Helical structures of poly(D-L-peptides). A conformational energy analysis.

Conformational energy calculations are reported for a number of possible helical structures of poly(D-L-peptides): the alpha helix, two single-stranded piDL, and five double-stranded pipiDL helices. For a poly(D-alanine-L-alanine) sequence, the energies of the various helices are found to differ by less than 1 kcal/(mol residue). For some helices (especially the piDL ones) two structural variants are predicted. These variants, called "goniomers", are characterized by reversed sequences of conformational angles but have the same screw sense and similar helical parameters. A biological implication of these goniomers is suggested, and their usefulness as a critical test for energy calculations is considered.