4‐Chloroprolines: Synthesis, conformational analysis, and effect on the collagen triple helix

Abstract

Collagen is an abundant, triple-helical protein comprising three strands of the repeating sequence: Xaa–Yaa–Gly. (2S)-Proline and (2S,4R)-4-hydroxyproline (Hyp) are common in the primary structure of collagen. Here, we use nonnatural proline derivatives to reveal determinants of collagen stability. Specifically, we report high-yielding syntheses of (2S,4S)-4-chloroproline (clp) and (2S,4R)-4-chloroproline (Clp). We find that the molecular structure of Ac-Clp-OMe in the solid state is virtually identical to that of Ac-Hyp-OMe. In contrast, the conformational properties of Ac-clp-OMe are similar to those of Ac-Pro-OMe. Ac-Clp-OMe has a stronger preference for a trans amide bond than does Ac-Pro-OMe, whereas Ac-clp-OMe has a weaker preference. (Pro–Clp–Gly)₁₀ forms triple helices that are significantly more stable than those of (Pro–Pro–Gly)₁₀. Triple helices of (clp–Pro–Gly)₁₀ have stability similar to those of (Pro–Pro–Gly)₁₀. Unlike (Pro–Clp–Gly)₁₀ and (clp–Pro–Gly)₁₀, (clp–Clp–Gly)₁₀ does not form a stable triple helix, presumably due to a deleterious steric interaction between proximal chlorines on different strands. These data, which are consistent with previous work on 4-fluoroprolines and 4-methylprolines, support the importance of stereoelectronic and steric effects in the stability of the collagen triple helix and provide another means to modulate that stability. © 2007 Wiley Periodicals, Inc. Biopolymers 89: 443–454, 2008. This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com