Helix‐coil stability constants for the naturally occurring amino acids in water. XXIV. Half‐cystine parameters from random poly(hydroxybutylglutamine‐CO‐S‐methylthio‐L‐cysteine)

Water‐soluble, random copolymers containing N 5 ‐(4‐hydroxybutyl)‐ L ‐glutamine (host) and S ‐methylthio‐ L ‐cysteine (guest) have been prepared, fractionated, and characterized, with S ‐methylthio‐ L ‐cysteine serving as a model for cystine residues in proteins. From the thermally induced helix‐coil transition curves of these copolymers in water at neutral pH, the Zimm‐Bragg parameters σ and s for the helix‐coil transition of “poly( L ‐cystine)” were deduced. The results show that the cystine model acts as a weak helix‐breaker over the entire temperature range from 0 to 60°C. The implications of this finding are evaluated in the context of a general discussion of the Zimm‐Bragg parameters for all the 20 naturally occurring amino acids.