Chemistry of carotenoid neutral radicals

Abstract

Proton loss from the carotenoid radical cations (Car(+)) to form neutral radicals (#Car) was investigated by numerous electrochemical, EPR, ENDOR and DFT studies described herein. The radical cation and neutral radicals were formed in solution electrochemically and stabilized on solid silica-alumina and MCM-41 matrices. Carotenoid neutral radicals were recently identified in Arabidopsis thaliana plant and photosystem II samples. Deprotonation at the terminal ends of a zeaxanthin radical cation could provide a secondary photoprotection pathway which involves quenching excited state chlorophyll by the long-lived zeaxanthin neutral radicals formed.

Photo Protection of Haematococcus pluvialis Algae by Astaxanthin: Unique Properties of Astaxanthin Deduced by EPR, Optical and Electrochemical Studies

“…Each #Zea • would become a potent free radical trap for large numbers of excited Chl, potentially making a major contribution to qE even if its quantum yield is small. This supports the idea of a possible additional mechanism for quenching Chl’s excess energy in which the longer lived proton loss neutral radical of the carotenoid plays the role of the quencher [ 25 ].…”

“…Their neutral radicals formed under high light intensity can be very efficient quenchers of excited singlet and triplet states of chlorophyll. Neutral radicals quench the excited states of chlorophyll in a secondary photoprotection pathway [ 24 , 25 ]. Astaxanthin and its monoester can also trap metal ions in the open ponds forming metal complexes with large stability constants [ 12 ].…”

“…Carotenoid radicals such as radical cations (Car • + ) formed by electron transfer from carotenoid molecules (see Figure 4 ) and proton loss neutral radicals #Car • formed under high light intensity by deprotonation of Car • + at the most favorable position have been found to occur not only in solution, but in photosynthetic media. The proton loss neutral radical, first observed to occur in irradiated PSII samples as #β-Car • (beta-carotene neutral radical) [ 23 ] and then in the Arabidopsis thaliana plant [ 24 ] as #Zea • (zeaxanthin neutral radical), has been suggested [ 25 , 26 ] to provide additional photo protection of plants to that provided by Car • + . Since the radical cation Zea • + is a weak acid (with pK a 4–7), loss of a proton will occur to a water acceptor and the proton loss neutral radical, #Zea • , is formed.…”

“…During the charge separation, both #Zea • and Zea • + can encounter and quench Chl excited states. To reform the charge transfer complex and return to the molecular ground state, the cation Zea • + and anion Chl •− need to come together again [ 25 ].…”

“…In a solid matrix proton loss occurs not only from the cyclohexene ends but also from less energetically favorable positions such as the methyl groups attached to the polyene chain. As an alternative for the study of their properties, carotenoid radicals, both Car • + and #Car • , were long-term stabilized on solid supports such as silica-alumina, silica gel, and molecular sieves like MCM-41 and metal-substituted MCM-41 where they are stable for as long as hours to days [ 25 ]. Published EPR literature has shown that the yield of these radicals (Car • + and #Car • ) formed on such solid supports is largely increased during light irradiation and in the MCM-41’s molecular framework.…”