Conformational effects of chiral alpha,alpha-dialkyl amino acids. I. C-terminal tetrapeptides of emerimicin containing alpha-ethylalanine.

The syntheses and the crystal structures of the C-terminal tetrapeptide fragments of emerimicin IV and III, Boc-R-EtA-Hyp(Bzl)-Ala-Phol and Boc-R-EtA-Hyp(Bzl)-MeA-Phol, containing the chiral alpha,alpha-dialkyl amino acid, R-alpha-ethylalanine (R-EtA) are reported. The two peptides are isomorphous and assume a 3(10)-helical conformation in the crystal. A comparison of the crystal data on alpha,alpha-dialkyl amino acids indicates that alkyl substituents larger than a methyl group do not preclude peptides containing these amino acids from assuming the conformations associated with minima which have been well characterized for alpha-methylalanine.