanyl amide were obtained by SCF ab initio calculations on the 4‐21G level, with gradient geometry optimization at each point. The calculations indicate that no barrier exists at this computational level between α R and β′. The variation of geometry (bond distances and bond angles) with conformation is analyzed in detail, and the most important geometrical parameters that should be treated as variables in both empirical energy calculations and in the fitting of polypeptide chains in proteins by x‐ray methods are identified. In addition to the ϕ,ψ correlation discussed previously for the helical state, a correlation of these dihedral angles in the β‐region is described.