Kinetic studies of the photosensitized oxidation (O-2((1)Delta(g))-mediated) of tryptophan-alkyl esters in Triton X-100 micellar solutions

The kinetics of the sensitized (O-2((1) Delta(g))-mediated) photo-oxidation of tryptophan (trp) and a series of trp-alkyl esters were studied in Triton X-100 micellar solutions employing time-resolved phosphorescence detection of O-2((1) Delta(g)) and static photolysis. The characterization of the solute-micelle interaction indicates that trp-methyl ester, trp-butyl ester and trp-octyl ester bind Triton X-100 micelles to a different extent, depending on the hydrocarbon length of their ester chains. Trp remains in the aqueous phase. The partition constants vary from 380 for the free amino acid to 11 200 for the octyl-ester derivative. An important decrease in the photo-oxidation quantum efficiency of the esterified compounds was observed due to the presence of the micellar medium. The magnitude of the effect for these solutes, which occupy hydrophobic environments in the micellar structure, is extremely sensitive to the location of the probe and increases as the length of the ester hydrocarbon chain increases. The results, interpreted in terms of the competition between the local oxygen concentration and the solvent polarity effect (the latter representing the driving force),predict a protection against photodynamic damage for trp-containing hydrophobic assemblies in organized biological environments compared with the same processes in pure water. The protection increases as the probe becomes more hydrophobic. (C) 1997 Elsevier Science S.A.