Helix-coil stability constants for the naturally occurring amino acids in water. XXIII. Proline parameters from...

Water-soluble random copolymers containing L-proline and N5-(4-hydroxybutyl)-L-glutamine were synthesized by copolymerization of the tripeptides H-L-Glu(OBzl)-L-Glu(OBzl)-L-Glu(OBzl)-OH and H-L-Glu(OBzl)-L-Pro-L-Glu(OBzl)-OH, using benzotriazolyl-N-oxy-tris(dimethylamino)-phosphonium hexafluorophosphate as condensing reagent, and subsequent aminolysis of the Bzl ester groups with 4-amino-1-butanol. These copolymers were found to contain significant amounts of N5-(4-hydroxybutyl)-D-glutamine, thus requiring the synthesis of a binary copolymer containing only D- and L-N5-(4-hydroxybutyl)glutamine residues in order to evaluate the possible effects of the D-residues on the conformational properties of poly(hydroxybutylglutamine-co-L-proline). The different copolymers were fractionated, and their thermally induced helix-coil transition curves were obtained in water at neutral pH. When proper corrections were applied for the helix-destabilizing properties of N5-(4-hydroxybutyl)-D-glutamine, the Zimm-Bragg parameters sigma and s for L-proline could be deduced from the melting curves of poly(hydroxybutylglutamine-co-L-proline). The results indicate that L-proline acts as a very strong helix breaker over the entire temperature range from 0 to 60 degrees C.