A cyclic beta-strand tripeptide with an alpha-helix like CD spectrum.

A protein alpha-helix is defined by 3.6 amino acids per turn. Cyclization of the tripeptide Alanine-Leucine-Glutamate through a side chain to the N-terminus lactam bond produces cyclo-(1,3)-[ALE]-NH(2) which displays a circular dichroism spectrum typical of an alpha-helix backbone. However, proton NMR spectra show a novel cyclic peptide featuring two non-hydrogen-bonded antiparallel beta-strands connected by an Ala-Leu cis-amide bond. This example highlights that the common practice of characterizing alpha-helices by CD spectra alone can be misleading.