Photodissociative decay pathways of the flavin mononucleotide anion and its complexes with tryptophan and glutamic acid

Abstract

Flavin mononucleotide (FMN) is a highly versatile biological chromophore involved in a range of biochemical pathways including blue-light sensing proteins and the control of circadian rhythms. Questions exist about the effect of local amino acids on the electronic properties and photophysics of the chromophore. Using gas-phase anion laser photodissociation spectroscopy, we have measured the intrinsic electronic spectroscopy (3.1–5.7 eV) and accompanying photodissociative decay pathways of the native deprotonated form of FMN, i.e. [FMN-H]⁻ complexed with the amino acids tryptophan (TRP) and glutamic acid (GLU), i.e. [FMN-H]⁻·TRP and [FMN-H]⁻·GLU, to investigate the extent to which these amino acids perturb the electronic properties and photodynamics of the [FMN-H]⁻ chromophore. The overall photodepletion profiles of [FMN-H]⁻·TRP and [FMN-H]⁻·GLU are similar to that of the monomer, revealing that amino acid complexation occurs without significant spectral shifting of the [FMN-H]⁻ electronic excitations over this region. Both [FMN-H]⁻·TRP and [FMN-H]⁻·GLU are observed to decay by non-statistical photodecay pathways, although the behaviour of [FMN-H]⁻·TRP is closer to statistical fragmentation. Long-lived FMN excited states (triplet) are therefore relatively quenched when TRP binds to [FMN-H]⁻. Importantly, we find that [FMN-H]⁻, [FMN-H]⁻·TRP and [FMN-H]⁻·GLU all decay predominantly via electron detachment following photoexcitation of the flavin chromophore, with amino acid complexation appearing not to inhibit this decay channel. The strong propensity for electron detachment is attributed to excited-state proton transfer within FMN, with proton transfer from a ribose alcohol to the phosphate preceding electron detachment.