New insights into alpha-lactalbumin behavior upon interaction with resveratrol and curcumin by spectroscopic and molecular modeling techniques: binary and ternary system comparison

The interaction between -lactalbumin (a-LA) with resveratrol (RES) and curcumin (CUR) was studied by multiple spectroscopic as well as the zeta potential and molecular modeling techniques. Fluorescence spectroscopy showed that RES could quench the a-LA fluorescence with a static mechanism and this quenching effect became more significant when both CUR and RES coexisted. The synchronized fluorescence spectroscopy indicated that the conformation of a-LA and the distance between RES and Trp residues was altered. On the other hand, in the binary system, the RES site was closer to Trp than to Tyr, which proved the significant contribution of the Trp residues to the fluorescence of a-LA. Using red edge excitation shift, we found that the mobility restriction around the Trp residues was unchanged, demonstrating that the rigidity of the environment of Trp residue remained the same. The binding distance between RES and the Trp residues of a-LA was obtained according to Forster's theory of nonradioactive energy transfer and was found to be 2.67nm and 2.78nm for the binary and ternary systems, respectively. This confirmed the existence of static quenching for the binary and ternary systems alike. The quantitative analysis data from the circular dichroism spectra demonstrated that the binding of RES to a-LA induced conformational changes in a-LA. Moreover, the -helix content in a-LA greatly decreased in the presence of RES and in the ternary system this decrease was significant. Protein-ligand docking suggested that the affinity of RES to the (a-LA-CUR) complex was lower than for RES to a-LA.