Coenzyme models. Part 23. Formation and reactivity of the stable 'quinone form' of flavin in cationic polymer matrices

The flavin covalently linked to quaternised poly(4-vinylpyridine) via 8α-position afforded, in alkaline solution, a blue species absorbing at 622 nm. On the basis of spectral examination, the species is assigned to the 'quinone form' of the flavin. The regioselective proton abstraction from the 8α-methylene is due to the enriched OH^- concentration along the polymer chain, and probably also due to the stabilisation of the delocalised quinone form by the polyelectrolyte environment. The quinone flavin was insensitive to molecular oxygen and could oxidise neither NADH analogues nor thiols, indicating that the reactivity is markedly different from the normal flavin form.