Papain-Catalyzed, Sequence-Dependent Polymerization Yields Polypeptides Containing Periodic Histidine Residues

His-containing polypeptides, including polyHis, are attractive materials due to the unique characteristics of the imidazole ring of the His residue. In particular, His-containing polypeptides with repetitive sequences have a variety of distinctive features based on their periodic structure. In this study, chemoenzymatic polymerization of ethyl ester monomers with sequences His, GlyHis, HisGly, and GlyHisGly with hydrophobic side chains on the imidazole ring was performed using papain as a catalyst. Sequence dependence in chemoenzymatic polymerization was observed for GlyHis-and HisGly-based monomers: GlyHis-based monomers did not undergo polymerization, whereas polymerization of HisGly-based monomers afforded polypeptides with a degree of polymerization from 6 to 38 and from 5 to 31 and a number-average degree of polymerization of 16.4 and 12.4 for poly(HisGly) and poly[His(Bu)Gly], respectively. The difference in polymerizability of these dipeptide monomers was supported by a docking simulation between these monomers and papain, where the ester group of the HisGly-based monomer was closer to the catalytic center of papain than that of the GlyHis-based monomer. Infrared spectroscopy and synchrotron wide-angle X-ray diffraction measurements indicated that poly(HisGly) formed a beta-sheet structure whose crystallinity was 41.6%, whereas the other tripeptide-based polypeptides were more amorphous showing 19.6- 30.7% of crystallinity. Poly(HisGly) exhibited the highest thermal stability among all of the polypeptides in the thermogravimetric analysis, reflecting the difference in the secondary structures.

Automated summary

His-containing polypeptides with repetitive sequences were polymerized using chemoenzymatic methods, resulting in polypeptides with different structures and thermal stability.