Modulation of the helical stability of a model peptide by ionic residues.

The mean residue ellipticity of the helical host peptide, acetyl-YEAAAKEAXAKEAAAKA-amide containing guest residues at position X, was measured as a function of pH and ionic strength at 0 degree C. Changes in ellipticity at 222 nm were interpreted in terms of a two-state helix/coil transition of a monomeric peptide. Variable pH measurements in low concentrations of KCl defined changes in helix stability resulting from 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 contributions. These ellipticity values were used to calculate a helix propagation parameter for each form of a guest residue using the Lifson-Roig algorithm and assuming a universal nucleation parameter. In all cases, the propagation parameter of a residue is either unaffected or decreased by ionization of its side chain.