The first crystal structure of a molybdenum complex 9 with a hydrogenated pterin and a sulfur ligand contributes to the discussion about the active center of molybdenum and tungsten enzymes containing a molybdopterin cofactor. Complex 9 was synthesized through a redox reaction of [MoV'02(LN-S,)] (8; LN-S, = pyridine-2,6-bis(methanethiolato)) with 5,6,7&tetrahydropterin (7)'2 HCl (H,Ptr '2 HC1). The complex crystallizes, with a non-coordinating CI-atom acting as a counterion, in the monoclinic space group C2/c (No. 15) with cell dimensions a = 22.900(5), b = 10.716(2), c = 17.551(4)A,P = 120.36(3)0, a n d Z = 8. Weinterpret 9as [Mo"O(LN-S2)(Hf-q-H,Ptr)]Cl (q = quinonoid; H,Ptr = dihydropterin), i e . , a Mo" monooxo center coordinated by a pyridine-2,6-bis(methanethiolat0) ligand and a protonated dihydropterin. The spectroscopic properties of this new complex are comparable to those of other crystalline molybdenum complexes of hydrogenated pterins without additional S-coordination. The slightly H20-soluble complex 9 reacts with the natural enzyme subsuate DMSO very slowly, possibly due to the lack ofeasily dissociable ligands at the metal center.Introduction. -The chemistry and structure of metalloenzymes containing hydrogenated pterins') are currently intensive fields of research. The 6/?-5,6,7,8-tetrahydro-~-biopterin') (1; H4Bip3)) plays an important role as a cofactor in the iron-or copper-dependent phenylalanine hydroxylases and other amino-acid hydroxylases [2] and in the ironcontaining NO synthase [3]. A large group of molybdoenzymes utilizes molybdopterin, a hydrogenated pterin component, in the molybdenum cofactor 2 (Mo-co 2). These molybdenum enzymes catalyze 0-transfer reactions such as those of dimethyl-sulfoxide reductase, sulfite oxidase, and xanthine oxidase [4]. Mo-co is also present in the formylmethanofuran dehydrogenase of methanogenic bacteria [5], and recently molybdopterin has been identified as a component of the active center of several tungsten-containing enzymes [6]. The proposed structures of Mo-co 2 had two main features: a hydrogenated pterin and coordination to the molybdenum ion by an enedithiol group attached as a side chain to the pterin. The oxidation state of the pterin in Mo-co has been a point of
