Binding of reactive brilliant red to human serum albumin: insights into the molecular toxicity of sulfonic azo dyes.

The non-covalent interaction of reactive brilliant red (RBR) as a representative of sulfonic azo compounds with human serum albumin (HSA) was investigated by a combination of UV-VIS spectrometry, fluorophotometry, circular dichroism (CD) and isothermal titration calorimetry (ITC) technique. The thermodynamic characterization of the interaction was performed. The saturation binding numbers of RBR on peptide chains were determined and the effects of electrolytes and temperature were investigated. The ionic interaction induced a combination of multiple non-covalent bonds including hydrogen bonds, hydrophobic interactions and van der Waals force. A three-step binding model of RBR was revealed. The binding of RBR molecules might occur on the external surface of HSA via electric interaction when the mole ratio of RBR to HSA was less than 40 and RBR molecules entered the hydrophobic intracavity of HSA when ratio was more than 40. Moreover, RBR binding resulted in a conformational change in the structure of HSA or even the precipitation of HSA and inhibited its function accordingly. The possible binding site and the conformational transition of HSA were hypothesized and illustrated. This work provides a new insight into non-covalent interactions between sulfonic azo compounds and proteins, which may be further used to investigate the potential toxicity mechanism of azo dyes.