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The chemical and electronic structure of the neutral flavin radical as revealed by electron spin resonance spectroscopy of chemically and isotopically substituted derivatives.

The neutral flavosemiquinone has been studied in detail by electron spectroscopy. Isotopic (15N, 2H) and chemical substitutions have been carried out. A hyperfine coupling scheme of the neutral flavosemiquinone is described where N(5), N(10), CH3(10), CH3(8) and H(6) are involved. The highest spin density is located at N(5) as has also been found for the anionic and chelated form of the flavosemiquinone. The exchangeable proton in the neutral flavosemiquinone is attached to N(5) having a coupling constant of 7.6 G which is about the same magnitude as the coupling constant to N(5). The hyperfine couplings are interpreted in terms of spin densities and comparison is made with available quantum chemical calculations. Evidence is presented that two tautomeric forms of the neutral flavosemiquinone have to be considered which can be obtained in a pure form by suitable substitution of flavin derivatives. The two types of neutral flavosemiquinones are easily distinguishable by their electron spin resonance and light absorptions.