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, T rp, 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 ban ds. A tyrosine or tryptophan residue is needed in each peptide in orde r to measure accurately the peptide concentration and the mean residue ellipticity. The N- or C-terminal position is chosen because helix fr aying is greatest at each end and the Tyr or Trp residue should influe nce the helix content of the peptide least at these positions. Amide p roton exchange measurements by proton nuclear magnetic resonance spect roscopy indicate that the Tyr, Trp, and Ala peptides possess similar a mounts of H-bonded secondary structure. Comparison of the far-ultravio let circular dichroism and absorption spectra of these peptides sugges ts 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 tbe induced aromatic band to insignificant levels. Using this procedure of inserting Gly residues between the Tyr and the rest of the helical se quence, we remeasured the helix propensity of Gly. We find that the Al a:Gly ratio of helix propensities is 40, as opposed to our previous es timate of 100 determined using the Tyr peptide without considering the aromatic contribution of Tyr in the analysis [Chakrabartty, A., Schel lman, J. A., & Baldwin, R. L. (1991) Nature 351, 586-588].