Structural and spectroscopic studies of vanadium complexes that model vanadium-protein coordination.

摘要

The complex ion [VVO(HIBA)]- [H 4HIBA is 1,2-bis(2-hydroxy-2-methylpropanamido)benzene], the first crystallographically characterized vanadium(V) complex where the vanadium is directly bonded to a deprotonated amide nitrogen, provides a model for interactions between vanadium and the amide nitrogen of the protein backbone. The amide contribution to the EPR parallel coupling constant was determined from the vanadium(IV) analog of this complex, [VIVO(HIBA)]2-, to be Az,amide = 37 x 10-4 cm -1. This value can be used in an EPR additivity relationship for square pyramidal oxovanadium(IV) complexes to determine the coordination sphere of vanadium complexes. Through a series of structurally characterized vanadium-Schiff base complexes of varying geometry, this EPR additivity relationship was shown to be effective even for highly distorted trigonal bipyramidal molecules. The EPR parallel hyperfine coupling constant ( A∥ or Az) does not change appreciably with structural distortions and can therefore be used to propose or refute a coordination environment for vanadium. The perpendicular region of the EPR spectrum, however, gives a good indication of the geometric distortion of the molecule. Electron paramagnetic resonance spectroscopy can now be used in a wide variety of vanadium-biomolecule situations to help elucidate the binding mode of and relative geometry about vanadium.; Vanadium binds to protein tyrosine phosphatase (PTP) through a cysteinal sulfur at the active site to form a five-coordinate trigonal bipyramidal vanadium adduct, where the sulfur is in an axial position and the balance of the coordination sphere is composed of oxygen. Phosphorus coordinates to the ligand mbp 2S2- [H2mbp2S is 2,2 '-thiobis(4-methyl-6-tert-butylphenol)] such that the sulfide sulfur is in an axial position. Since vanadium can behave as a trigonal bipyramidal transition state analog in phosphate binding sites, it was anticipated that vanadate would assume a similar structure with mbp2S2- and provide a model for V-PTP binding. Instead, the resulting complex was an alkoxide bridged dimer with each vanadium(V) coordinated to one mbp 2S ligand, an oxo oxygen, and the two bridging alkoxide oxygens. Oxygenation of the sulfide complex to a sulfoxide monomer occurred indirectly via an oxoperoxovanadium intermediate.