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Interplay of two low-barrier hydrogen bonds in long-distance proton-coupled electron transfer for water oxidation.

ABSTRACT: D1-Tyr161 (TyrZ) forms a low-barrier H-bond with D1-His190 and functions as a redox-active group in photosystem II. When oxidized to the radical form (TyrZ-O^•), it accepts an electron from the oxygen-evolving Mn₄CaO₅ cluster, facilitating an increase in the oxidation state (S_n; n = 0-3). In this study, we investigated the mechanism of how TyrZ-O^• drives proton-coupled electron transfer during the S₂ to S₃ transition using a quantum mechanical/molecular mechanical approach. In response to TyrZ-O^• formation and subsequent loss of the low-barrier H-bond, the ligand water molecule at the Ca²⁺ site (W4) reorients away from TyrZ and donates an H-bond to D1-Glu189 at Mn4 of Mn₄CaO₅ together with an adjacent water molecule. The H-bond donation to the Mn₄CaO₅ cluster triggers the release of the proton from the lowest pK_a site (W1 at Mn4) along the W1…D1-Asp61 low-barrier H-bond, leading to protonation of D1-Asp61. The interplay of the two low-barrier H-bonds, involving the Ca²⁺ interface and forming the extended Grotthuss-like network [TyrZ…D1-His190]-[Mn₄CaO₅]-[W1…D1-Asp61], rather than the direct electrostatic interaction, is likely a basis of the apparent long-distance interaction (11.4 Å) between TyrZ-O^• formation and D1-Asp61 protonation.

SUBMITTER: Saito K

PROVIDER: S-EPMC10733176 | biostudies-literature | 2023 Dec

REPOSITORIES: biostudies-literature

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Interplay of two low-barrier hydrogen bonds in long-distance proton-coupled electron transfer for water oxidation.

Saito Keisuke K Nishio Shunya S Ishikita Hiroshi H

PNAS nexus 20231207 12

D1-Tyr161 (TyrZ) forms a low-barrier H-bond with D1-His190 and functions as a redox-active group in photosystem II. When oxidized to the radical form (TyrZ-O•), it accepts an electron from the oxygen-evolving Mn4CaO5 cluster, facilitating an increase in the oxidation state (Sn; n = 0-3). In this study, we investigated the mechanism of how TyrZ-O• drives proton-coupled electron transfer during the S2 to S3 ...[more]

PMID: 38130665

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Project description:Despite advances in hydrogen atom transfer (HAT) catalysis, there are currently no molecular HAT catalysts that are capable of homolysing the strong nitrogen-hydrogen (N-H) bonds of N-alkyl amides. The motivation to develop amide homolysis protocols stems from the utility of the resultant amidyl radicals, which are involved in various synthetically useful transformations, including olefin amination and directed carbon-hydrogen (C-H) bond functionalization. In the latter process-a subset of the classical Hofmann-Löffler-Freytag reaction-amidyl radicals remove hydrogen atoms from unactivated aliphatic C-H bonds. Although powerful, these transformations typically require oxidative N-prefunctionalization of the amide starting materials to achieve efficient amidyl generation. Moreover, because these N-activating groups are often incorporated into the final products, these methods are generally not amenable to the direct construction of carbon-carbon (C-C) bonds. Here we report an approach that overcomes these limitations by homolysing the N-H bonds of N-alkyl amides via proton-coupled electron transfer. In this protocol, an excited-state iridium photocatalyst and a weak phosphate base cooperatively serve to remove both a proton and an electron from an amide substrate in a concerted elementary step. The resultant amidyl radical intermediates are shown to promote subsequent C-H abstraction and radical alkylation steps. This C-H alkylation represents a catalytic variant of the Hofmann-Löffler-Freytag reaction, using simple, unfunctionalized amides to direct the formation of new C-C bonds. Given the prevalence of amides in pharmaceuticals and natural products, we anticipate that this method will simplify the synthesis and structural elaboration of amine-containing targets. Moreover, this study demonstrates that concerted proton-coupled electron transfer can enable homolytic activation of common organic functional groups that are energetically inaccessible using traditional HAT-based approaches.

Dataset Information

Interplay of two low-barrier hydrogen bonds in long-distance proton-coupled electron transfer for water oxidation.

Publications

Interplay of two low-barrier hydrogen bonds in long-distance proton-coupled electron transfer for water oxidation.

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