Cyclic voltammetry studies of metalloflavin complexes in aqueous solution

Abstract

ciation of the ion pair will result in an apparent decrease of AV*. Such an effect is significant in those solvents with larger dielectric constants, e.g., PC. No information is available, however, concerning the pressure effect on the ion pair formation in organic solvents.DMF with the largest DN gives a negative AV*. The molecule is not as large as PC and can go deep into the coordination sphere to overcompensate the increase in volume at the transition state caused by the break of one end of the ligands. The complex undergoes solvolysis in this solvent. However, since the solvolysis proceeds more slowly than the decrease in CD strength by a factor of more than 10, the encountered experimental error will remain less than ca. 10%.If one of the acac" ligands leaves the coordination sphere to give bis(acetylacetonato)bis(dimethylformamide)germanium-(IV) ion on solvolysis, the product can be of either cis or trans isomerism. If the trans isomer were formed, it cannot be optically active and the experimental error comes only from the apparent molar extinction coefficient e, whenever the ratio Atm/e2v is used in place of the CD strength itself for the first-order kinetic plot. If ds-[Ge(acac)2(DMF)2] + were' formed with retention of the configuration, the product could contribute to the CD strength at 305 nm. However, bis(/3diketonato) complexes of metal ions have UV absorption maxima due to the exciton band at a lower wavenumber region than the tris-type complexes do.17 The CD strength at the negative exciton peak of this bis(S-diketonato) complex should shift to the lower wavenumber region for the present isomer, which has a negative CD peak at a lower wavenumber side. The ( decreases more steeply than the molar extinction coefficient does at the higher wavenumber side of the peaks. Therefore, the contribution of ds-[Ge(acac)2(DMF)2]+ to the <305/<287 ratio cannot be large. The In (Ae305/í287) vs. time plots, in fact, remained linear until 70% of the initial [Ge-(acac) 3] + racemized. This fact supports indirectly the legitimacy of our kinetic treatment.Conformity of the racemization mechanism in donating and less donating solvents was also discussed on the basis of the rather continuous change of AH* and AS* values.8 The present results support that the racemization proceeds via an intermediate with one unidentate ligand that was formed by a solvent-assisted bond break. However, a less donating solvent assists the bond break to only a very small extent.Acknowledgment. The authors thank the Ministry of Education of the Japanese Government for Grant-in-Aid No. 434029.Registry No. (-)589-[Ge(acac)3]Ci04, 76370-51-9.

Electronic properties of hydroquinone-containing ruthenium complexes in different oxidation states

“…Such decomposition has been reported elsewhere for phenolatebound complexes. 12 Proton NMR spectroscopy (see below) and elemental analysis indicate that in the complex the free OH group is protonated and that as expected the co-ordinated hydroxyl group is deprotonated.…”

Pterin chemistry and its relationship to the molybdenum cofactor

“…The evidence was structural distortion of flavin observed crystallographically, consistent with partial Ru(+3)-flavinsemiquinone character [105]. Clarke’s group also determined that Ru-pterin complexation shifted subsequent protonation from N1 to N8, due to electronic delocalization from Ru(+2) to the pterin anti-bonding pi system (i.e., back bonding), making the pyrazine more electron rich and basic [106].…”

Revelation of Varying Bonding Motif of Alloxazine, a Flavin Analogue, in Selected Ruthenium(II/III) Frameworks

“…Although the metal ions in the vicinity of flavoenzymes facilitate the intraprotein electron-transfer processes, the coordination of flavin with the metal ions in the enzymes has yet not been recognized . This indeed has spurred the development of model metal complex frameworks of flavin and its tricyclic analogue alloxazine or isoalloxazine (Scheme ) in order to understand (i) their coordinating mode(s), (ii) metal ion prompted electron-transfer processes including the effect of noncovalent interactions (hydrogen bonding and π–π interaction), and (iii) accessibility of the intermediate radical state. , …”