Characteristics of the interaction mechanism between tannic acid and sodium caseinate using multispectroscopic and thermodynamics methods

Abstract

The spectroscopy and thermodynamics method was applied to study the binding of tannic acid (TA) to sodium caseinate (SC) in a phosphate buffer solution at pH 7.0. The turbidity of TA/SC complexes in phosphate buffer solution increased with increasing of TA/SC mass ratio. It tended to equilibrium until the TA/SC mass ratio was over the value of 1:1. The presence of TA changed the particle size of TA/SC complexes, which decreased with increasing of TA/SC mass ratio (TA/SC mass ratio less than 1:1). The intrinsic fluorescence and synchronous fluorescence spectra measurements demonstrated the complexes of SC with TA, which leads to the formation of TA/SC complexes. The far-UV CD results indicated that the TA induced a progressive increased in proportion of β-sheet structure, however, the proportion of α-helical, β-turn and random coil all decreased. Fourier transform infrared spectroscopy (FTIR) further confirmed that TA induced the transformation of SC structure. Isothermal titration calorimetry (ITC) measurements, which were used to characterize the binding mechanism of TA/SC complexes in phosphate buffer solution (pH 7.0) at 25 °C, clearly showed that the binding between TA and SC was mainly driven by the noncovalent interaction (e.g. H-bonding).