Light absorption of flavosemiquinone metal chelates.

Abstract

The light absorption of flavosemiquinone metal chelates (Zn2+, Cd2+, Mn2+, Co2+, Ni2+, Fe2+, Mo5+, and Cu+) in aprotic polar solvents has been studied. The light absorption of flavosemiquinone metal chelates, the uncomplexed cation and anion free radicals, and the corresponding, very unstable, flavoquinone metal chelates are compared. The visible spectra of the flavosemiquinone metal chelates, irrespective of the metal, are very similar to that of the free radical anion. In addition, they show a rather unique, sharp transition in the near infrared range (650 to 850 nm). The extinction coefficients for d1, d5, d6, d7, d8, and d10 metal centres are between 10 and 2 × 103 M−1cm−1. Solvents that act as quaternary ligands (pyridine, acetonitrile, but not 2,6-lutidine) strongly affect this near infrared transition, but not the visible spectrum. We therefore conclude that the near infrared absorption reflects the symmetry of the flavosemiquinone-metal-H2O-solvent complex. The molar extinction coeffcients in the near infrared are independent of chelate concentration, solvent polarity, and ionic strength, and the optical densites are proportional to total spin concentration as monitored by electron spin resonance (for systems of total spin 1/2, i. e. with d10 central ions). This excludes association phenomena. Altering the flavin moiety, even slightly, effects the light absorption spectrum in the visible and near infrared regions. This near infrared absorption may be a useful tool for detecting flavin-metal chelation in enzyme systems.