Electronic Structure of the Lowest Triplet State of Flavin Mononucleotide

The electronic structure of flavin mononucleotide (FMN), an organic cofactor that plays a role in many important enzymatic reactions, has been investigated by electron paramagnetic resonance (EPR) spectroscopy, optical spectroscopy, and quantum chemistry. In particular, the triplet state of FMN, which is paramagnetic (total spin S = 1), allows an investigation of the zero field splitting parameters D and E, which are directly related to the two singly occupied molecular orbitals. Triplet EPR spectra and optical absorption spectra at different pH values in combination with time dependent density functional theory (TDDFT) reveal that the highest occupied orbital (HOMO) and lowest unoccupied orbital (LUMO) of FMN are largely unaffected by changes in the protonation state of FMN. Rather, the orbital structure of the lower lying doubly occupied orbitals changes dramatically. Additional EPR experiments have been carried out in the presence of AgNO3, which allows the formation of an Ag-FMN triplet state with different zero field splitting parameters and population and depopulation rates. Addition of AgNO3 only induces small changes in the optical spectrum, indicating that the Ag+ ion only contributes to the zero field splitting by second order spin-rbit coupling and leaves the orbital structure unaffected. By a combination of the three employed methods, the observed bands in the UV/vis spectra of FMN at different pH values are assigned to electronic transitions.