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Singlet–Triplet Fission of Carotenoid Excitation in Light-Harvesting LH2 Complexes of Purple Phototrophic Bacteria

I. B. Klenina*, Z. K. Makhneva, A. A. Moskalenko, N. D. Gudkov, M. A. Bolshakov, E. A. Pavlova, and I. I. Proskuryakov

Institute of Basic Biological Problems, Russian Academy of Sciences, ul. Institutskaya 2, 142290 Pushchino, Moscow Region, Russia; fax: 8 (4967) 330-532; E-mail: ikleonik@yandex.ru

* To whom correspondence should be addressed.

Received November 13, 2013; Revision received December 9, 2013
The current generally accepted structure of light-harvesting LH2 complexes from purple phototrophic bacteria conflicts with the observation of singlet–triplet carotenoid excitation fission in these complexes. In LH2 complexes from the purple bacterium Allochromatium minutissimum, a drop in the efficiency of carotenoid triplet generation is demonstrated, which correlates with the extent of selective photooxidation of bacteriochlorophylls absorbing at ~850 nm. We conclude that singlet–triplet fission of carotenoid excitation proceeds with participation of these excitonically coupled bacteriochlorophylls. In the framework of the proposed mechanism, the contradiction between LH2 structure and photophysical properties of carotenoids is eliminated. The possibility of singlet–triplet excitation fission involving a third mediator molecule was not considered earlier.
KEY WORDS: purple phototrophic bacteria, light-harvesting complexes, structural organization, carotenoids, singlet–triplet excitation fission

DOI: 10.1134/S0006297914030092