A. Chakrabartty et al., AROMATIC SIDE-CHAIN CONTRIBUTION TO FAR-ULTRAVIOLET CIRCULAR-DICHROISM OF HELICAL PEPTIDES AND ITS EFFECT ON MEASUREMENT OF HELIX PROPENSITIES, Biochemistry, 32(21), 1993, pp. 5560-5565
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].