The Synthesis and Circular Dichroism of a Series of Peptides Possessing the Structure ( l ‐Tyrosyl‐ l ‐alanyl‐ l ‐glutamyl) n

A series of peptides possessing the structure ( l ‐Tyr‐ l ‐Ala‐ l ‐Glu) n , with n = 1,2,3,4,7,9, and 13 was synthesized. The oligomers up to n = 4 were prepared by stepwise synthesis using the N ‐hydroxysuccinimide ester of the tripeptide derivative for the elongation of the chain. The oligomers with n = 7,9, 13 were prepared by a polycondensation technique, followed by gel filtration. The circular dichroism spectra of the above oligopeptides were measured in the wavelength range 200–330 nm in a solution of 0.15 M sodium chloride –0.02 M sodium phosphate, pH 7.4, conditions at which the high molecular weight polytripeptide (Tyr‐Ala‐Glu) n exists in a helical conformation. The circular dichroic spectrum of the high molecular weight polymer exhibits two negative ellipticity bands: an intense band at 220 nm ([Θ] max = 8700) and a weak band at 273 nm ([Θ] max = 360). The tripeptide (n = 1) exhibits a positive ellipticity band at 227 nm ([Θ] max = 4000) and a broad, positive, weak band at 270 nm ([Θ] max = 115). The other oligopeptides (n = 2 to 13) exhibit positive bands in the 227 nm region, with ellipticity values that decrease as the degree of polymerization increases. Except for (Tyr‐Ala‐Glu) 13 , which has a negative peak centered at 216 nm, circular dichroic spectra of the other oligomers (n = 1 to 9) in the 200–210 nm region resemble that of random coils. In the 260–290 nm region, the circular dichroic curves of the oligopeptides gradually approach the 272 nm band of the polypeptide (Tyr‐Ala‐Glu) n , as the degree of polymerization increases. Under physiological conditions, only the (Tyr‐Ala‐Glu) 13 shows an indication of helical content.