Collagen-like Triple Helices Research Articles

The synthetic polytripeptides (Pro-Pro-Gly) 10 and (Pro-Pro-Gly) 20 form micro-crystalline structures similar to segmental structures formed by collagen

Previous studies demonstrated that (Pro-Pro-Gly) 10 and (Pro-Pro-Gly) 20 prepared by a specific modification of the Merrifield technique form triple-helical structures in aqueous solution similar to tropocollagen. In the present studies we used electron microscopy to demonstrate that at about 4 °C the same polytripeptides also form larger structures which we consider to be micro-crystalline segments. Examination of the micro-crystalline segments by negative staining with sodium silicotungstate demonstrated that one lateral dimension was relatively constant and was approximately equal to the theoretical length of the polytripeptides in a collagen-like triple helix. Also, when (Pro-Pro-Gly) 20 was negatively stained with uranyl acetate, smaller structures were seen which were of the same length as the “regular dimension” of the micro-crystalline segments and which probably were individual triple helices. Selective positive staining of either the amino-terminal or the carboxy-terminal groups indicated that the ends of the polytripeptide chains were packed in an anti-parallel fashion within the segments. The results obtained by both negative and positive staining are best explained by a scheme in which (a) the polytripeptide chains within triple helices are in register and parallel, and (b) the micro-crystalline segments are formed by the lateral aggregation of triple helices in an anti-parallel fashion.

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

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.