Aromatic side-chain contribution to far-ultraviolet circular dichroism of helical peptides and its effect on measurement of helix propensities.

Peptides of the sequence Ac-XKAAAAKAAAAKAAAAK-amide, where X is Tyr, Trp, or Ala, produce circular dichroism spectra that are typical of the alpha-helix; there are, however, significant differences between the Tyr, Trp, or Ala peptides in the magnitudes of the far-ultraviolet bands. A tyrosine or tryptophan residue is needed in each peptide in order to measure accurately the peptide concentration and the mean residue ellipticity. The N- or C-terminal position is chosen because helix fraying is greatest at each end and the Tyr or Trp residue should influence the helix content of the peptide least at these positions. Amide proton exchange measurements by proton nuclear magnetic resonance spectroscopy indicate that the Tyr, Trp, and Ala peptides possess similar amounts of H-bonded secondary structure. Comparison of the far-ultraviolet circular dichroism and absorption spectra of these peptides suggests that the differences in circular dichroism arise in each case from an induced aromatic circular dichroism band, which is positive for Tyr and negative for Trp. Insertion of one to three Gly residues between the aromatic residue and the rest of the helical sequence reduces the induced aromatic band to insignificant levels. Using this procedure of inserting Gly residues between the Tyr and the rest of the helical sequence, we remeasured the helix propensity of Gly. We find that the Ala:Gly ratio of helix propensities is 40, as opposed to our previous estimate of 100 determined using the Tyr peptide without considering the aromatic contribution of Tyr in the analysis [Chakrabartty, A., Schellman, J. A., & Baldwin, R. L. (1991) Nature 351, 586-588].