Abstract: In order to explore the effect of pH on the interaction of caffeic acid β-phenethyl ester (CAPE) with human serum albumin (HSA), the quenching effect of CAPE on the fluorescence of HSA under different pH (3.0, 5.0 and 7.4) conditions was investigated using fluorescence emission spectroscopy (FES) and synchronous fluorescence spectroscopy (SFS), and the quenching constants, binding constants and thermodynamic parameters were calculated. Meanwhile, the fluorescence quenching mechanism was analyzed, and the binding site was determined by site marker experiment. The fluorescence emission spectra showed that CAPE could quench the intrinsic fluorescence of HSA under all pH conditions tested. The quenching rate was the highest and the binding constant was the largest at physiological pH 7.4. The thermodynamic results suggested that the binding of CAPE to HSA was mainly driven by van der Waals force and hydrogen bonds. SFS indicated that binding to CAPE and the change of pH jointly induced the conformational changes of HSA. The results of site marker experiment showed that the binding site of CAPE to HSA was located near Sudlow’s site I. Altogether, the results revealed that the combination of CAPE with HSA could be promoted by the increase of pH, which can provide a theoretical basis for activity homeostasis maintenance and targeted delivery of CAPE.

Key words: human serum albumin; caffeic acid β-phenethyl ester; fluorescence quenching; pH