pH dependence of the absorption and emission behaviour of riboflavin in aqueous solution

The absorption and fluorescence behaviour of riboflavin is investigated in aqueous solution over a wide range of pH values from pH = - 1.1 to pH = 13.4. Absorption spectra, fluorescence quantum distributions, fluorescence quantum yields, and fluorescence signal decays are measured. Riboflavin exists in three different forms depending on the pH of the solutions. There is an equilibrium between the cationic form (RFl(ox)H(2)(+)) and the neutral form (RFl(ox)H) at low pH (pK(c) = 0.4), and between the neutral form and the anionic form (RFl(ox)(-)) at high pH (pK(a) = 9.75). The cationic, neutral, and anionic species have different absorption spectra. The fluorescence quantum yields and fluorescence lifetimes of the various forms are separated. The cationic form is non-fluorescent (fluorescence quantum yield phi(F) < 5 x 10(-5)), and the anionic form has a low fluorescence quantum yield of phi(F) approximate to 1.2 x 10(-3). The fluorescence quantum yield of the neutral form is around phi(F) approximate to 0.26 above pH = 4 and decreases strongly below pH = 4 because of [H+]-dependent excited-state reaction of RFl(ox)H* to RFl(ox)H(2)(+)*. At low pH the fluorescence signal decay is single-exponential since only the neutral form, RFl(ox)H, is emitting. At high pH a bi-exponential fluorescence decay occurs because RFl(ox)H and RFl(ox)(-) emit with different time constants. (C) 2002 Published by Elsevier Science B.V.