Studies on the interaction of gallic acid with human serum albumin in membrane mimetic environments

In this study the interaction between gallic acid and human serum albumin (HSA) in AOT/isooctane/water microemulsions was characterized for the first time using fluorescence quenching technique in combination with UV absorption spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, circular dichroism (CD) spectroscopy and dynamic light scattering (DLS) technique. In water–surfactant molar ratio (ωo) = 20 microemulsions fluorescence data revealed the presence of one binding site of gallic acid on HSA and its binding constants (K) were (1.18 ± 0.02) × 104, (1.13 ± 0.02) × 104, (1.03 ± 0.02) × 104, (0.95 ± 0.02) × 104, (0.87 ± 0.02) × 104 and (0.82 ± 0.03) × 104 M−1 at 282, 289, 296, 303, 310 and 317 K, respectively. The affinities in microemulsions were much higher than that in buffer solution. FT-IR and CD data suggested that the protein conformations were altered with the reductions of α-helices from 54–56% for free HSA in buffer to 40–41% for free HSA in microemulsion. After binding with gallic acid, the α-helices of HSA in microemulsion increased 2–7% for different drug–protein molar ratio. The thermodynamic functions standard enthalpy (ΔH0) and standard entropy (ΔS0) for the reaction were calculated to be −8.10 kJ mol−1 and 49.42 J mol−1 K−1. These results indicated that gallic acid bound to HSA mainly by hydrophobic interaction and electrostatic interaction in microemulsions. In addition, the displacement experiments confirmed that gallic acid could bind to the site I of HSA, which was approved by the molecular modeling study. Furthermore, the DLS data suggested that HSA may locate at the interface of the microemulsion and gallic acid could interact with them.