Mapping the Franck-Condon Active Modes of Flavin

Flavins are a class of organic molecules which share a tricyclic isoalloxazine ring as part of their structure. Several flavins are biologically important, including riboflavin (RF, also known as vitamin B2), flavin mononucleotide (FMN), and flavin adenine dinucleotide (FAD). FMN and FAD, in particular, are notoriously cofactors in a large group of proteins known as flavoproteins. Enzymologists and biochemists frequently utilize spectroscopic tools such as ultraviolet-visible (UV-vis) spectroscopy to study the reactivity and kinetics of these enzymes. Therefore, it is useful to understand the origin of spectral features of flavins and how they are affected by the protein environment. The shape and broadening of the UV-vis bands are attributed to a combination of electronic and vibrational excitations, which can be understood through the Franck-Condon (FC) principle. The FC principle states that since electronic transitions occur on a much faster timescale than nuclear ones, excitation is essentially a “vertical” process that occurs without changes in the positions of the nuclei. This implies that the vertical excitation energy should match well with the wavelength at which flavin absorbs most strongly (λmax). However, this approximation does not appear to work well in flavins. In this study, we employed electronic structure calculations on lumiflavin, a smaller molecule sharing the same tricyclic isoalloxazine structure and UV-vis spectrum as RF, FMN, and FAD, to understand its electronic and vibrational energies. The inspection of the ground and excited state energies for lumiflavin along its vibration modes was used to determine which modes are FC active and to shed light on why flavins do not follow the vertical transition approximation. The calculations provide insights into the spectroscopic behavior of the broader class of flavin molecules and provides a foundation for explaining the unique photophysical properties of flavins.