THE ROLE OF PII CONFORMATIONS IN THE CALCULATION OF PEPTIDE FRACTIONAL HELIX CONTENT

Changes in the temperature, pH, ionic strength, or denaturant concentr ation of aqueous solutions of the monomeric non-a-helical peptide acet yl YEAAAKEAPAKEAAAKAamide generate changes in its dichroic spectrum ch aracteristic for a conformational transition. This transition has the characteristic features of a residue PII/unstructured conformational e quilibrium in which PII denotes an extended left-handed helical confor mation and unstructured denotes all the remaining conformations in a r andom coil ensemble. Replacement of the proline residue facilitates po pulation of residues in an cu-helical conformation. However, the ellip tcity values for these non-proline peptides merge with the ellipticity of the proline peptide as the population of residues in the a-helix c onformation 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 ma y be estimated by using a two-state calculation in which the elliptici ty for the non-a-helix conformations is provided by a peptide having a central proline guest residue.