Structural and electronic effects resulting from metal-flavin ligation.

The synthesis and physical properties of a model metalloflavin complex, (10-methylisoalloxazine)-(NH3)4Ru2 . 2H2O are reported. The structure of this stable, enantiomeric compound was elucidated by x-ray diffraction methods with a final unweighted R value of 0.054. Crystals belong to the triclinic space group P1 with unit cell dimensions: a = 9.631, b = 10.618, c = 13.216 A; alpha = 113.86, beta = 100.19, gamma = 94.12 degrees. Chelation of the metal ion to the flavin occurs at the N(5) and O(4) positions, with a short Ru-N(5) bond distance of 1.979 A. Steric and electronic factors induce a 9.9 degrees bend in the isoalloxazine ring system and a significant lengthening of the C(4a)-N(5) bond. The flavin absorption bands shift significantly toward lower energy on complexation and a new band occurs at 617 nm. These absorptions are pH-dependent and pK alpha values for the complex are 0.6 and 7.4. The spectra of this complex exhibit similarities to that of metallosemiquinone species and arguments are made that a significant amount of electron density is donated to the pi-system of the flavin. Proton NMR studies suggest enhanced electron density at the N(10) position, probably occurring through backbonding interactions. Cyclic voltametry studies are also consistent with substantial metal to ligand pi-electron donation, since it is significantly more difficult to reduce the coordinated flavin relative to the free ligand under the same conditions. Moreover, the complexed flavin accepts electrons in 1-electron rather than 2-electron steps. Spectroelectrochemical studies on the 1-electron reduced complex indicate a similarity with other M(II)-F1 species.