Spectroscopic Characterization of the Photolysis of Riboflavin (Vitamin B2) via Time-Resolved Mass Spectrometry and IRMPD Spectroscopy.

Riboflavin, Vitamin B2, is a key photoactive biomolecule that has important uses as a food additive and as a photocatalyst. While riboflavin's photodegradation pathways have been studied extensively, open questions exist about the effect of the chemical environment on riboflavin photodegradation and the nature of the subsequent photoproducts. Here, we use time-resolved mass spectrometry (TRMS) and gas-phase infrared multiple-photon dissociation (IRMPD) spectroscopy to characterize 365 nm online photolysis of riboflavin under basic conditions. TRMS allowed for monitoring of the light-induced decay of deprotonated riboflavin along with the formation of photoproducts and photolysis intermediates. IRMPD spectroscopy was performed over the fingerprint region (1100-1800 cm) at the FELIX free-electron laser facility, to obtain the first gas-phase IR spectrum of deprotonated riboflavin, the isolated chromophore, along with the IRMPD spectrum of the deprotonated riboflavin dimer. In addition, spectroscopic characterization was performed for the photoproducts lumichrome and lumiflavin, as well as the photolysis intermediates formylmethylflavin and the riboflavin-lumichrome dimer. Our experiments reveal that 365 nm photolysis of the riboflavin dimer is enhanced compared with the monomer, potentially due to spectral shifting of the chromophore upon complexation. The clear propensity for formation of the dimer that we observe for riboflavin and its photolysis behavior indicates that aggregates play a significant role in accelerating photodegradation of riboflavin. This is the first time, to our knowledge, that such an effect has been identified in flavin photochemistry and provides new insight into why photodegradation of riboflavin is particularly sensitive to solution conditions.