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Aza- and oxadithiolates are probable proton relays in functional models for the [FeFe]-hydrogenases.

ABSTRACT: The dithiolate cofactor for the [FeFe]-hydrogenase models, Fe(2)(xdt)(CO)(2)(dppv)(2) (where xdt = 1,3-propanedithiolate (pdt), azadithiolate (adt), (SCH(2))(2)NH, and oxadithiolate (odt), (SCH(2))(2)O; dppv = cis-1,2-bis(diphenylphosphino)ethylene) have been probed for their functionality as proton relays enabling formation and deprotonation of terminal hydrides. Compared to the propanedithiolate derivative, the azadithiolate and oxaditiholate show enhanced rates of proton transfer between solution and the terminal site on one Fe center. The results are consistent with the heteroatom of the dithiolate serving a gating role for both protonation and deprotonation. The pK(a) of the transiently formed ammonium (pK(CD(2))(Cl(2)) 5.7-8.2) or oxonium (pK(CD(2))(Cl(2)) -4.7-1.6) regulates the proton transfer. As a consequence, only the azadithiolate is capable of yielding the terminal hydride from weak acids. The aza- and oxadithiolates manifested the advantages of proton relays: the odt derivative proved to be a faster catalyst for hydrogen evolution than the pdt derivative as indicated from cyclic voltammetry plots of i(c)/i(p) vs [H(+)]. The adt derivative was capable of proton reduction from the weak acid [HPMe(2)Ph]BF(4) (pK(CD(2))(Cl(2)) = 5.7). The proton relay function does not apply to the isomeric bridged-hydrides [Fe(2)(xdt)(mu-H)(CO)(2)(dppv)(2)](+), where the hydride is too distant and too basic to interact to be affected by the heteroatomic relay site. None of these mu-H species can be deprotonated.

SUBMITTER: Barton BE

PROVIDER: S-EPMC2646893 | biostudies-literature | 2008 Dec

REPOSITORIES: biostudies-literature

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Aza- and oxadithiolates are probable proton relays in functional models for the [FeFe]-hydrogenases.

Barton Bryan E BE Olsen Matthew T MT Rauchfuss Thomas B TB

Journal of the American Chemical Society 20081201 50

The dithiolate cofactor for the [FeFe]-hydrogenase models, Fe(2)(xdt)(CO)(2)(dppv)(2) (where xdt = 1,3-propanedithiolate (pdt), azadithiolate (adt), (SCH(2))(2)NH, and oxadithiolate (odt), (SCH(2))(2)O; dppv = cis-1,2-bis(diphenylphosphino)ethylene) have been probed for their functionality as proton relays enabling formation and deprotonation of terminal hydrides. Compared to the propanedithiolate derivative, the azadithiolate and oxaditiholate show enhanced rates of proton transfer between solu ...[more]

PMID: 19053433

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Project description:The [FeFe]-hydrogenases are enzymes that catalyze the reversible activation of H2 coupled to the reduction-oxidation of electron carriers. Members of the different taxonomic groups of [FeFe]-hydrogenases display a wide range of preference, or bias, for H2 oxidation or H2 production reactions, despite sharing a common catalytic cofactor, or H-cluster. Identifying the properties that control reactivity remains an active area of investigation, and models have emerged that include diversity in the catalytic site coordination environments and compositions of electron transfer chains. The kinetics of proton-coupled electron transfer at the H-cluster might be expected to be a point of control of reactivity. To test this hypothesis, systematic changes were made to the conserved cysteine residue that functions in proton exchange with the H-cluster in the three model enzymes: CaI, CpII, and CrHydA1. CaI and CpII both employ electron transfer accessory clusters but differ in bias, whereas CrHydA1 lacks accessory clusters having only the H-cluster. Changing from cysteine to either serine (more basic) or aspartate (more acidic) modifies the sidechain pKa and thus the barrier for the proton exchange step. The reaction rates for H2 oxidation or H2 evolution were surveyed and measured for model [FeFe]-hydrogenases, and the results show that the initial proton-transfer step in [FeFe]-hydrogenase is tightly coupled to the control of reactivity; a change from cysteine to more basic serine favored H2 oxidation in all enzymes, whereas a change to more acidic aspartate caused a shift in preference toward H2 evolution. Overall, the changes in reactivity profiles were profound, spanning 105 in ratio of the H2 oxidation-to-H2 evolution rates. The fact that the change in reactivity follows a common trend implies that the effect of changing the proton-transfer residue pKa may also be framed as an effect on the scaling relationship between the H-cluster di(thiolmethyl)amine (DTMA) ligand pKa and E m values of the H-cluster. Experimental observations that support this relationship, and how it relates to catalytic function in [FeFe]-hydrogenases, are discussed.

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Dataset Information

Aza- and oxadithiolates are probable proton relays in functional models for the [FeFe]-hydrogenases.

Publications

Aza- and oxadithiolates are probable proton relays in functional models for the [FeFe]-hydrogenases.

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