Helix‐coil stability constants for the naturally occurring amino acids in water. XXIII. Proline parameters from random poly(hydroxybutylglutamine‐ CO ‐ L ‐proline)

Water‐soluble random copolymers containing L ‐proline and N 5 ‐(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 ben‐zotriazolyl‐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 N 5 ‐(4‐hydroxybutyl)‐ D ‐glutamine, thus requiring the synthesis of a binary copolymer containing only D ‐ and L ‐N 5 ‐(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 N 5 ‐(4‐hydroxybutyl)‐ D ‐glutamine, the Zimm‐Bragg parameters σ 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°C.