No abstract

Experimental time-resolved spectral and photon counting kinetic results confirm formation of an isoalloxazinic excited state via excited-state double proton transfer (ESDPT) catalyzed by a carboxylic acid molecule that forms a hydrogen-bond complex with the parent alloxazine molecule. This isoalloxazinic tautomer manifests itself as a distinct long-lived emissive species formed only in such alloxazine derivatives that were not substituted at the N1 nitrogen atom, being a product of the excited-state reaction occurring from the alloxazinic excited state. Theoretical calculations support the idea that the ESDPT occurs by the concerted mechanism. The calculated activation barrier in the excited state is much lower than the same barrier in the ground state and even disappears for the HOMO-1 to LUMO excitation, which explains the fact that the reaction takes place in the excited-state only. The reaction rate estimated from the emission kinetics is ca. 1.4 x 10(8) dm3 mol(-1) s(-1) in ethanolic solutions of lumichrome with added acetic acid.

In this short review we provide some insights to the main processes that riboflavin is involved in upon absorption of a photon. We describe riboflavin properties in its interactions with oxygen, comparing them to the properties of some other singlet oxygen sensitizers. Data are provided on riboflavin photosensitizing properties in vivo and in vitro, and its properties as an endogenous singlet oxygen sensitizer are discussed. We additionally report flavin catalytic role in organic synthesis and photochemical reactivity in solutions of riboflavin and some of its derivatives.

Light-induced degradation of the riboflavin derivatives is reported, including 5-deaza-riboflavin, iso-6,7-riboflavin, 3-methyl-tetra-acetyl-riboflavin (3MeTARF), and 3-benzyl-lumiflavin. The studied compounds undergo photolytic processes with considerable quantum yields in methanolic solutions (φ ≈ 10(-3)-10(-4) mol einst(-1)). Photolysis in anaerobic conditions is more efficient than that in the presence of oxygen. Experiments show that triplet excited state of flavin molecules is involved in the reactions. The main photoproducts are the respective alloxazinic and isoalloxazinic derivatives of the substrates. 3MeTARF reveals higher photostability than riboflavin. The values of photodegradation quantum yields were estimated for the four studied derivatives.