Further insight into the molecular basis of carotenoid-albumin interactions: Circular dichroism and electronic absorption study on different crocetin-albumin complexes

TitleFurther insight into the molecular basis of carotenoid-albumin interactions: Circular dichroism and electronic absorption study on different crocetin-albumin complexes
Publication TypeJournal Article
Year of Publication2002
AuthorsZsila, F, Bikádi, Z, Simonyi, M
JournalTetrahedron Asymmetry
Volume13
Issue3
Pagination273 - 283
Date Published2002
Abstract

Binding of the achiral carotenoid crocetin to the serum albumins of seven different species in borate buffer at pH 8.5 was studied in detail by UV-vis circular dichroism and absorption spectroscopy. The circular dichroism spectra yielded new information about this carotenoid-protein interaction showing the inter- and intramolecular types of induced chirality. In the visible region, human and pig albumin-crocetin complexes exhibited intense, bisignate, vibrationally coupled circular dichroism bands with opposite chirality proving intermolecular excitonic interaction between the bound crocetin molecules, left-handed for human and right-handed for pig albumin, respectively. This sign inversion and the very weak excitonic bands found with dog, horse and rat albumin were interpreted with the model of two carotenoid molecules fitted to the fatty acid binding sites of human albumin in subdomain IIIA. Positive and negative Cotton effects measured around 315-325 nm indicate that the cis-isomers of crocetin also bind to serum albumin and they become optically active due to the distortion around the cis-bond induced by the surrounding protein structure and producing inherently chiral, non-planar conformers. To determine the sense of the resulting molecular helicity the C2-chirality rule was applied. In all cases, albumin binding caused a bathochromic shift of the visible absorption band of crocetin due to dispersion interactions although the bandwidth and the vibrational fine structure were distinctly altered depending on the kind of albumin used. For human, bovine, rabbit and rat serum albumins the better resolved vibrational structure suggests a few, high affinity binding sites with rigid environments restricting conformational mobility of their ligands while the increased bandwidth and the suppressed fine structure obtained by titration of crocetin solution with horse, pig and dog albumins point to heterogeneous binding sites with lower affinity. © 2002 Elsevier Science Ltd. All rights reserved.

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