Synthesis and characterization of six sequential polypeptides containing tyrosine, glutamic acid, alanine, and glycine by circular dichroism and difference spectroscopy

Six sequential polytetrapeptides containing equimolar amounts of tyrosine, glutamic acid, alanine, and glycine were characterized by CD and difference spectroscopy over a wide range of pH. As the pH was lowered from physiological values, each of the polymers underwent pH‐sensitive transitions. The CD spectra indicated that two polymers, poly(Tyr‐Glu‐Ala‐Gly) and poly(Tyr‐Ala‐Glu‐Gly), had some α‐helical conformation at pH 7.0 and approached maximum helicity around pH 6.0; two others, poly(Ala‐Tyr‐Glu‐Gly) and poly(Glu‐Ala‐Tyr‐Gly), had no α‐helical conformation at pH 7.0 and about one‐third of the ellipticities of the above two polymers at pH 5.5; and the remaining two, poly(Ala‐Glu‐Tyr‐Gly) and poly(Glu‐Tyr‐Ala‐Gly) had little or no α‐helix, even at pH 5.5. Difference spectroscopy at 286 nm yielded results quite different. The molar extinction coefficients for poly(Tyr‐Glu‐Ala‐Gly) and poly(Tyr‐Ala‐Glu‐Gly) continued to change, even below pH 5.5, and the total changes in absorbance between pH 8.0 and 4.5 were of intermediate magnitudes among the six polymers. Poly(Ala‐Tyr‐Glu‐Gly) and poly(Glu‐Ala‐Tyr‐Gly), which had similar CD spectra, had the lowest and highest pH‐related changes in the molar extinction coefficients. It thus appears that amino acid composition alone cannot account for the apparent differences in conformation among the polytetrapeptides. Other factors, such as amno acid sequence, must play a major role in the determination of conformation. The intrinsic viscosity of poly(Tyr‐Glu‐Ala‐Gly) increased markedly between pH 6.0 and 5.5, which was below the pH of the CD transition but above the pH at which the largest absorption perturbation change, at 286 nm, took place. The model that can best account for the relatively low pH at which the absorption transition of tyrosine occurred is a progressive immobilization of side chains in the α‐helix as the pH decreases.