Helix-coil stability constants for the naturally occurring amino acids in water. 16. Aspartic acid parameters from random poly(hydroxybutylglutamine-co-L-aspartic acid)

The synthesis and characterization of water-soluble random copolymers containing L-aspartic acid with N5-(4-hydroxybutyl)-l-glutamine, and the thermally induced helix-coil transitions of these copolymers in water and in 0.1 N KCl, are described. The incorporation of L-aspartic acid was found to decrease the helix content of the polymer at both high and low pH, in water and also in 0.1 N KCl. The Zimm-Bragg parameters sigma and s for the helix-coil transition in poly(L-aspartic acid) in water and in 0.1 N KCl were deduced from an analysis of the melting curves of the copolymers in the manner described in earlier papers. Corrections were made for the presence of a small amount of racemized aspartic acid, using data from random copolymers containing D-aspartic acid as the guest residue. The computed values of s indicate that L-aspartic acid destabilizes helical sequences at all temperatures in the range of 0-70 degrees C. Titrations of the copolymers and of N-acetyl-N'-methyl-L-aspartic acid amide in 0.1 N KCl are described.