Singlet Molecular Oxygen Quenching Ability of Carotenoids in a Reverse-micelle Membrane Mimetic System¶

Abstract

The influence of the medium heterogeneity upon the bimolecular rate constants for the physical quenching, kq, and chemical quenching, kr, of singlet molecular oxygen O2(1deltag) by seven natural and three synthetic carotenoids (CAR) with different substituent patterns was studied in a reverse micelle system of sodium bis(2-ethylhexyl)sulfosuccinate, hexane and water. Because O2(1deltag) was generated inside the water pools of the reverse micelles by photosensitization of the water-soluble dye rose bengal and the CAR are mainly located in the external hexane pseudophase, the quenching process was interpreted using a pseudophase model for the partition of 02(1deltag) between the water pools and the organic pseudophases. The kq values were mainly dependent on the extent of the double-bond conjugation of the CAR, as demonstrated by a good empirical relationship between log(k(q)) and the energy E(S) of the longest wavelength transition pi-->pi* of the CAR. In contrast, the kr values were almost independent of the extent of the double-bond-conjugated system and about four orders of magnitude lower than kq. However, in all cases, CAR photobleaching was observed with the formation of various oxidation products, depending on the photosensitization time. Chromatographic and spectroscopic product analysis for the reaction products of beta-carotene with O2(1deltag) indicated the formation of the beta-carotene-5,8-endoperoxide as the primary oxidation product.

Chemical Quenching of Singlet Oxygen by Carotenoids in Plants

“…High-light stress induced an increase in the b-carotene endoperoxide levels, but this effect was not more pronounced in the cat2 mutant compared with the wild type, confirming that this compound is specific to 1 O 2 attack on b-carotene. This study has identified a rather large range of products generated during the in vitro oxidation of b-carotene and xanthophylls by 1 O 2 , thus extending previous studies (Stratton et al, 1993;Yamauchi et al, 1998;Fiedor et al, 2001;Montenegro et al, 2002). More precisely, the presented results confirm the formation of an endoperoxide that appears to be a major oxidation product for both b-carotene and xanthophylls.…”

“…In this previous study, the major oxidation product was identified as b-carotene endoperoxide. This compound was also reported to be a major product generated in vitro from b-carotene by (bacterio) chlorophyll-sensitized photooxidation (Yamauchi et al, 1998;Fiedor et al, 2001) and was even identified as the primary oxidation products in early stage photolyzed solution of b-carotene (Montenegro et al, 2002). We collected the compound eluted in peak 6 and measured its absorbance spectrum in methanol/hexane (Fig.…”

“…Nevertheless, in vitro experiments have shown that carotenoids in solvent can be oxidized by 1 O 2 , thus indicating the existence of a chemical quenching capacity for those compounds (Foote and Denny, 1968;Stratton et al, 1993;Yamauchi et al, 1998;Fiedor et al, 2001;Montenegro et al, 2002). However, the occurrence of this mechanism in vivo during exposure of plants to high-light stress is not documented.…”

Diversifying Carotenoid Biosynthetic Pathways by Directed Evolution

“…The polyene chromophores of carotenoids, which absorb light in the 400 to 550-nm range, provide the basis for their characteristic yellowto-red colors and their ability to quench singlet oxygen (5,74,79,95,103,137,218,230). Carotenoids act as antioxidants in vivo (170,183,210), and many beneficial effects of carotenoids on human health have been reported, ranging from cancer prevention and tumor suppression to upregulation of immune function, reduction of the risk of coronary heart disease or age-related degeneration, and cataract prevention (19,21,42,47,83,108).…”

Determination of physical (kq) and chemical (kr) rate constants for singlet oxygen quenching using the thermolysis of a naphthalenic endoperoxide in H2O and D2O

“…[6][7][8][9][10] In all these processes, the scavenging of 1 O 2 by compounds involves two pathways : physical quenching which occurs through an energy transfer with the rate constant k q and the chemical quenching which results in the formation of oxidation products with the rate constant k r . 11 Overall rate constants (k r + k q ) have been determined for thousands of compounds 12 since they can readily be measured either by continuous or flash photolysis [13][14][15][16][17][18][19][20][21][22] or by using a standardized chemical source of 1 O 2 such as naphthalenic endoperoxides. [23][24][25][26][27][28][29][30] Reported values for the chemical rate constants k r are scarcer 12 since usual methods require the precise measurement of the amount of oxidation products formed when the starting material is submitted to a given amount of 1 O 2 .…”