Photoacidity and solvent-dependent fluorescence of 7,8-didemethyl-8-hydroxy-5-deazariboflavin

7,8-didemethyl-8-hydroxy-5-deazariboflavin (8-HDF or F₀) is a chromophore involved into electronic energy transfer processes in DNA-photolyases, cryptochromes and bioluminescent reaction of the Siberian earthworm, Henlea sp. We studied the impact of the solvent properties (polarity, hydrogen bonding ability, etc.) on F₀ fluorescence using the Catalan multiparametric approach and molecular dynamics technique. Analysis of the absorption and fluorescence spectra, as well as fluorescence decays and decay-associated spectra, of F₀ in 14 solvents revealed the presence of a mixture of protonated and deprotonated forms - F₀H and F₀^-, respectively, in the majority of solvents. We performed a correlation analysis of the spectral maxima of excitation/emission spectra and fluorescence lifetimes with the Catalan parameters of the solvents. A stronger dependence of spectral maxima of F₀^- than of F₀H on the solvent polarizability (SP) was found. For both F₀H and F₀^-, a weaker effect of solvent basicity (SB) was oppositely directed on excitation and emission spectra, which resulted in a stronger effect on the Stokes shifts. In water solutions with low pH (2.9-4.5), absorption and fluorescence spectra of F₀ indicated the proton transfer during the excited-state lifetime (ESPT). For this process, ΔpK_a = 3.67 ± 0.04 was first obtained, characterizing F₀ as a weak photoacid. All-atom molecular dynamics simulations of F₀H and F₀^- in water, methanol, DMSO, and acetone demonstrated that solvent hydrogen-bonding interactions influence the fluorophore's conformational dynamics and could alter F₀ photophysical properties. A special role of the N3 atom as a donor of the hydrogen bonds with solvents or proteins was proposed.