Applications of EPR Spectroscopy to Understanding Carotenoid Radicals - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 167

9.7 SUPRAMOLECULAR COMPLEX FORMATION
The scavenging rates of canthaxathin and 7 -carotene toward •OOH after supra-

molecular complex formation between carotenoids and a triterpene glycoside,

′ ,7 ′ -dicyano-7 ′ -apo- β

-glycyrrhizic acid
(GA) have been estimated using the EPR spin trapping technique. Complexation with GA results
in an increase of scavenging rates by more than 10 times for both carotenoids, however complex
formation had no affect on zeaxanthin. This effect was found to be due to an increase in oxida-
tion potential upon complexation (Polyahov et al. 2006). Optical studies (Polyakov et al. 2006a,b)
revealed that a 1:2 complex was formed with GA. The complex is a cyclic-like dimer of GA encap-
sulating a carotenoid molecule (Scheme 9.5). The stability constants are near 10 4 M −1 . In addition,
GA forms inclusion complexes with carotenoid radical cations, which results in their stabilization.
Spin trapping methods were also used to show that when carotenoid-

β

-cyclodextrin 1:1 inclusion
complex is formed (Polyakov et al. 2004), cyclodextrin does not prevent the reaction of carotenoids
with Fe 3+ ions but does reduce their scavenging rate toward • OOH radicals. This implies that dif-
ferent sites of the carotenoid interact with free radicals and the Fe 3+ ions. Presumably, the • OOH
radical attacks only the cyclohexene ring of the carotenoid. This indicates that the torus-shaped
cyclodextrins, Scheme 9.6, protects the incorporated carotenoids from reactive oxygen species.
Since cyclodextrins are widely used as carriers and stabilizers of dietary carotenoids, this demon-
strates a mechanism for their safe delivery to the cell membrane before reaction with oxygen species
occurs.

β

COOH

H

O

COOH
O
OH

O

HO

COOH

Glycyrrhizic acid (GA)

O

O

OH

HO

OH

SCHEME 9.5 Suggested structures of the GA dimer and their inclusion complex with the carotenoid. Light
gray: hydrogen atoms, dark gray: carbon atoms, black: oxygen atoms. (From Polyakov, N.E., J. Phys. Chem.
B , 110, 6991, 2000. With permission.)