The solvent effect on the fluorescence and light absorption of riboflavin and lumiflavin
ABSTRACT Changes in fluorescence intensity of riboflavin and lumiflavin with the nature of the solvent were studied. A series of mixtures of ethanol, acetone and dioxane with water were used as solvents. Changes in absorption and fluorescence spectra expressed as transition energies, apparent absorption coefficients and quantum yields of fluorescence were correlated with each other, with dielectric constants and with the Z-values, expressing solvent polarity of the solvent mixtures used. Rough linearity was observed for all parameters except of dielectric constants for flavin solutions containing no more than 90% of organic solvent. In riboflavin solutions containing higher concentrations of organic solvents deviations from linearity were observed. The possibility is discussed, that in such higher concentrations of organic solvents relative measurements accepted do not adequately reflect changes in flavin molecules. In solvent mixtures of lowest polarity riboflavin and, to a greater extent lumiflavin, were photodecomposed much faster than in aqueous solutions. It is suggested that the decrease of the stability of flavin molecules is caused by lowering of the degree of association with water molecules, and by a secondary solvent polarity effect on the electron system of light-excited flavin molecules.
- SourceAvailable from: Fumio TanakaJournal of Photochemistry and Photobiology A Chemistry 12/2012; 250:6-17. · 2.42 Impact Factor
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ABSTRACT: The spectroscopic behaviour of lumiflavin (7,8,10-trimethyl-isoalloxazine, oxidized form LFox) in aqueous solutions of pH range −1.08 to 14.6 is studied. Absorption spectra, fluorescence quantum distributions, quantum yields and lifetimes are determined. The ionization stage of ground-state LFox changes from cationic (LFoxH2+) at low pH (pKc ≈ 0.38) via neutral (LFoxH) to anionic (LFox−) at high pH (pKa ≈ 10.8). The cationic, neutral, and anionic forms are identified by their different absorption spectra. LFoxH in neutral aqueous solution is reasonably fluorescent (fluorescence quantum yield ϕF ≈ 0.29, fluorescence lifetime τF ≈ 5.2 ns), while LFox− is weakly fluorescent (ϕF ≈ 0.0042, τF ≈ 90 ps), and LFoxH2+ is nearly non-fluorescent (ϕF ≈ 3.6 × 10−5, τF ≈ 0.4 ps).A theory of the pH dependent equilibration of cationic, neutral and anionic molecules in the ground state and their dynamics in the excited state is developed. For lumiflavin in aqueous solution in the excited state no equilibrium distributions are reached between the cationic, neutral, and anionic forms. Some neutral excited lumiflavin transforms to the cationic ground-state form at low pH by intermolecular photo-induced proton transfer from H3O+ to LFoxH*. At high pH no photo-induced intermolecular proton transfer takes place.Journal of Photochemistry and Photobiology A: Chemistry. 01/2010;
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ABSTRACT: Individuals with keratoconus form a significant proportion of patients for a practitioner specialising in corneal diseases. Yet it is a disease where the pathogenesis is poorly understood, and until recently, there has been no treatment apart from transplantation that could be offered that was curative or even capable of slowing the progression of the disease. Collagen cross-linking treatment using riboflavin and UV light has been developed to address this need, and the initial results are promising. The purpose of this review is to critically evaluate this treatment in light of the scientific basis for cross-linking, to highlight the strengths and limitations of the evidence in terms of efficacy and long-term safety, and finally to identify areas for future research in this area with a significant potential to change the way we treat our keratoconus patients. In addition, we hope that our unbiased review for the first time would bring together, in a concise fashion, scientific information for a practitioner contemplating on offering this treatment and to help inform their patients of its potential risks and benefits.The British journal of ophthalmology 09/2009; 94(8):965-70. · 2.92 Impact Factor