Binding of naturally occurring hydroxycinnamic acids to bovine serum albumin

Hydroxycinnamic acids (HCAs) possess numerous biological effects including antioxidant, antiallergic, antimicrobial, and immunomodulatory activities and due to these properties are widely used in folk medicine. Nevertheless, they can interact with protein molecules and cause some structural and functional changes. The possibility of HCAs binding to bovine serum albumin (BSA) under physiological conditions was investigated by the UV-VIS absorption spectroscopy and fluorescence quenching method. Apart from rosmarinic acid, all tested HCAs quenched tryptophan fluorescence of BSA in the studied range of concentrations (0-20 μM) mainly by static quenching mechanism (formation of nonfluorescent HCA-BSA complexes). The binding constants, number of binding sites and free energy changes were determined. The binding affinities of HCAs were ranked in the order: chlorogenic acid > sinapic acid ≥ caffeic acid > ferulic acid > o-coumaric acid > p-coumaric acid ≥ m-coumaric acid, which was confirmed by spectral overlaps of BSA emission spectrum with absorption spectrum of HCA. All free energy changes possessed negative sign indicating the spontaneity of HCA-BSA interaction.