Quenching of singlet oxygen by a carotenoid-cyclodextrin complex: the importance of aggregate formation

The Girard's reagent P derivative of canthaxanthin ((GRP)(2)-canthaxanthin), a dicationic carotenoid, forms a highly water-dispersible complex with (2-hydroxypropyl)-gamma-cyclodextrin. The UV-visible light spectrum of the complex is consistent with some degree of aggregation, but the spectrum is independent of concentration from 7.5 to 750 mum. Stern-Vomer plots for singlet-oxygen quenching by the complex are linear over a concentration range of 0-20 mum. In the presence of 1 mm (2-hydroxypropyl)-gamma-cyclodextrin, the singlet-oxygen quenching constant for the complex is 7.9 +/- 0.9 x 10(8) m(-1)s(-1). This is about an order of magnitude lower than the singlet-oxygen quenching constants for (GRP)(2)-canthaxanthin in various organic solvents. The properties of the complex are also compared with the properties of (GRP)(2)-canthaxanthin solubilized in neat water and in water containing various detergents. The singlet-oxygen quenching constant for (GRP)(2)-canthaxanthin in micelles depends strongly on the specific detergent used, varying from 9.4 x 10(8) m(-1)s(-1) for hexadecyltrimethylammonium bromide (CTAB) to 1.24 +/- 0.4 x 10(10) m(-1)s(-1) for sodium dodecyl sulfate. The small quenching constant in CTAB micelles correlates with spectroscopic evidence for aggregation of the (GRP)(2)-canthaxanthin in this detergent.