Deep ultraviolet initiated excited state dynamics of riboflavin and flavin mononucleotide

Abstract

Flavins are cofactors in several light-activated enzymes and therefore their excited states are found to involve in many photobiological processes. Excited state dynamics of flavin compounds corresponding to their first singlet state (S-1) has been studied using a plethora of techniques, whereas studies related to highly absorbing ultraviolet excited states are lacking. Here, we study the ultrafast excited state dynamics of riboflavin and flavin mononucleotide using resonance Raman intensity analysis upon photoexcitation into their most intense absorption band centered at 266nm. Resonance Raman cross sections of each flavin band are quantitatively measured across the 266-nm absorption band (257-280nm), and Raman excitation profiles are constructed. We have used Lee and Heller's time-dependent wave packet theory to simulate the experimental Raman cross sections in a self-consistent manner. The simulation results in instantaneous structural changes along with solvation dynamics, through linewidth broadening within tens of femtoseconds following photoexcitation. Major structural changes were observed through contraction and elongation of several ring stretching coordinates, affecting at a different site when compared with the S-1 excitation. The value of the total reorganization energy was determined to be 1,665cm(-1) (and 1,602cm(-1) for flavin mononucleotide) with a contribution of 1,310cm(-1) from the inertial response of water. We find upon excitation, the first solvation shell inertially responds with an ultrafast timescale of <30fs for both the molecules. Our results can be useful to determine the structure and dynamics of flavoenzymes by using flavin as a probe following excitation within their 266-nm absorption band.