Absorption and Emission Spectroscopic Characterization of Lumichrome in Aqueous Solutions^†

Abstract

The spectroscopic behavior of lumichrome (7,8-dimethyl-alloxazine, LC) in aqueous solutions in a pH range from -1.08 to 14.6 is studied. Absorption spectra, fluorescence quantum distributions, quantum yields, and lifetimes are determined. The ionization stage of ground-state LC changes with rising pH from the cationic form (LCH(2)(+)) to the neutral form (LCH) with a mid-point pH of pK(c) ≈ -0.53, and to the anionic form (LC(-)) with a mid-point pH of pK(a) ≈ 12.5. Above pH 7 a partial ground-state tautomerization of LCH to 7,8-dimethyl-isoalloxazine (IAH) occurs by N1-N10 intra-molecular proton transfer. For pH > pK(a) ≈ 12.5 LCH and IAH change to the anionic forms LC(-) and IA(-), and above pH 14 LC(-) tautomerizes completely to IA(-). In the excited state some neutral lumichrome (LCH*) converts to cationic lumichrome (LCH(2)(+)) at low pH by proton transfer from H(3)O(+) to LCH*. No photoinduced excited-state tautomerization of lumichrome was observed. LCH for pH > 3 and IAH are reasonably fluorescent. The fluorescence efficiencies of LC(-) and IA(-) are lower than those of LCH and IAH. The fluorescence of LCH(2)(+) is strongly quenched likely by intra-molecular diabatic charge transfer and excited-state relaxation by potential surface touching with the ground state.

Loss of Tryptophan Fluorescence Correlates With Mechanical Stiffness Following Photo-Crosslinking Treatment of Rabbit Cornea

“…Riboflavin is subject to photodegradation under UV irradiation, but its degradation products, specifically lumichrome and lumiflavin, have also been shown to similarly photosensitize tryptophan. 20,21 In the dextran-plus-UV control group, this decrease in the 290/340 signal is not seen in either the half or standard treatment times. In the half-dose group, the dextran group signal at 290/340 excitation/emission increased, although this difference was not statistically different from controls.…”

Novel Riboflavin-Inspired Conjugated Bio-Organic Semiconductors

“…Flavin-like compounds are well known to possess the formation of tautomers in the solution by means of the proton transfer [ 67 ] resulting in the formation of the iso -form. Thus, in our case, the influence of the conjugated functional “tail” on the proton transfer would be noticeable in the absorption-emission optical spectra in DMSO.…”

“…In this case the possible impurities, which could remain after the recrystallization should not affect the absorption spectra. Moreover, the formation of the tautomers in other alloxazine derivatives, i.e., lumichrome [ 67 ] or 6,7-dicyanolumazine [ 40 ] (compound 4 ) are well known. As an example, it has been reported, that in aqueous solution of the 6,7-dicyanolumazine at neutral pH, it possesses two-chromophoric behavior with two different absorption bands at 350 and 400 nm.…”

Photophysics, Excited‐state Double‐Proton Transfer and Hydrogen‐bonding Properties of 5‐Deazaalloxazines

“…Although extensive studies have been performed on the spectral and photophysical properties of a series of isoalloxazines (benzo[g]pteridine‐2,4(3 H ,10 H )‐diones) and alloxazines (benzo[ g ]pteridine‐2,4(1 H ,3 H )‐diones), including lumichrome (7,8‐dimethylbenzo[ g ]pteridine‐2,4(1 H ,3 H )‐dione) , there is very little data on such studies on 5‐deazaalloxazines.…”