Influence of pH and micellar systems on the sensitized photo-oxidation of bovine serum albumin.

Photosensitized oxidation of bovine serum albumin (BSA), by using perinaphtenone as a sensitizer, has been studied at pH 7.4 and 11. The selected sensitizer does not present ground-state complexation with BSA and ensures that the mechanism is mediated by O-2((1)o(g)). Strong dependence between BSA-O-2((1)o(g)) photo-oxidation and the pH of the medium has been found. The relative oxygen uptake rate (v(- o O2)) and the total quenching rate constant (k(t)) values are higher at pH 11 than pH 7.4. The enhancement in the alkaline condition is due to conformational changes in the protein and the reactivity of tyrosinate anion with O-2((1)o(g)). Even when the tendency with the pH in the presence of sodium dodecyl sulfate (SDS) micelles is similar to that observed in homogeneous media, an increment on the k(t) value is detected. This effect may be attributable to the strong interaction of BSA-SDS, which leads to the protein unfolding and could leave more exposed photo-oxidizable amino acids. A protective effect against the O-2((1)o(g))-mediated photo-oxidation was observed in reverse micelles (RMs) of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) by comparing the k(t) values obtained at W = 10 with respect to the one obtain in homogeneous media. The latter could be mainly explained by the modification in the solvent polarity. Also, another important observation was found, the internal pH inside RMs of AOT sensed through tyrosine absorption was independent of the one used for the formation of the water pool. Hence, the k(t) values observed at both pH, are quite similar.