Protein Prosthesis:  1,5-Disubstituted1,2,3triazoles as cis-Peptide Bond Surrogates

Proline is unique among the natural amino acids in the similar propensity of its peptide bond to be in the cis or trans conformation. This attribute affects many processes, including the rate at which proteins fold, their structures, and their activities. Other aliphatic amino acids can serve as mimics for proline residues with trans -peptide bonds. In contrast, chemical synthesis is needed to create surrogates for cis -prolyl peptide bonds. Here, 1,5-disubstituted1,2,3triazoles were assessed as cis -peptide bond surrogates. Huisgen's 1,3-dipolar cycloaddition reaction of amino alkynes and azido acids and a Ru(II) catalyst was used to synthesize a variety of Xaa-1,5-triazole-Ala modules in moderate-to-high yields. Two of these modules, along with their 1,4-triazole regioisomers, were installed in a turn region of bovine pancreatic ribonuclease by using expressed protein ligation. The resulting semisynthetic enzymes displayed full enzymatic activity, indicating the maintenance of native structure. The 1,5-triazole surrogates instilled conformational stability that was comparable to that of Xaa- cis -Pro segments, whereas the 1,4-triazoles conferred markedly less stability. The stability conferred by both surrogates was independent of the Xaa residue, eliminating an uncertainty in protein design. We conclude that Xaa-1,5-triazole-Ala modules can serve as viable mimics of Xaa- cis -Pro segments. The possibility of synthesizing this surrogate by the ligation of fragments in situ and the emergence of biocompatible catalysts for that process portends its widespread use.