The role of PII conformations in the calculation of peptide fractional helix content

Abstract

Changes in the temperature, pH, ionic strength, or denaturant concentration of aqueous solutions of the monomeric non-α-helical peptide acety1YEAAAKEAPAKEAAAKAamide generate changes in its dichroic spectrum characteristic for a conformational transition. This transition has the characteristic features of a residue PII/unstructured conformational equilibrium in which PII denotes an extended left-handed helical conformation and unstructured denotes all the remaining conformations in a random coil ensemble. Replacement of the proline residue facilitates population of residues in an α-helical conformation. However, the elliptcity values for these non-proline peptides merge with the ellipticity of the proline peptide as the population of residues in the α-helix conformation is diminished. This convergence suggests that all residues in a host/guest peptide series of the same length share a common PII/unstructured conformational equilibrium in a given solvent. We propose that the fractional helix content of peptides within such a series may be estimated by using a two-state calculation in which the ellipticity for the non-α-helix conformations is provided by a peptide having a central proline guest residue.