The role of dehydro-alanine in the design of peptides.

X-ray crystallography, NMR spectroscopy and theoretical studies on some oligopeptides containing dehydro-alanine (delta Ala) have indicated that delta Ala adopts an extended conformation and also induces a definite conformation in the preceding saturated residue. In order to evaluate the conformational constraints imposed by delta Ala on the neighbouring saturated residues, we have undertaken a systematic, theoretical study of the preferred conformations of tripeptide sequences of the type N-Ac-X-delta Ala-NHCH3 and N-Ac-delta Ala-X-NHCH3 (X = Gly, L-Ala, L-Val, L-Ile and L-Phe). The methodology and parameters used have been standardized against sequences with known crystal structures. The significant findings of this study are that delta Ala always adopts an extended conformation and induces in both the preceding and the succeeding neighbouring saturated residues a conformation in which phi approximately 140 degrees and psi approximately -40 degrees. These results have a direct application in the design of peptide sequences for specific biological activity.