Thermodynamic study of the interaction between hen egg-white lysozyme and curcumin

Lysozyme is a small protein (~ 14 kDa) formed by 129 amino acid residues that are distributed in hydrophilic and hydrophobic regions within the protein structure. Due to the latter region, lysozyme presents the ability to interact with hydrophobic bioactive compounds, such as curcumin, forming a complex with improved solubility and storage stability that maintains its original pharmacological features, such as antioxidant, anticancer, antibacterial, and anti-inflammatory activities. Therefore, this study aimed to investigate the binding affinity between curcumin and lysozyme by steady-state fluorescence spectroscopy technique, at pH 6.5, and a temperature range of 20 to 40 ºC. The fluorescence results revealed that the addition of curcumin caused quenching of intrinsic fluorescence of lysozyme, mostly by the static quenching mechanism, thus the complex lysozyme-curcumin was formed. The binding constants were of the order of 10-5 M-1, with a stoichiometry value around 1. The standard Gibbs free energy values were negative at all temperatures (i.e., ΔG° ≈ - 31,35 kJ.mol-1 ), indicating that the chemical reaction equilibrium favors the complex formation. The standard enthalpy (ΔH°) and entropy (TΔS°) change values exhibited a linear relationship (R² = 0.99), increasing with the increase in the temperature, being at 40 ºC and 24 ºC, these parameters became positive, respectively. The negative ΔH° values suggested that Van der Waals force and hydrogen interactions occur between the functional groups of curcumin and lysozyme at low temperatures. The increase in the TΔS° values is owing to the hydrophobic effect, which leads to an increase in configurational entropy of the system, caused by lysozyme and curcumin desolvating, releasing water. This work confirms the formation of the lysozyme-curcumin complex and its dependence on temperature, besides helping on the understanding of the binding behavior of lysozyme, which could be used as a potential carrier in functional foods.