1H NMR study of the hetero-association of flavin-mononucleotide with mutagenic dyes: ethidium bromide and proflavine

The hetero-association of the vitamin B₂ derivative, flavin-mononucleotide (FMN), with a mutagenic dye, ethidium bromide (EB) or proflavine (PF), has been studied by 1D and 2D 500?MHz ¹H NMR spectroscopy. The variations of proton chemical shifts of both the vitamin and dye as a function of concentration and temperature were analysed in terms of the structural and thermodynamical properties of the FMN–EB and FMN–PF complexes in solution. The structures of the complexes were also investigated by observed intermolecular ROE contacts and molecular mechanics calculations. The results show that the 1?:?1 hetero-association complexes in solution are more stable than the self-association complexes, which is consistent with formation of an intermolecular hydrogen-bond in the hetero-complexes of FMN–EB and FMN–PF. Hence it is possible that the toxicity of aromatic molecules such as EB and PF may be reduced in vitro by the presence of FMN, partly because of the known antimutagenic action of FMN and partly because it has been shown in this work that there is an effective intermolecular association between the mutagens and the vitamin.