Generation of a Diligand Complex of Bovine Serum Albumin with Quercetin and Carbon Nanotubes for the Protection of Bioactive Quercetin and Reduction of Cytotoxicity

The interactions between proteins and bioactive ligands (such as flavonoids and nanomaterials) are vital to the design of effective protein carriers for the protection of bioactive molecules and reduction of the cytotoxicity of nanotubes. Bovine serum albumin (BSA) can bind various bioactive components and subsequently form protein-ligand complexes. Herein, the binding of BSA to quercetin and single-walled carbon nanotubes (SWCNTs) was investigated by using experimental and molecular-docking methods. The fluorescence intensity of BSA was decreased by both quercetin and SWCNTs in static quenching mode (i.e., compound formation), which was authenticated by Stern-Volmer calculations. Although quercetin showed a higher affinity for BSA than SWCNTs, the binding of both components to BSA was located in site I (subdomain IIA). BSA-diligand complexes were successfully generated when SWCNTs and quercetin, in that sequence, were added. The cytotoxicity of SWCNTs and the formation of reactive oxygen species in endothelial cells were decreased with the BSA-diligand complexes relative to those of SWCNTs or BSA-SWCNT corona, whereas the stability problems of quercetin were ameliorated in the BSA-diligand complex relative to in the free flavonoid. The BSA-diligand complex showed a better inhibitive effect on the cytotoxicity of SWCNTs than the BSA-SWCNT complex, and thus the coexistence of quercetin played a crucial role. These data demonstrate the advantages and possibility of designing BSA carriers for the protection of bioactive ligands and reduction of the cytotoxicity of nanotubes in functional-food and biomedical applications.