Interaction of the disodium disuccinate derivative of meso-Astaxanthin with human serum albumin: from chiral complexation to self-Assembly

To exploit the promising biochemical activities of naturally-occurring and synthetic hydrophobic carotenoids, it is necessary to improve their aqueous solubility. The disodium disuccinate derivative of synthetic meso-astaxanthin was prepared, and its behavior in pH 7.4 buffer solutions in both the presence and absence of fatty acid-free human serum albumin (HSA) was evaluated. The induced circular dichroism (CD) spectra and red-shifted absorption band of the optically inactive ligand as well as the fluorescence quenching of HSA indicated that at low ligand/protein ratios (less than approximately 1:1 ligand/protein), the meso-carotenoid bound to albumin in monomeric form. Based on the current experimental and available structural data for HSA, the binding site was tentatively localized to the large interdomain cleft of HSA. Around a 1:1 meso-carotenoid/HSA molar ratio, characteristic positive-negative bands appeared in the visible region of the CD spectrum, whose amplitudes increased in parallel with the increasing concentration of the ligand. These oppositely-signed Cotton effects are typical for chiral intermolecular exciton coupling between adjacent polyene chains arranged in right-handed assembly. Surprisingly, the magnitude of these induced CD bands continued to increase at high ligand/protein ratios (up to 13:1 meso-carotenoid/HSA). These results suggest the formation of unique, mixed-type carotenoid–albumin assemblies in which the HSA molecules themselves serve as chiral templates for the generation of supramolecular assemblies.