Electron paramagnetic resonance detection of carotenoid triplet states

Abstract

Triplet states of carotenoids have been detected by X-band electron paramagnetic resonance (EPR) and are reported here for the first time. The systems in which carotenoid triplets are observed include cells of photosynthetic bacteria, isolated bacteriochlorophyll-protein complexes, and detergent micelles which contain ..beta..-carotene. It is well known that if electron transfer is blocked following the initial acceptor in the bacterial photochemical reaction center, back reaction of the primary radical pair produces a bacteriochlorophyll dimer triplet. Previous optical studies have shown that in reaction centers containing carotenoids the bacteriochlorophyll dimer triplet sensitizes the carotenoid triplet. We have observed this carotenoid triplet state by EPR in reaction centers of Rhodopseudomonas sphaeroides, strain 2.4.1 (wild type), which contain the carotenoid spheroidene. The zero-field splitting parameters of the triplet spectrum are /D/ = 0.0290 +- 0.0005 cm/sup -1/ and /E/ = 0.0044 +-0.0006 cm/sup -1/, in contrast with the parameters of the bacteriochlorophyll dimer triplet, which are /D/ = 0.0189 +- 0.0004 cm/sup -1/ and /E/ = 0.0032 +- 0.004 cm/sup -1/. Bacteriochlorophyll in a light harvesting protein complex from Rps. sphaeroides, wild type, also sensitizes carotenoid triplet formation. In whole cells the EPR spectra vary with temperature between 100 and 10 K. Carotenoidmore » triplets also have been observed by EPR in whole cells of Rps. sphaeroides and cells of Rhodospirillum rubrum which contain the carotenoid spirilloxanthin. Attempts to observe the triplet state EPR spectrum of ..beta..-carotene in numerous organic solvents failed. However, in nonionic detergent micelles and in phospholipid bilayer vesicles ..beta..-carotene gives a triplet state spectrum with /D/ = 0.0333 +- 0.0010 cm/sup -1/ and /E/ = 0.0037 +- 0.0010 cm/sup -1/. 6 figures, 1 table.« less