Flavins are photoenzymatic cofactors often exploiting the absorption of light to energize photoinduced redox chemistry in a variety of contexts. Both flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) are used for this function. The study of these photoenzymes has been facilitated using flavin analogs. Most of these analogs involve modification of the flavin ring, and there is recent evidence that adenine (Ade)-modified FAD can affect enzyme turnover, but so far this has only been shown for enzymes where the adenine and flavin rings are close to each other in a stacked conformation. FAD is also stacked in aqueous solution, and its photodynamics are quite different from unstacked FAD or FMN. Oxidized photoexcited FAD decays rapidly, presumably through PET with Ade as donor and Fl* as acceptor. Definitive identification of the spectral signatures of Ade(center dot+) and Fl(center dot-) radicals is elusive. Here we use the FAD analog Flavin 1,N-6-Ethenoadenine Dinucleotide (epsilon FAD) to study how different photochemical outcomes depend on the identity of the Ade moiety in stacked FAD and its analog epsilon FAD. We have used UV-Vis transient absorption spectroscopy complemented by TD-DFT calculations to investigate the excited state evolution of the flavins. In FAD*, no radicals were observed, suggesting that FAD* does not undergo PET. epsilon FAD* kinetics showed a broad absorption band that suggests a charge transfer state exists upon photoexcitation with evidence for radical pair formation. Surprisingly, significant triplet flavin was produced from epsilon FAD* We hypothesize that the dipolar (epsilon)Ade moieties differentially modulate the singlet-triplet energy gap, resulting in different intersystem crossing rates. The additional electron density on the etheno group of epsilon FAD supplies better orbital overlap with the flavin S-1 state, accelerating charge transfer in that molecule.
