The conformation of oligomers of beta-amino acids of the general type Ac-[b
eta-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 employi
ng two quantum-mechanical self-consistent reaction field models. The result
s show a wide variety of possibilities for the formation of characteristic
elements of secondary structure in beta-peptides. Most of them can be deriv
ed from the monomer units of blocked beta-peptides with n = 1. The stabilit
y and geometries of the beta-peptide structures are considerably influenced
by the side-chain positions, by the configurations at the C-alpha- and C-b
eta-atoms of the beta-amino acid constituents, and especially by environmen
tal effects. Structure peculiarities of beta-peptides, in particular those
of various helix alternatives, are discussed in relation to typical element
s of secondary structure in alpha-peptides. (C) 1999 John Wiley & Sons, Inc
.