New Insights into the Ultrafast Photophysics of Oxidized and Reduced FAD in Solution
Résumé
The ultrafast photophysics of oxidized and reduced flavin adenine dinucleotide (FAD) in aqueous solution was studied by broadband UV-Vis femtosecond transient absorption spectroscopy. We observed that oxidized FAD (FADox) in solution readily aggregates at sub-millimolar concentration. Upon excitation of FADox, three excited-state lifetimes were found and assigned to three different species: the closed (stacked) conformation of the monomer (~5.4 ps), the open (extended) conformation of the monomer (~2.8 ns), and the dimer (~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 timescale, namely 390 fs. We additionally demonstrate that deprotonated reduced flavin (FADHminus) undergoes biphotonic ionization under high excitation fluence, and dissociates into a hydrated electron and the neutral semiquinone radical FADHdot.