Fluorescence quenching of flavin adenine dinucleotide in aqueous solution by pH dependent isomerisation and photo-induced electron transfer

Abstract

Abstract The fluorescence behaviour of flavin adenine dinucleotide (FAD) in aqueous solution is investigated over a wide hydrogen ion range from pH=1 to pH=11.3. The accessible pH range is limited by room temperature hydrolysis of FAD to flavin-mononucleotide and adenosine-monophosphate at high and low pH. Fluorescence quantum distributions, fluorescence quantum yields, and fluorescence signal decays are measured. The results are compared with the pH dependence of riboflavin in aqueous solution. In the range from pH ≈ 4 to pH ≈ 9 the fluorescence quantum yield is approximately constant at φ F ≈0.033. Below pH=4 the fluorescence rises up to φ F ≈0.13 around pH=2.5 and then reduces similar as riboflavin. The fluorescence behaviour is discussed in terms of a two conformation system. In the range from pH ≈ 3.5 to pH ≈ 11 the stacked (closed) isoalloxazine–adenine conformation dominates over the un-stacked (open) conformation in the S 0 ground-state. At higher and lower pH the ground-state isomerisation equilibrium shifts towards un-stacked conformation dominance. S 1 state photo-isomerisation converts un-stacked molecules to stacked molecules, especially in the range from pH ≈ 3 to pH ≈ 11. The stacked molecules are practically non-fluorescent due to reductive photo-induced electron transfer from the adenine moiety to the isoalloxazine moiety.