The synthesis and circular dichroism of a series of peptides possessing the structure (L-Tyrosyl-L-alanyl-L-glutamyl)_n

Abstract

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.