Synthesis and physicochemical properties in aqueous solution of the sequential polypeptide poly(Tyr-Ala-Glu)

Abstract

A polypeptide having the repealing sequence (Tyr-Ala-Glu)_n was synthesized by the polymerization of the N-hydroxysuccinimide ester of O-benzyl-L-tyrosyl-L-alanyl-benzyl-L-glutamate, followed by the removal of the benzyl groups by means of hydrogen bromide. The main fraction obtained on gel filtration had an average molecular weight of over 60, 000, corresponding to over 500 amino acid residues per polypcptide chain. The polymer is soluble in water above pH 5.5, and precipitates on lowering the pH. The x-ray powder photographs show features of an α-helical structure. The dependence of the ultraviolet absorption spectrum, the optical rotatory dispersion, and the fluorescence of poly(Tyr-Ala-Glu) on pH, in salt-free as well as in salt-containing aqueous solutions, was compared with the corresponding properties of a copolymer containing equimolar proportions of tyrosine, alanine, and glutamic acid in a random sequence. From these measurements it was concluded that poly(Tyr-Ala-Glu ) has a helical con formation at low pH and a random coil conformation at high pH, the transition taking place at pH 6 in the absence of salt and pH II in the presence of salt. Thus, in the range pH 7 to l0. random coil-to-helix transition can be achieved by merely increasing the ionic strength. A model is proposed for the structure of the helical poly peptide which accounts for the Stability of the helical conformation by assuming hydrogen bonding between the carboxylate group of the ith glutamic acid residue and the hydroxyl group of the (i+4)th tyrosine residue. The complex ORD of helical poly(Tyr-Ala-Glu) is explained as being due to a superposition of the ORD of an α-helix and that of a regular array of phenolic ehroniopholes originating from the immobilization of the aromatic rings in the specific structure of the polymer.