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-degrees-C. The changes observed in the dichroic spectra are ch aracteristic for a two-state helix/coil transition. The mean residue e llipticity at 222 nm exhibits a curvilinear dependence on ionic streng th 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 t he mean residue ellipticity of the peptide solution freed from both el ectrostatic and lyotropic contributions which affect the helical stabi lity of the host peptide. An extrapolated mean residue ellipticity val ue was obtained for each host peptide having a different amino acid gu est residue at position 9 in the peptide sequence. These values were u sed to calculate a propagation parameter, s, for each residue using th e Lifson-Roig algorithm for peptide helical content and assuming a com mon nucleation parameter of 0.003. The ability of these minimally dete rmined residue parameters to predict the helical content of a variety of peptides is encouraging. Estimates were also made of the DELTAG val ues for the electrostatic interactions within the host peptide and for the additional interactions generated by ionic guest residues.