Riboflavin-induced photodegradation of tryptophan in sodium sulfosuccinate (AOT)-iso-octane-water

HUGO A. PEREZ, FAUSTINO E. MORÁN VIEYRA, LORENA VALLE AND CLAUDIO D. BORSARELLI

Mendoza

Congreso; INTER-AMERICAN-PHOTOCHEMICAL-SOCIETY (IAPS); 2011

Flavins play an important role in photosensitized reactions that include a wide range of electron donor substrates, such as amino acids. In aqueous solutions, triplet state of flavins reacts with amino acids such as Cys, Trp and Tyr with bimolecular quenching constants between 1-10´109 M-1 s-1 [[i]]. In aerobic conditions, the flavin photosensitized oxidation of aminoacids was attributed to H2O2 produced by dismutation of O2•-, generated by the interaction of O2 with the triplet of the flavins and/or radical species formed by flavin triplet reaction with the aminoacids [[ii], [iii]]. However, to our knowledge, there are few studies about flavin-photosensitized oxidation of amino acid in microenvironments which mimic biological system compartmentalization. In this work we study the riboflavin (Rf)-mediated reaction of tryptophan in nanodroplets made of sodium sulfosuccinate (AOT)-iso-octane-water by continuous blue-LED (460 nm) and laser pulsed (355 nm) photolysis, using both stationary and dynamic absorption and emission spectroscopies. The absorption and emission spectra of Rf and Trp indicated that both molecules are located in the inner water-surfactant interface of the reverse micelles. The fluorescence quenching of Rf by Trp is a mixing of static (Perrin) and dynamic mechanism. With the increment of the water content in the reversed micelles (W = [H2O]/[AOT]) the fraction of static quenching is reduced while the dynamic component increases. These results are in agreement with the larger micelar size and fluidity as W increased. The UV-Vis absorption and fluorescence spectra showed the riboflavin conversion to lumichrome and the photoproducts of the indole-flavin type. The rate of formation of photoproducts, measured with the absorbance increment at 320nm was proportional with the increment of the quenching rate constant of 3Rf* by Trp, e.g. kQT= 2.6´107, 6.1´107and 3.3´109 M-1s-1, for W =3, W= 20 and phosphate buffer solution, respectively, indicating that the photodegradation reaction is mainly govern by triplet state photochemistry. In reverse micelles the product formation is almost six-times lower that in bulk buffer solution. This indicates that the much rigid micellar environment reduces the Rf-mediated transformation of Trp. [i]. A. M. Edwards. General properties of flavins in Flavins: photochemistry and photobiology (Ed. E. Silva and A. M. Edwards), Comprehensive series in Photochemical and Photobiological Science, Vol 6, Royal Soc. Chemistry, UK, Chapter 2, pp 13 – 40 (2006). [ii]. L. Chang- Yuan, L. Yan- Yun. Electron transfer oxidation of tryptophan and tyrosine by triplet states and oxidized radicals of flavin sensitizers: a laser flash photolysis study. Biochim. Biophys. Acta. 1571, 71 – 76 (2002). [iii]. H. Gorner. Oxygen uptake after electrón transfer from amines, amino acids and ascorbic acid to triplet flavins in air – saturated aqueous solution. J. Photochem. Photobiol. B. 87, 73 – 80 (2007).