Modeling the tungsten sites of inactive and active forms of hyperthermophilic Pyrococcus furiosus aldehyde ferredoxin oxidoreductase

Abstract

The complex [Et₄N]₂[W^VIO₂(mnt)₂] (1), [Et₄N]₂[WIVO(mnt)₂] (2), and [Et₄N]₂[WVIO(S₂)(mnt)₂] (3) (mnt²⁻= 1,2-dicyanoethylenedithiolate) have been synthesized as possible models for the tungsten cofactor of inactive red tungsten protein (RTP) and the active aldehyde ferredoxin oxidoreductase (AOR) of the hyperthermophilic archaeon Pyrococcus furiosus. The [Ph4P]⁺ salt of the complex anion of 1·2H₂O crystallizes in space group Pbcn, with a=20.526(3) Å , b=15.791(3) Å , c=17.641(3) Å , and Z=4. The W^VIO₂S₄ core of [Ph₄P]2[W^VIO₂(mnt)₂]·2H₂O has distorted octahedral geometry with cis dioxo groups. 2 crystallizes in space group P2₁2₁2, with a =14.78(3) Å, b=30.08(2) Å , c=7.37(4) Å , and Z=4. The complex anion of 2 has a distorted square-pyramidal structure with an axial WO bond. 3 crystallizes in space group P21/a, with a=12.238(3) Å , b=18.873(2) Å , c=15.026(2)Å , β=102.84(2)° , and Z=4. The anion of 3 with a terminal oxo group and a dihapto disulfido ligand in an adjacent position is the first example of a seven-coordinate W(VI) species with bis-dithiolene coordination. The complexes 1-3 have been characterized by IR, UV-visible, ¹³C NMR, negative ion FAB mass spectra, and electrochemical properties. Complex 1 reacts with H₂S, PhSH, 1,4-dithiothreitol (DTT), or dithionite (S₂O^42-) to yield 2 with the oxidation of these reducing agents suggesting intramolecular electron transfer in the respective intermediates across the W(VI)-sulfur bond. Participation of this type of redox reaction, seemingly unrealistic from the point of view of real reduction potential values of 1 and of these reductants, is best explained by the formation of a precursor complex. This relates to the essential formation of a Michaelis (enzyme-substrate) complex wherein the individual chemical identity of the free enzyme and unbound substrate is lost. Subsequent atom transfer reaction embodies internal electron transfer between the two redox partners 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 responds to a metal exchange reaction with MoO₄²⁻ to form [MoIVO(mnt)2]2− which is similar to in vitro reconstitution of the molybdenum cofactor by MoO₄²⁻ in tungsten formate dehydrogenase (W-FDH). The model reaction between 2 and MoO₄²⁻ involves a stepwise one-electron transfer reaction from W(IV) to Mo(VI) with the intermediate formation of 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 Ph₃P in a second-order process (A+2B type) to yield 2 and Ph₃PS with the observed rate constant k₂=4.3 (± 0.06) M⁻¹ s⁻¹ at 25 ° C (Δ H=5.14 (± 0.46) kcal/mol, Δ S=−38.35 (± 1.5) cal/(deg·mol)). A cyclic voltammetric study suggests the attack of Ph₃P across the W-S bond in the WS₂ moiety of 3. 2 catalyzes the reactions Ph₃P + S → Ph₃PS and H₂+S → H₂S, demonstrating its sulfur reductase activity. No such reaction is observed in the absence of 2. Formaldehyde reduces 3 to 2. Crotonaldehyde reacts with 3 to yield 2 and crotonic acid in MeCN containing 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.