Influence of nanoenvironment in reverse micelles on the fluorescence of flavins

Riboflavin (RF) and flavin adenine dinucleotide (FAD) are two chromophores in ubiquitous blue-light photoreceptors where molecular motions are partially constrained. However, most previous studies on their fluorescence were performed in solutions. In this work, we studied their fluorescence in Aerosol-OT (AOT) and cetyltrimethylammonium bromide (CTAB) reverse micelles (RMs) by using steady-state UV–visible spectroscopy and femtosecond transient absorption spectroscopy. Compared with in aqueous solution, fluorescence of RF in AOT RMs remains nearly constant, but it is quenched in CTAB RMs induced by intersystem crossing promoted by bromide anion. In addition, fluorescence of FAD in AOT RMs is strengthened because the stacked conformation is less favorable in AOT RMs than in aqueous solution. Moreover, the fluorescence quenching of FAD in CTAB RMs by intersystem crossing mainly affects the unstacked FAD because the intramolecular charge transfer in stacked FAD is faster than intersystem crossing by more than 40 folds. These results determine the dependence of fluorescence of flavins on the ions, polarity and steric constraints in nanoenvironment.