Proton-coupled electron transfer of macrocyclic ring hydrogenation: The chlorinphlorin.

Sun, Rui; Liu, Mengran; Zheng, Shao-Liang; Dogutan, Dilek K; Costentin, Cyrille; Nocera, Daniel G · Proc Natl Acad Sci U S A · 2022

basic_science · Level V

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Abstract

SignificanceThe chemical reduction of unsaturated bonds occurs by hydrogenation with H<sub>2</sub> as the reductant. Conversely, in biology, the unavailability of H<sub>2</sub> engenders the typical reduction of unsaturated bonds with electrons and protons from different cofactors, requiring olefin hydrogenation to occur by proton-coupled electron transfer (PCET). Moreover, the redox noninnocence of tetrapyrrole macrocycles furnishes unusual PCET intermediates, including the phlorin, which is an intermediate in tetrapyrrole ring reductions. Whereas the phlorin of a porphyrin is well established, the phlorin of a chlorin is enigmatic. By controlling the PCET reactivity of a chlorin, including the use of a hangman functionality to manage the proton transfer, the formation of a chlorinphlorin by PCET is realized, and the mechanism for its formation is defined.