title: Protonation and Proton-Coupled Electron Transfer at S-Ligated [4Fe-4S] Clusters.
abstract: Biological [Fe-S] clusters are increasingly recognized to undergo proton-coupled electron transfer (PCET), but the site of protonation, mechanism, and role for PCET remains largely unknown. Here we explore this reactivity with synthetic model clusters. Protonation of the arylthiolate-ligated [4Fe-4S] cluster [Fe4 S4 (SAr)4 ](2-) (1, SAr=S-2,4-6-(iPr)3 C6 H2 ) leads to thiol dissociation, reversibly forming [Fe4 S4 (SAr)3 L](1-) (2) and ArSH (L=solvent, and/or conjugate base). Solutions of 2+ArSH react with the nitroxyl radical TEMPO to give [Fe4 S4 (SAr)4 ](1-) (1ox ) and TEMPOH. This reaction involves PCET coupled to thiolate association and may proceed via the unobserved protonated cluster [Fe4 S4 (SAr)3 (HSAr)](1-) (1-H). Similar reactions with this and related clusters proceed comparably. An understanding of the PCET thermochemistry of this cluster system has been developed, encompassing three different redox levels and two protonation states.
authors.affiliation: Department of Chemistry, University of Washington, Box 351700, Seattle, WA 98195 (USA). caroline.saouma@utah.edu.|Department of Chemistry, University of Utah, 315 S 1400 E Room 2020, Salt Lake City, UT 84112 (USA). caroline.saouma@utah.edu.
authors.forename: Caroline T
authors.initials: CT
authors.lastname: Saouma
authors.affiliation: Department of Chemistry, Yale University, 225 Prospect St., PO Box 208107, New Haven, CT 06520 (USA).
authors.forename: Wesley D
authors.initials: WD
authors.lastname: Morris
authors.affiliation: Department of Chemistry, Yale University, 225 Prospect St., PO Box 208107, New Haven, CT 06520 (USA).
authors.forename: Julia W
authors.initials: JW
authors.lastname: Darcy
authors.affiliation: Department of Chemistry, University of Washington, Box 351700, Seattle, WA 98195 (USA). james.mayer@yale.edu.|Department of Chemistry, Yale University, 225 Prospect St., PO Box 208107, New Haven, CT 06520 (USA). james.mayer@yale.edu.
authors.forename: James M
authors.initials: JM
authors.lastname: Mayer
mesh_terms: D001686:Biological Phenomena; D004579:Electron Transport; D004583:Electrons; D007501:Iron / Q000737:chemistry* / Q000378:metabolism; D007506:Iron-Sulfur Proteins / Q000737:chemistry* / Q000378:metabolism; D009589:Nitrogen Oxides / Q000737:chemistry; D066244:Proton Magnetic Resonance Spectroscopy; D011522:Protons
chemical_list: D007506:Iron-Sulfur Proteins; D009589:Nitrogen Oxides; D011522:Protons; D007501:Iron; C039900:nitroxyl
keywords: PCET; [Fe-S] cluster; electron transfer; enzyme mechanism; iron