Optical and Infrared Spectroscopy of Lumiflavin Protomers in a Cryogenic Ion Trap

Flavins are multifunctional molecules that serve as essential cofactors for redox reactions and light-sensing proteins, and protonation is often relevant for these biochemical processes. We characterize herein the ground-state protonation sites of lumiflavin, the simplest member of the flavin family, in a cryogenic ion trap using infrared photodissociation (IRPD) spectroscopy as well as various types of isomer-selective vibrational and electronic double-resonance spectroscopies. The spectra are interpreted by complementary (time-dependent) density functional calculations. Infrared spectra in the 3 μm range reveal a predominant population of the most stable O2-cis protomer, while the less stable N1 isomer provides only a minor contribution. The relative abundances of the two protomers depend slightly on the solvent used for electrospray ionization (methanol or acetonitrile), indicating kinetic trapping effects. The optical spectrum observed near 430 nm is assigned to the vibronic transitions into the S₁ state of the O2-cis protomer, while contributions of the N1 protomer suggested previously can be excluded in this spectral range. The IRPD spectra provide the first definitive identification of the N1 protomer in the gas phase.