Carotenoid cation radical resonance spectroscopy

Under particular conditions, P-carotene (1) serves as an alternative electron donor in the PS2 reaction centre, coping with highly oxidised intermediates formed during water oxidation. This redox role of P-carotene (1) in PS2 is unique amongst photosynthetic reaction centres. An overview on the electron transfer processes in PS2, emphasising those involving carotenoids, has recently been presented [97]. Methods employed for studies of the P-carotene cation radical (12) in PS2 include Resonance Raman [36,98,99], FT-IR [100], ENDOR [38] and EPR [37] spectroscopy, see under Methods below. 

The principles, scope and limitations of Raman spectroscopy and its application in the carotenoid field has been discussed [134], Time-resolved resonance Raman spectroscopy provides useful information on carotenoids in the excited state [135]. Differences between the resonance Raman spectra of a neutral carotenoid and its cation radical provide structural information on the cation radical. 

This chapter reviews the work ofthe last five to six years on paramagnetic states of carotenoids using electron magnetic resonance. Mainly radical cation and neutral molecular triplet states are treated. Part of this ch te deals with paramagnetic states of carotenoids in model systems. These have been synthesized in order to mimic both electron and energy transfer processes in the natural photosynthetic systems. Consequently, the electron magnetic resonance (EMR) spectroscopy of carotenoid triplet and radical states yields important information about their photochemistry. Finally, the EMR spectroscopy on carotenoid radicals is reviewed. It serves to establish the database on their intrinsic properties which is necessary for the analysis of carotenoid radicals in vivo. 

Tinkler JH, Tavender SM, Parker AW McGarvey DJ, Mukoy L and Truscott TG (1996) Investigation of carotenoid radical cations and triplet states by laser flash photolysis and time-resolved resonance Raman spectroscopy Observation of... 

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