Induced chirality upon crocetin binding to human serum albumin: origin and nature

Binding to human serum albumin (HSA) of the natural, achiral carotenoid crocetin, having hypocholesterolemic and antitumour effects, was investigated in detail by circular dichroism (CD) and absorption spectroscopy. It has been shown that in the visible absorption region the crocetin–HSA complex exhibits a well-defined induced circular dichroic spectrum with two major bands of opposite sign, proving excitonic interaction between carotenoids bound in a left-handed chiral arrangement on the albumin molecule. In the course of CD titration experiments, palmitic acid gradually decreased the exciton band intensities indicating that crocetin and palmitic acid have common binding sites on HSA. To investigate potential sources of the intermolecular excitonic interaction, molecular modeling studies were performed fitting crocetin molecules to the long-chain fatty acid binding sites of HSA, determined recently by X-ray crystallographic measurements. The results suggest that binding of crocetin to domain III of the albumin might be responsible for the observed intermolecular exciton coupling. Crocetin binding was accompanied by a significant red shift in the visible absorption spectrum which has showed no excitonic contribution but rather indicates the higher polarizability of the protein environment.