LUTEIN-LYSOZYME INTERACTION: A THERMODYNAMIC PERSPECTIVE BY FLUORESCENCE SPECTROSCOPY

Lutein (Lut) is a carotenoid with antioxidant and anti-inflammatory properties, but its available levels to absorption can be reduced by several factors, such as thermal treatment and light exposition. Thus, the Lut interaction with a protein to improve its stability is strategic. Lysozyme (Lys), a protein with bactericidal and bacteriostatic properties, can act as a natural nanovehicle for different small molecules. Therefore, we investigated the thermodynamics of interaction between these two bioactive molecules using fluorescence spectroscopy. The solutions of Lut and Lys were prepared in potassium phosphate buffer (pH 7.4) with 3% of ethanol. The Lys concentration (6.0x10-7 M-1) was maintained constant and Lut varied from 0 to 6.9x10-6 M-1. The fluorescence of Lys was measured at 295 nm and at 20, 25, 30, and 40 °C. The obtained binding constants were in the order of 105 and it reduced as the temperature increased. The binding stoichiometry suggested that there was around one Lut for each binding site in a Lys molecule. The standard Gibbs free energy (∆G°), enthalpy (∆H°) and entropy (∆S°) changes were determined. The negative ∆G° values indicated that the chemical equilibrium favored the Lut-Lys complex formation. The ∆H° and T∆S° values decreased with the increase in the temperature and indicated that the complex formation process was enthalpically and entropically driven (∆H°= -10.7 kJ∙mol-1 and T∆S°= 16.0 kJ∙mol-1, at 25 °C). These results showed the role of van der Waals forces and hydrogen bonds between hydroxyls in Lut and carboxylic groups of Lys, and hydrophobic interactions, which contributes to the entropic gain of the system due to release of water molecules from solvating layers of Lut and Lys. Our study revealed the nature of the Lut-Lys interaction and its dependence on the temperature, contributing to the improvement of the application of this bioactive complex in different formulations.