Conformational effects of chiral α,α-dialkyl amino acids. I: C-Terminal tetrapeptides of emerimicin containing α-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 α,α-dialkyl amino acid, r-α-ethylalanine (r-EtA) are reported. The two peptides are isomorphous and assume a 310-helical conformation in the crystal. A comparison of the crystal data on α,α-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 α-methylalanine.