Steric and stereoelectronic effects play a defining role in molecular conformation and reactivity. In small molecules, steric and stereoelectronic effects often have dichotomous consequences. For example, the anomeric effect in glycosides yields axial substituents that are disfavored by sterics.1 Similarly, replacing the steric effect of a methyl group with the stereoelectronic effect of a fluoro group enables a â-peptide to fold.2 Stereoelectronic effects contribute markedly to the conformational stability of an abundant protein: collagen.3-5 Collagen is a fibrous protein comprising a bundle of three parallel strands folded into polyproline type II helices.6 Each strand consists of 300 repeats of the unit: Xaa-Yaa-Gly, where Xaa is often (2S)-proline (Pro) and Yaa is often (2S,4R)-4-hydroxyproline (Hyp). The pyrrolidine rings in the Xaa and Yaa positions have Cç-endo and Cç-exo ring puckers, respectively.7 These puckers can be preordained by
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