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, ΔpKa = 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.