Study of photophysical properties of 5-deazaalloxazine and 1,3-dimethyl-5-deazaalloxazine in dependence of pH using different spectral techniques

Abstract

The photophysical properties of 5-deazaalloxazine and 1,3-dimethyl-5-deazalloxazine at different pH values were characterized using absorption spectra, fluorescence emission spectra, fluorescence excitation spectra, synchronous fluorescence spectra and total fluorescence spectra. Their ionised and/or neutral forms were discussed in comparison with those obtained for other derivatives of 5-deazaalloxazine and/or 5-deazaisoalloxazine. Steady-state and time-resolved techniques were used to study the protonation/deprotonation equilibria between cationic and neutral forms of both compounds and between neutral and monoanionic forms of 5-deazalloxazine, as well as between monoanionic forms of this compound and its dianion. We estimated pK_a values for these equilibria both in the ground and excited states. Our steady-state and time-resolved measurements indicate that the cation of 5-deazaalloxazine in its isoalloxazinic form exhibits fluorescence that is quenched by protons in a dynamic process. Contrary to that, the cation of 1,3-dimethyl-5-deazaalloxazine has almost no fluorescence. Additionally, we found that the neutral forms of 5-deazalloxazine and 1,3-methyl-5-deazalloxazine are also quenched in acidic conditions by protons. In basic conditions, 5-deazaalloxazine forms two structurally different anions, namely the alloxazinic monoanion and the isoalloxazinic monoanion; both simultaneously dissociate into the isoalloxazinic dianion at even higher pH values. The synchronous fluorescence spectra and total fluorescence spectra demonstrated their suitability to characterize and differentiate different fluorescent forms of 5-deazalloxazine, namely: the cation, the neutral form, two monoanions, and the dianion, in a wide pH range.