MODULATION OF THE HELICAL STABILITY OF A MODEL PEPTIDE BY IONIC RESIDUES

The mean residue ellipticity of the helical host peptide, acetyl-YEAAA KEAXAKEAAAKA-amide containing guest residues at position X, was measur ed as a function of pH and ionic strength at 0-degrees-C. Changes in e llipticity at 222 nm were interpreted in terms of a two-state helix/co il transition of a monomeric peptide. Variable pH measurements in low concentrations of KCl defined changes in helix stability resulting fro m the ionization of each guest residue. Variable [KCl] measurements at fixed pH generated ellipticity values for the neutral and ionic forms of each guest residue free of electrostatic and lyotropic contributio ns. These ellipticity values were used to calculate a helix propagatio n parameter for each form of a guest residue using the Lifson-Roig alg orithm and assuming a universal nucleation parameter. In all cases, th e propagation parameter of a residue is either unaffected or decreased by ionization of its side chain.