Kinetics of the photosensitized oxidation of chymotrypsin in different media

Abstract

The kinetic aspects of the Perinaphthenone-sensitized photooxidation (singlet molecular oxygen [O2(1Dg)]-mediated) of a-Chymotrypsin (a-Chymo) have been studied at pH 8 and pH 11 as well in reverse micelles (RMs) of sodium 1, 4 bis (2-ethylhexyl) sulfosuccinate (AOT) in n-heptane. The rate constant values for both overall (kt) and chemical (kr) quenching of O2(1Dg) by a-Chymo in homogeneous media were higher at pH=11 than pH=8, indicating that the OH-ionized tyrosine (Tyr) residues, clearly dominate the quenching process. Besides, the rate constants in water were higher than those determined in RMs, demonstrating that the organized medium protects the protein against photooxidation, probably due to a diminution in both, the accessibility towards oxidizable amino acid residues and the polarity inside the aggregate, as compared to water. The effect of both, surfactant concentration and variation of the ratio ([H2O]/[AOT])=W on the reactive rate constant was also investigated. The process does not depend significantly on micelles concentration while the kr values increase as W increases. Furthermore, at W=30, the highest W studied, kr tends to the value obtained in aqueous medium.