Ryosuke Masuda

Abstract Selenocysteine (Sec)‐derived cyclic selenenyl amides, formed by the intramolecular cyclization of Sec selenenic acids (Sec–SeOHs), have been postulated to function as protective forms in the bypass mechanism of glutathione peroxidase (GPx). However, their chemical properties have not been experimentally elucidated in proteins or small‐molecule systems. Recently, we reported the first nuclear magnetic resonance observation of Sec–SeOHs and their cyclization to the corresponding cyclic selenenyl amides by using selenopeptide model systems incorporated in a molecular cradle. Herein, we elucidate the structures and reactivities of Sec‐derived cyclic selenenyl amides. The crystal structures and reactions toward a cysteine thiol or a 1,3‐diketone‐type chemical probe indicated the highly electrophilic character of cyclic selenenyl amides. This suggests that they can serve not only as protective forms to suppress the inactivation of Sec–SeOHs in GPx but also as highly electrophilic intermediates in the reactions of selenoproteins.

Although much attention has been paid to chemical elucidation of the catalytic cycle of glutathione peroxidase (GPx), it has been hampered by instability of selenocysteine selenenic acid (Sec–SeOH) intermediates. In this study, not only chemical processes of the canonical catalytic cycle but also those involved in the bypass mechanism, including the intramolecular cyclization of a Sec–SeOH to the corresponding five-membered ring selenenyl amide were demonstrated experimentally by utilizing selenopeptide model systems in which reactive intermediates can be stabilized by a nano-sized molecular cradle. The resulting cyclic selenenyl amide exhibited higher durability under oxidative conditions than in the state of a Sec–SeOH, corroborating its role as the protective form of GPx. The cyclization of Sec–SeOHs of the Sec-Gly-Thr and Sec-Gly-Lys models, which mimic the catalytic site of isozymes GPx1 and GPx4, respectively, was found to proceed at lower temperature than in the Sec-Gly-Gly model, which corresponds to the generalized form of the tripeptides in the catalytic site of GPx. The role of the hydrogen-bond accepting moieties in the cyclization process was elucidated by DFT calculation. It was indicated that, if the selenocysteine centers are incorporated in appropriate microenvironments, the bypass mechanism can function efficiently.

Late-stage functionalization of the periphery of oligophenylene dendrimers was efficiently achieved via site-selective C-H activation of a preconstructed, readily accessible dendron. By fourfold iridium-catalyzed C-H borylation followed by Suzuki-Miyaura cross-coupling, various arene units were introduced into the end points of the 1,3,5-phenylene-based hydrocarbon dendron. Coupling of the modified dendrons with a core unit, such as 2,6-dibromobenzoic acid derivatives, afforded the periphery-functionalized dendrimers that also have an endohedral functionality at the core position.

Although selenocysteine selenenic acids (Sec–SeOHs) have been recognized as key intermediates in the catalytic cycle of glutathione peroxidase (GPx), examples of the direct observation of Sec–SeOH in either protein or small-molecule systems have remained elusive so far, mostly due to their instability. Here, we report the first direct spectroscopic (1H and 77Se NMR) evidence for the formation of Sec–SeOH in small-molecule selenocysteine and selenopeptide model systems with a cradle-type protective group. The catalytic cycle of GPx was investigated using NMR-observable Sec–SeOH models. All the hitherto proposed chemical processes, i.e., not only those of the canonical catalytic cycle but also those involved in the bypass mechanism, including the intramolecular cyclization of Sec–SeOH to the corresponding five-membered ring selenenyl amide, were examined in a stepwise manner.

An isolable small-molecule cysteine sulfenic acid (Cys-SOH) protected by a molecular cradle was synthesized by direct oxidation of the corresponding cysteine thiol and its structure was established by X-ray crystallographic analysis. Studies on biologically relevant reactivity indicated its usefulness as a biorepresentative small-molecule sulfenic acid model.

A stable primary-alkyl-substituted selenenic acid was developed as a synthetic model of selenocysteine-derived selenenic acid (Sec-SeOH) by taking advantage of a huge cavity-shaped substituent. The primary-alkyl model compound was successfully applied to model studies on the trapping reaction of protein Sec-SeOH generated in the active site of selenoenzymes with several reagents.