Polypeptides. 53. Water-soluble copolypeptides of L-glutamic acid, L-lysine, and L-alanine

The synthesis and some of the physical-chemical properties of tricopolymers of L -glutamic acid, L -lysine, and L -alanine are reported here. The molar ratios of the glutamyl: lysyl: alanyl residues were 1:1:X or 3:2:X, where the alanyl content X was increased in regular steps. The α-helix content calculated from the optical rotatory dispersion of the polypeptides is compared with a predicted helix content estimated from the composition of the polymers and the known behavior of the homopolypeptides at pH 3, 8, and 12. At pH 3 copolypeptides containing 20 mole-% or more alanine exhibit a helix content equal to the sum of their alanyl and glutamyl residue contents. At pH 8 the helix content equals the alanyl content when the latter was 40 mole-% or higher; at lower alanyl contents the electrostatic interaction between charged glutamyl and lysyl residues makes some contribution. At pH 12 the amount of helix observed is proportional to the mole ratio of alanine residues present in the polymer. The helix content of a tricopolymer containing 1:1:3 mole ratios of glutamyl: lysyl: alanyl residues was determined in solutions of lithium bromide and in urea solutions. Both reagents led to a decrease in helix content at pH 3 and 8 to a minimum of approximately 20% helix in 8M urea or 5.5M LiBr. The helix–random chain transition curves at pH 3 and 8 are parallel when the urea concentration is varied, but differ in shape when the lithium bromide concentration is varied at pH 3 and 8. The mode of action of these two “denaturing” reagents may thus be different. Heating the same tricopolypeptide at pH 3 or 8 from 5 to 80°C. also led to a helix–random chain transition centered at approximately 45°C.