Desoxo molybdenum(IV) and tungsten(IV) bis(dithiolene) complexes: monomer-dimer interconversion involving reversible thiol bridge formation.

Two series of thiol-bridged dimeric desoxo molybdenum(IV) and tungsten(IV) bis(dithiolene) complexes, [Et(4)N](2)[M(IV)(2)(SR)(2)(mnt)(4)] [M = Mo, R = (1) -Ph, (2) -CH(2)Ph, (3) -CH(2)CH(3), (4) -CH(2)CH(2)OH; M = W, R = (1a) -Ph, (2a) -CH(2)Ph, (3a) -CH(2)CH(3), (4a) -CH(2)CH(2)OH] and one monomeric desoxo complex, [Et(4)N](2)[WIV(SPh)(2)(mnt)(2)] (5a) are reported. These complexes are diamagnetic, and crystal structures of each of the complex (except 5a) exhibits a dimeric {M(IV)(2)(SR)(2)} core without any metal-metal bond where each metal atom possesses hexa coordination. The M-SR distance ranges from 2.437 to 2.484 Angstrom in molybdenum complexes and from 2.418 to 2.469 Angstrom in tungsten complexes. These complexes display Mo-S(R)-Mo angles ranging from 92.84 degrees to 96.20 degrees in the case of 1-4 and W-S(R)-W angles ranging from 91.20 degrees to 96.25 degrees in the case of 1a-4a. Interestingly, both the series of Mo(IV) and W(IV) dimeric complexes respond to an unprecedented interconversion between the dimer and the corresponding hexacoordinated monomer upon change of pH. This pH-dependent interconversion establishes the fact that even the pentacoordinated Mo(IV) and W(IV) bis(dithiolene) moieties are forced to dimerize; these can easily be reverted back to the corresponding monomeric complex, reflecting the utility of dithiolene ligand in stabilizing the Mo(IV)/W(IV) moiety in synthesized complexes similar to the active sites present in native proteins.

Symphoria: the success of modeling the active site function of oxo-molybdoenzymes.

The success of modeling the active site function of oxomolybdoenzymes have been claimed generally on the basis of reactivity of the synthetic analogues towards PPh(3) or DMSO (dimethyl sulfoxide). Here it has been shown that the success of modeling the active site function of these enzymes may not be determined by the ability of a model to undergo oxotransfer with PPh(3) or DMSO (except for the modeling of DMSO reductase) and one should adhere to the criteria accepted by the bioinorganic community. A critical evaluation of two of those criteria which requires a synthetic analogue (a) should react with the enzyme substrate (b) should follow the same rate law as does the enzyme, has been presented in this paper. We have shown that the fulfillment of criterion (b) and the inhibition phenomena to that effect both are dictated by symphoria (from sympherin in Greek: the bringing together of reactants into the proper spatial relationship) on the basis of kinetic studies of the reactivity of enzyme substrate the HSO(3)(-) and its analogues (anions of oxyacids of phosphorous) towards a functional model sulfite oxidase [Bu(4)N](2)[Mo(VI)O(2)(mnt)(2)] (mnt(2-)=1,2-dicyanoethylenedithiolate) but with the caveat that the mechanistic inference drawn from such studies may not be the same as in the case of native enzyme. In view of this ambiguity it has been pointed out that the fulfillment of this criterion is not a definitive conclusion towards our understanding of the structure-function relationship of an enzyme and, therefore, the criterion of a 'structural analogue' and 'functional analogue' have been revised subject to an amendment of criterion (a) to include substrate analogues. It has also been shown for the first time on the basis of kinetic studies that the effect of medium can lead to substrate - inhibitor type dualism and hence the effect of medium is also a factor that can play a key role for the success of modeling the active site function of an enzyme. Here we also provide the details of the inhibition mechanisms proposed in our earlier report with an indirect proof to that effect.

Structural and functional analogue of the active site of polysulfide reductase from Wolinella succinogenes.

Synthesis of [PPh4]2[Mo(SPh)2(S2C2(CN)2)2] (2) from [PPh4]2[MoO(S2C2(CN)2)2] (1) has been achieved to mimic the postulated [Mo(S)6] core of polysulfide reductase with two thiolates and two bis(ene-dithiolate) ligands. Compound 2 reacts with polysulfide to yield H2S, modeling the function of polysulfide reductase. The facile conversion of 2 back to 1 in moist solvent suggests that the interconversion of the [MoIV = O] and [MoIV - X] (X = O-Ser, S-Cys, Se-Cys) moieties might occur in the DMSO reductase class of enzymes under appropriate hydrophobic/hydrophilic conditions.