Consequences of incorporating thiaproline and its oxidized derivatives into collagen triple helices.

(2R)-4-thiaproline (Thp) is an analog of proline, replacing C in the pyrrolidine ring with sulfur. Its thiazolidine ring easily interconverts between endo and exo puckers due to a small energy barrier, which leads to destabilize polyproline helices. Collagen, composed of three polyproline II helices, mainly consists of X-Y-Gly triplets, where X is often proline and Y is frequently (2S,4R)-hydroxyproline.