New Insights into the Ultrafast Photophysics of Oxidized and Reduced FAD ins Solution

The ultrafast photophysid of oxidized and reduced flavin adenine dinucleotide (FAD) M aqueous solution was studied by broadband UV-vis, femtosecond transient absorption spectroscopy. We observed that oxidized FAD (FAD(ox)) in solution readily aggregates at submillimolar concentration. Upon excitation of FAD(ox), three excited-state lifetimes were found and assigned to three different species: the Closed (stacked) conformation of the monomer (similar to 5.4 ps), the open (extended) conformation of the monomer (similar to 2.8 ns), and the dimer (similar to 27 ps). In the case of the stacked conformation of the monomer, we show that intramolecular electron transfer from the adenine to the isoalloxazine ring occurs with a time constant of 5.4 ps and is followed by charge recombination on a faster time scale, namely, 390 fs. We additionally demonstrate that deprotonated reduced flavin (FADH(-)) undergoes biphotonic ionization under high excitation fluence and dissociates into a hydrated electron and the neutral semiquinone radical FADH(center dot).