Investigation of Flavonoids Bearing Different Substituents on Ring C and Their Cu2+ Complex Binding with Bovine Serum Albumin: Structure-Affinity Relationship Aspects

The effects of 1:1 flavonoid-Cu2+ complexes of four flavonoids with different C-ring substituents, quercetin (QU), luteolin (LU), taxifolin (TA), and (+)-catechin (CA), on bovine serum albumin (BSA) were investigated and compared with corresponding free flavonoids by spectroscopic analysis in an attempt to characterize the chemical association taking place. The results indicated that all of the quenching mechanisms were based on static quenching combined with nonradiative energy transfer. Cu2+ chelation changed the binding constants for BSA depending on the structures of flavonoids and the detected concentrations. The reduced hydroxyl groups, increased steric hindrance, and hydrophilicity of Cu2+ chelation may be the main reasons for the reduced binding constants, whereas the formation of stable flavonoid-Cu2+ complexes and synergistic action could increase the binding constants. The changed trends of critical energy transfer distance (R-0) for Cu2+ chelation were contrary to those of binding constants.