MODELING THE TUNGSTEN SITES OF INACTIVE AND ACTIVE FORMS OF HYPERTHERMOPHILIC PYROCOCCUS-FURIOSUS ALDEHYDE FERREDOXIN OXIDOREDUCTASE

The complex [Et(4)N](2)[(WO2)-O-VI(mnt)(2)] (1), [Et(4)N](2)[(WO)-O-IV (mnt)(2)] (2), and [Et(4)N](2)[(WO)-O-VI(S-2)(mnt)2] (3) (mnt(2-) = 1, 2-dicyanoethylenedithiolate) have been synthesized as possible models for the tungsten cofactor of inactive red tungsten protein (RTP) and t he active aldehyde ferredoxin oxidoreductase (AOR) of the hyperthermop hilic archaeon Pyrococcus furiosus. The [Ph(4)P](+) salt of the comple x anion of 1 . 2H(2)O crystallizes in space group Pbcn, with a = 20.52 6(3) Angstrom, b = 15.791(3) Angstrom, c = 17.641(3) Angstrom, and Z = 4. The (WO2S4)-O-VI core of [Ph(4)P](2)[(WO2)-O-VI(mnt)(2)]. 2H(2)O h as distorted octahedral geometry with cis dioxo groups. 2 crystallizes in space group P2(1)2(1)2, with a = 14.78(3) Angstrom, b = 30.08(2) A ngstrom, c = 7.37(4) Angstrom, and Z = 4. The complex anion of 2 has a distorted square-pyramidal structure with an axial W=O bond. 3 crysta llizes in space group P2(1)/a, with a = 12.238(3) Angstrom, b = 18.873 (2) Angstrom, c = 15.026(2) Angstrom, beta = 102.84(2)degrees, and Z = 4. The anion of 3 with a terminal oxo group and a dihapto disulfido l igand in an adjacent position is the first example of a seven-coordina te W(VI) species with bis-dithiolene coordination. The complexes 1-3 h ave been characterized by IR, UV-visible, C-13 NMR, negative ion FAB m ass spectra, and electrochemical properties. Complex 1 reacts with H2S , PhSH, 1,4-dithiothreitol (DTT), or dithionite (S2O42-) to yield 2 wi th the oxidation of these reducing agents suggesting intramolecular el ectron transfer in the respective intermediates across the W(VI)-sulfu r bond. Participation of this type of redox reaction, seemingly unreal istic from the point of view of real reduction potential values of 1 a nd of these reductants, is best explained by the formation of a precur sor complex. This relates to the essential formation of a Michaelis (e nzyme-substrate) complex wherein the individual chemical identity of t he free enzyme and unbound substrate is lost. Subsequent atom transfer reaction embodies internal electron transfer between the two redox pa rtners present in the enzyme-substrate complex. The terminal oxo group of 2 is readily protonated (pH < 4) to yield [W-IV(mnt)(3)](2-). 2 re sponds to a metal exchange reaction with MoO42- to form [(MoO)-O-IV(mn t)(2)](2-) which is similar to in vitro reconstitution of the molybden um cofactor by MoO42- in tungsten formate dehydrogenase (W-FDH). The m odel reaction between 2 and MoO42- involves a stepwise one-electron tr ansfer reaction from W(IV) to Mo(VI) with the intermediate formation o f EPR active W(V) species. Oxidative addition of elemental sulfur from 2 affords 3, which gives sulfur atom transfer reactions with several thiophiles. 3 reacts with Ph(3)P in a second-order process (A + 2B typ e) to yield 2 and Ph(3)PS with the observed rate constant k(2) = 4.3 ( +/- 0.06) M(-1) s(-1) at 25 degrees C (Delta H = 5.14 (+/- 0.46) kcal /mol, Delta S = -38.35 (+/- 1.5) cal/(deg . mol)). A cyclic voltammet ric study suggests the attack of Ph(3)P across the W-S bond in the WS2 moiety of 3. 2 catalyzes the reactions Ph(3)P + S --> Ph(3)PS and H-2 + S --> H2S, demonstrating its sulfur reductase activity. No such rea ction is observed in the absence of 2. Formaldehyde reduces 3 to 2. Cr otonaldehyde reacts with 3 to yield 2 and crotonic acid in MeCN contai ning H2O (5% v/v) or in CH2Cl2 medium which demonstrates that aldehyde oxidase activity of 3 is similar to that of the active AOR enzyme of P. furiosus.