Titration and Melting Curves of the Collagen-like Triple Helices Formed from (Pro-Pro-Gly)10 in Aqueous Solution

Abstract

Previous reports demonstrated that synthetic polytripeptides of the structure (Pro-Pro-Gly)10, prepared with a specific modification of the Merrifield technique, formed triple helical structures in aqueous solution which were similar to the triple helical structures of tropocollagen. The studies presented here were undertaken in order to answer the question of whether the individual polytripeptide chains within the triple helices were packed in a parallel fashion similar to vertebrate collagen or in an antiparallel fashion similar to the structure which has been suggested for collagen from the cuticle of the round worm, Ascaris lumbricoides. It was found that electrostatic interactions between the COOH-terminal and NH-terminal groups of (Pro-Pro-Gly)10 could be used to probe the detailed structure of triple helices formed by the polytripeptide. Titration of (Pro-Pro-Gly)10 in triple helical conformation indicated that the pK of the NH-terminal group was 0.59 units lower than the pK for the same group in nonhelical (Pro-Pro-Gly)5. The titration of the NH-terminal group of helical (Pro-Pro-Gly)10 was significantly broader than the theoretically predicted curve, and the pK increased after the polytripeptide was heated and quenched. Furthermore, the melting curve for the helix coil transition of (Pro-Pro-Gly)10 was changed by protonation of the NH-terminal or COOH-terminal groups. The Tm for the dipolar ion form was about 10° lower than for either the fully protonated or unprotonated forms of the helical polytripeptide. The results indicate that in the triple helical structure formed by (Pro-Pro-Gly)10 the three chains are in register and parallel.