Abstract: Fluorescence quenching and synchronous fluorescence methods were used to study the interactions of fluorescence-active quercetin (Qct) with casein (Cas) and bovine serum albumin (BSA) in phosphate buffer solution (PBS, pH=7.4) with or without coexisting carbon nanotubes (CNTs). Formulae for binding constant (K) and molar binding ratio (n)were established formethods 1 (fixing protein concentration, changing Qct concentration, andmonitoring the fluorescence of protein) and 2 (fixing Qct concentration, changing protein concentration, and monitoring the fluorescence of Qct), to which values of K and n were calculated via nonlinear least-square fitting of the experimental data, and the“optical inner filtering induced fluorescence quenching”effect was thus quantitatively evaluated. The quenching effects of coexisting CNTs on the fluorescence of Qct, BSA, and Cas, as well as the effects of coexisting CNTs on Qct-BSA and Qct-Cas interactions, were examined. Synchronous fluorescence was also used to examine the effects of coexisting CNTs and Qct on conformations of BSA and Cas, with relevant K and n values for tyrosine (Tyr) and tryptophan (Trp) residues estimated. It was concluded that the CNTs mainly interact with the Trp residues locating near the protein surfaces, but small-sized Qct molecules could further interact with the Tyr residues locating inside the protein molecules.

Key words: Carbon nanotubes, Quercetin, Casein, Bovine serumalbumin, Fluorescence quenching, Synchronous fluorescence, Intermolecular interactions