A Kinetic Study of the Photodynamic Effect on Tryptophan Methyl Ester and Tryptophan Octyl Ester in DOPC Vesicles

The photodynamic effect on tryptophan methyl ester (trpME) and tryptophan octyl ester (trpOE), using the O-2(1 Delta(g))-photosensitizers Rose Bengal (RB) and Perinaphthenone (PN) has been studied in large unilamellar vesicles (LUVs) of the phospholipid 1,2-di-oleoyl-sn-glycero-3-phosphatidylcholine (DOPC) by stationary photolysis and time-resolved methods. This work reports on the influence of both the site (O-2(1 Delta(g))) generation and the location of the tryptophan derivatives (trpD), on the photo-oxidation process in a compartmentalized system. The apparent rate constant values for chemical quenching of O-2(1 Delta(g)) by trpOE (k(r,app)), was higher in vesicles than in water. Also, the ratio between apparent reactive and overall rate constant values for the deactivation of O-2(1 Delta(g)) (k(r,app)/k(t,app)), increases in vesicles as compared with water, when the oxidative species is generated in the lipidic region or at the interface. Nevertheless, this quotient is lower than the corresponding value in water when O-2(1 Delta(g)) is generated in the aqueous pseudophase. For trpME, the k(r,app)/k(t,app)values in vesicles and in water are quite similar, confirming the fact that trpME is located in the water pseudophase. Results are discussed in terms of relative protection against O-2(1 Delta(g)) attack in a microheterogeneous medium as compared with water.