Spectroscopic demonstration of sinapic acid methyl ester complexes with serum albumins.

The methyl ester of sinapic acid (MESA) is a molecule with confirmed antioxidant properties. It is important to establish whether it can be transported across humans and animals. Therefore, we investigated MESA interactions with serum albumins, namely, human serum albumin (HSA), bovine serum albumin (BSA), rabbit serum albumin (RSA), and sheep serum albumin (SSA). Experiments were performed in a pH range from 5.9 to 10.7 using absorption and fluorescence techniques. It was found that MESA formed complexes with every albumin in the entire pH range under examination, which was confirmed by the appearances of new absorption and fluorescence complex bands. Fluorescence intensities were much higher (up to 20 times) and lifetimes were up to 340 times as compared to those for unbound MESA. The quenching experiments at pH 7.4 showed that the stoichiometry for every albumin was 1 : 1; the binding constant was the highest for HSA, which reached 52 000 M. The obtained results suggested that MESA preferred the hydrophobic binding sites in albumins. The analysis of the fluorescence spectra and fluorescence lifetimes showed two possibly different binding sites in BSA, RSA, and SSA as well as three binding sites in HSA.