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The molecular complex 10-methylisoalloxazinium bromide: sesqui-naphthalene-2,7-diol monohydrate appears to be of charge-transfer nature and together with earlier work suggests that the flavin “chelate site,” CO(4)–N(5), may be significant in flavin–protein binding, even in the oxidized state.
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Langhoff et al. (1970) studied this question.