Effect of pH on the interaction of baicalein with lysozyme by spectroscopic approaches

The interaction mechanism of baicalein and lysozyme (Lys) has been characterized by fluorescence, synchronous fluorescence, ultraviolet–vis absorbance, and three-dimensional (3D) fluorescence. The structural characteristics of baicalein and Lys were probed, and their binding affinities were determined under different pH conditions (pH 7.4, 4.5, and 2.5). The results showed that the binding abilities of the drug to Lys increased under lower pH conditions (pH 4.5 and 2.5) due to the alterations of the protein secondary and tertiary structures or the structural change of baicalein. The effect of baicalein on the conformation of Lys was analyzed using UV, synchronous fluorescence and three-dimensional (3D) fluorescence under different pH conditions. These results indicate that the binding of baicalein to Lys causes apparent change in the secondary and tertiary structure of Lys. In the presence of Cu2+, the decrease of the binding constant in buffer solution of pH 2.5 may result from the competition of the metal ion and baicalein binding to Lys. In addition, the presence of Cu2+ increased the binding constants of baicalein–Lys complex under higher pH conditions (pH 7.4 and 4.5). The possible site of binding of baicalein to Lys has been proposed to explain these observations.