Residue helix parameters obtained from dichroic analysis of peptides of defined sequence

Circular dichroic measurements of the host peptide acetyl-Y(EAAAK)3A-amide were obtained in solutions of increasing ionic strength at pH 7.0 and 0 degree C. The changes observed in the dichroic spectra are characteristic for a two-state helix/coil transition. The mean residue ellipticity at 222 nm exhibits a curvilinear dependence on ionic strength which becomes linear at ionic strengths greater than 1 M. The slope of the linear portion is assumed to represent the lyotropic character of the salt, and its extrapolated intercept is assumed to represent the mean residue ellipticity of the peptide solution freed from both electrostatic and lyotropic contributions which affect the helical stability of the host peptide. An extrapolated mean residue ellipticity value was obtained for each host peptide having a different amino acid guest residue at position 9 in the peptide sequence. These values were used to calculate a propagation parameter, s, for each residue using the Lifson-Roig algorithm for peptide helical content and assuming a common nucleation parameter of 0.003. The ability of these minimally determined residue parameters to predict the helical content of a variety of peptides is encouraging. Estimates were also made of the delta G values for the electrostatic interactions within the host peptide and for the additional interactions generated by ionic guest residues.