Carotenoid-Bacteriochlorophyll Energy Transfer in LH2 Complexes studied with 10-fs Time Resolution

¹ Politecnico di Milano
² Osaka City University
³ University of Glasgow

^* To whom correspondence should be addressed. E-mail: dario.polli{at}polimi.it.

Submitted on June 23, 2005
Revised on August 22, 2005
Accepted on 5 January 2006

	Abstract

In this report we present a study of carotenoid-bacteriochlorophyll energy transfer processes in two peripheral light-harvesting complexes (known as LH2) from purple bacteria. We use transient absorption spectroscopy with 10 fs temporal resolution, which is necessary to observe the very fast energy relaxation processes. By comparing excited-state dynamics of the carotenoids in organic solvents and inside the LH2 complexes it has been possible to directly evaluate their energy transfer efficiency to the bacteriochlorophylls. In the case of okenone in the LH2 complex from Chromatium purpuratum, we obtained an energy transfer efficiency of _ET2 = 63±2.5% from the optically-active excited state (S₂) and _ET1 = 61±2% from the optically-dark state (S₁); for rhodopin glucoside contained in the LH2 complex from Rhodospeudomonas acidophila these values become _ET2 = 49.5±3.5% and _ET1 = 5.1±1%. The measurements also enabled us to observe vibrational energy relaxation in the carotenoids' S₁ state and real-time collective vibrational coherence initiated by the ultrashort pump pulses. Our results are important for understanding the dynamics of early events of photosynthesis and relating it to the structural arrangement of the chromophores.

Key Words: LH2 complexes, bacteriochlorophylls, carotenoids, energy transfer, purple bacteria, ultrafast spectroscopy