A structural model of mixed metal sulfide cluster of molybdenum and copper present in the orange protein of Desulfovibrio gigas

Abstract

[PPh₄][Et₄N]₂[S₂MoS₂CuS₂MoS₂] (2b) has been synthesized by the reaction between [PPh₄]₂[S₂MoS₂CuCl] (2a) and [Et₄N]₂[MoS₄] as a model of the heterometal sulfide core of an orange protein (ORP) isolated from the sulfate-reducing organism, Desulfovibrio gigas. 2a and 2b are structurally characterized by single crystal X-ray crystallography. Comparison of the structural data between ORP and 2b clearly suggests that the model complex ideally duplicates the reported structural features of the ORP metal cluster as analyzed by EXAFS. The Mo1cdots, three dots, centeredCucdots, three dots, centeredMo2 angle in 2b significantly deviates from linearity (17° ). This is related to the unequal bonding of the two chelated [MoS4]^2- ligands to the central Cu as is revealed by unequal Cu-S and Mo-S_br bond lengths in the X-ray structural data. The step-wise approach for the synthesis of 2a and 2b is essential as direct reaction of CuI, like other d10 metal ions, with [MoS4]^2- led to the formation of high nuclearity cluster which thermodynamically is more favorable. The structural instability of 2b and also of ORP may reflect the inability of ORP to act as a structurally stable entity in a manner similar to that in copper-molybdenum antagonism. One oxidation peak potential (E_pa = 0.26 V vs. Ag/AgCl) of 2b is observed by cyclic voltammetry. 2b by iodine oxidation produced an EPR active Cu^II species [g^II = 2.40, gperpendicular = 2.08, Ashort parallel = 112 (10⁴ cm^-1), Aperpendicular = 14 (10⁴ cm^-1)].