Remarkably, strong binding of lumichrome to fluoride and acetate anions has been found even in such a polar solvent as acetonitrile. Global analysis of the spectrophotometric titration data proved that both 1:1 and 1:2 lumichrome-anion complexes are formed. Interaction with the second anion in the ground state led to isoalloxazine-like absorption and fluorescence spectra because of the anion-promoted electron density redistribution in the heterocyclic rings. The lifetime and the quantum yield of the fluorescence for the excited 1:2 complex closely agreed with the value previously reported for lumiflavine, an analogous compound with isoalloxazine structure, providing further evidence for the anion-induced tautomerization. Effect of water on the complex formation was also revealed. (c) 2005 Elsevier B.V. All rights reserved.