Effect of amino acid ion pairs on peptide helicity

The three ER ion pairs in the peptide acetyl-W(EAAAR)3A-amide were replaced in turn with the ion pairs EK, EO, DR, DK, and DO, where O represents an ornithine residue. The far-ultraviolet circular dichroic spectra of the six peptides measured in 10 mM NaCl at pH 2 and 0 degrees C form a nested set having an isodichroic point at 203 nm of -17,000 deg cm2 dmol-1. The ellipticity values of the six peptides at 222 nm range from -31,600 to -7400 deg cm2 dmol-1 in the order listed. Changing the pH of each peptide solution from 2 to 13 also generates a nested set of dichroic spectra with the same isodichroic values. Increasing the pH from 2 to 7 differentially increases the ellipticity at 222 nm in a single transition having an apparent pK of 4.1 for the E-containing peptides are 3.6 for the D-containing peptides. Increasing the pH beyond neutrality differentially decreases the ellipticity at 222 nm in a single transition having an apparent pK of greater than or equal to 13.2 for the R-containing peptides, 11.1 for the K-containing peptides, and 10.7 for the O-containing peptides. It is proposed that the difference in the ellipticity of the six peptides chiefly reflects the helix preferences for the variable residues supplemented by intrahelical electrostatic interactions in the neutral pH range.