Probing berberine syringate-bovine serum albumin interactions by multi-spectroscopic, conductimetric, volumetric and molecular docking methods

Abstract

In this paper, a new berberine-based salt berberine syringate ([BBR][SA]) was synthesized and [BBR][SA]-bovine serum albumin (BSA) interactions were investigated by various techniques like UV–visible absorption, fluorescence spectroscopy, Fourier transform infrared spectroscopy (FT-IR), volumetry and conductimetry. Using the experimental values of density and conductivity, the specific volume of BSA and the critical aggregation concentration of [BBR][SA] were derived. From the variation trends of specific volume for BSA with [BBR][SA] concentrations, it was observed the predominate ionic interaction/dehydration effect at small [BBR][SA] concentration and the non-polar interaction/hydration phenomenon at higher [BBR][SA] concentration for the binding of [BBR][SA] with BSA, as well as hydration shell structure changes at different temperatures in the medium with different pH values. Aggregation of [BBR][SA] is slowed by increasing concentration of BSA and temperature. The thermodynamic parameters of aggregation process have been exploited to gain more information on the interaction of [BBR][SA] with BSA. UV–vis absorption, multiple fluorescence spectroscopy (steady-state spectroscopy, synchronous fluorescence, three-dimensional fluorescence), and FT-IR have been utilized to understand the interaction mechanisms and binding strength between the two molecules. [BBR][SA] quenches the fluorescence of BSA through static quenching mechanism. The 1:1 complex between [BBR][SA] and BSA with a strong affinity was formed. The changes in microenvironment and conformation of BSA caused by [BBR][SA] were revealed by synchronous fluorescence spectra, 3D fluorescence maps and FT-IR. The binding average distance from [BBR][SA] to BSA (2.83 nm) was obtained based on the theory of Förster energy transfer. Competitive binding experiment confirms [BBR][SA] binding to Sudlow site I (sub-domain IIA) of BSA. In addition, molecular docking was generated to provide valuable information about the available binding sites of [BBR][SA] on BSA and the types of binding forces.