Abstract
Second-coordination sphere effects such as hydrogen bonding and steric constraints that provide for specific geometric configurations play a critical role in tuning the electronic structure of metalloenzyme active sites and thus have a significant effect on their catalytic efficiency. Crystallographic characterization of vertebrate and plant sulfite oxidase (SO) suggests that an average Ooxo-Mo-SCys-C dihedral angle of ∼77° exists at the active site of these enzymes. This angle is slightly more acute (∼72°) in the bacterial sulfite dehydrogenase (SDH) from Starkeya novella. Here we report the synthesis, crystallographic, and electronic structural characterization of Tp*MoO(mba) (where Tp* = (3,5-dimethyltrispyrazol-1-yl)borate; mba = 2-mercaptobenzyl alcohol), the first oxomolybdenum monothiolate to possess an Oax-Mo-S thiolate-C dihedral angle of ∼90°. Sulfur X-ray absorption spectroscopy clearly shows that Oax-Mo-Sthiolate-C dihedral angles near 90° effectively eliminate covalency contributions to the Mo(xy) redox orbital from the thiolate sulfur. Sulfur K-pre-edge X-ray absorption spectroscopy intensity ratios for the spin-allowed S(1s) → Sv(p) + Mo(xy) and S(1s) → Sv(p) + Mo(xz,yz) transitions have been calibrated by a direct comparison of theory with experiment to yield thiolate Sv(p) orbital contributions, c i2, to the Mo(xy) redox orbital and the Mo(xz,yz) orbital set. Furthermore, these intensity ratios are related to a second coordination sphere structural parameter, the Ooxo-Mc-Sthiolate-C dihedral angle. The relationship between Mo-Sthiolate and Mo- dithiolate covalency in oxomolydenum systems is discussed, particularly with respect to electron-transfer regeneration in SO.
Original language | English |
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Pages (from-to) | 1259-1267 |
Number of pages | 9 |
Journal | Inorganic Chemistry |
Volume | 46 |
Issue number | 4 |
DOIs | |
State | Published - 19 Feb 2007 |