Light-absorption studies on neutral flavin radicals.

Quantitative light absorption data of the neutral free flavosemiquinone are reported. It is shown that the visible range of the spectrum of the radical is drastically dependent on the solvent. The spectra of the neutral blue flavosemiquinone are compared with those of the blue flavoprotein radicals and conclusions are drawn concerning the environment of the protein-bound flavosemiquinone. The stabilization of the neutral blue flavosemiquinone through N(5)-alkylation and/or n-complex formation with aromatic compounds is described. The light absorption characteristics of a tautomeric neutral, but red-coloured, flavosemiquinone are described. The results are compared with those obtained from the free anionic and protein-bound red flavosemiquinones. The two types of radicals cannot be distinguished from each other by electron spin resonance spectrometry but may be recognized by means of light absorption spectrophotometry. The possible biological significance of the neutral red flavosemiquinone is discussed. The optical properties of this radical are compared with those of free and protein-bound anionic flavosemiquinones.