Dependence of phthalocyanine-based fluorescence on albumin structure: A fluorescent probe for ascorbic acid

Abstract

In this study, we have investigated the environmental dependence of the fluorescent probe for detecting ascorbic acid. The fluorescent probe used herein, R2c, consists of silicon phthalocyanine and two 2,2,6,6-tetramethyl-1-piperidinyloxy radicals, and is encapsulated by the dimer of bovine serum albumin (BSA). Due to this encapsulation, the R2c@(BSA)2 complex was prevented from reacting with various redox species in biological systems, but reacted selectively with ascorbic acid. The fluorescence of R2c@(BSA)2 after ascorbic acid addition depended on the pH: the fluorescence intensity increased in the order pH 6 < pH 5 < pH 4 < pH 3, but decreased at pH 2 compared with that at pH 3. This pH dependence was analyzed in terms of the relationship between the shielding effects of BSA and the folding ↔ unfolding structural changes. Furthermore, the fluorescence intensity of R2c@(BSA)2 decreased with increasing temperature after ascorbic acid addition, which was explained by the change in relative proportions of α-helix and β-sheet in BSA. This result will be useful for developing a fluorescent probe using the structural changes of albumin.