Basic conformers in beta-peptides.

The conformation of oligomers of beta-amino acids of the general type Ac-[beta-Xaa]n-NHMe (beta-Xaa = beta-Ala, beta-Aib, and beta-Abu; n = 1-4) was systematically examined at different levels of ab initio molecular orbital theory (HF/6-31G*, HF/3-21G). The solvent influence was considered employing two quantum-mechanical self-consistent reaction field models. The results show a wide variety of possibilities for the formation of characteristic elements of secondary structure in beta-peptides. Most of them can be derived from the monomer units of blocked beta-peptides with n = 1. The stability and geometries of the beta-peptide structures are considerably influenced by the side-chain positions, by the configurations at the C alpha- and C beta-atoms of the beta-amino acid constituents, and especially by environmental effects. Structure peculiarities of beta-peptides, in particular those of various helix alternatives, are discussed in relation to typical elements of secondary structure in alpha-peptides.