Transient phenomena in the pulse radiolysis of retinyl polyenes. 6. Radical ions of retinal homologues

Carotenoid radical ions: A laser flash photolysis study

Laser excitation of a single precursor, namely 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone (HHEMP), has been used for generating the radical cations and radical anions of various carotenoids in methanol. In the presence of oxygen, laser excitation of HHEMP undergoes an efficient α-cleavage reaction (Norrish type I) to form acyl radicals, which react with O₂, in a nearly diffusion-controlled reaction, to form their corresponding strong oxidizing acylperoxyl radicals (RO₂^•) (E = ~1.1 V (v SHE)), which are capable of oxidizing almost all carotenoids. Under argon-saturated conditions and in the presence of strong base (0.01 M NaOH or tetrabutylammonium hydroxide (TBAOH)), the initially formed 2-hydroxy-2-propyl radical (ACH^•), generated after LFP of HHEMP, is deprotonated to form the strong reducing acetone ketyl radical (AC^•-) (E {acetone/ AC^•-} = -2.1 V (v SHE)), which is capable of reducing all carbonyl-containing carotenoids. To validate this new proposed approach, retinal and β-apo-8'-carotenal (APO), with known spectroscopic data, were investigated in methanol, acetonitrile and tetrahydrofuran (THF). In addition, the radical ions of newly investigated carotenoids, namely 4-oxo-β-apo-15'-carotenoic acid (4-oxo-15'), crocetindial, 4-oxo-β-apo-10'-carotenoic acid ethyl ester (4-oxo-10') and 4-oxo-β-apo-8'-carotenoic acid ethyl ester (4-oxo-8') have been reported. Moreover, the scope of this approach has been extended to investigate the radical ions of chlorophyll b.

Influence of pH on the decay of β-carotene radical cation in aqueous Triton X-100: A laser flash photolysis study

The identification of the spectral information of carotenoid neutral radicals is essential for studying their reactivities towards O2 and thereby evaluating their role in the antioxidant-prooxidant properties of the corresponding carotenoid. Recently, it was reported that β-carotene neutral radical (β-CAR) has an absorption maximum at 750 nm. This contradicts the results of many reports that show carotenoid neutral radicals (CAR) absorb in the same or near to the spectral region as their parent carotenoids. In this manuscript, the influence of pH on the decay of β-carotene radical cation (β-CAR-H(+)), generated in an aqueous solution of 2% Triton X-100 (TX-100), was investigated, employing laser flash photolysis (LFP) coupled with kinetic absorption spectroscopy, to identify the absorption bands of the β-carotene neutral radicals. By increasing the pH value of the solution, the decay of β-CAR-H(+) is enhanced and this enhancement is not associated with the formation of any positive absorption bands over the range 550-900 nm. By comparing these results with the literature, it can be concluded that β-carotene neutral radicals most probably absorb within the same spectral range as that of β-carotene. The reaction pathways of the reaction of β-CAR-H(+) with (-)OH have been discussed.

Studies of carotenoid one-electron reduction radicals

The relative reduction potentials of a variety of carotenoids have been established by monitoring the reaction of carotenoid radical anion (CAR1(*-)) with another carotenoid (CAR2) in hexane and benzene. This order is consistent with the reactivities of the carotenoid radical anions with porphyrins and oxygen in hexane. In addition, investigation of the reactions of carotenoids with reducing radicals in aqueous 2% Triton-X 100, such as carbon dioxide radical anion (CO2(*-)), acetone ketyl radical (AC(*-)) and the corresponding neutral radical (ACH(*)), reveals that the reduction potentials for beta-carotene and zeaxanthin lie in the range -1950 to -2100 mV and those for astaxanthin, canthaxanthin and beta-apo-8'-carotenal are more positive than -1450 mV. This illustrates that the presence of a carbonyl group causes the reducing ability to decrease. The radical cations have been previously shown to be strong oxidising agents and we now show that the radical anions are very strong reducing agents.

Carotenoid Radical Anions and Their Protonated Derivatives

In this study, we report the protonation reactions for astaxanthin and canthaxanthin radical anions in methanol, alkaline methanol, and aqueous 2% Triton X-100 at different pH values. The pKa values for the corresponding alpha-hydroxy radical derivatives of astaxanthin, canthaxanthin, and beta-apo-8'-carotenal were estimated in 2% Triton X-100. Also, the effects of the microenvironment and the structure of the carotenoids on the protonation rate constant are discussed.