Singlet−Singlet Annihilation and Local Heating in FMO Complexes

Abstract

Energy transfer in a trimeric Bchl a containing FMO pigment-protein complex from the green sulfur bacterium Chlorobium tepidum has been studied by means of picosecond transient absorption spectroscopy under highexcitation conditions. At room temperature the excited state absorption spectrum of the FMO complex was found to be similar to that of noninteracting Bchl molecules in solution, which suggests that the influence of exciton coupling on the spectroscopic properties of the FMO complex at room temperature is not substantial. Analysis of the excited state relaxation kinetics in singlet-singlet annihilation conditions shows that the energy transfer from the excited monomer to another excited monomer is independent of the oxidationreduction state of the complex and is slower than the intermonomer excitation migration rate. The difference spectrum at 77 K resembles the absorption spectrum, showing three exciton subbands. In addition to the singlet-singlet annihilation, the 7 ps rate of which is similar to that at room temperature, and to the intrinsic exciton decay, which is also temperature independent, energy redistribution between exciton states with a mean time of 26 ps is evident. This redistribution is explained as being due to local heating/cooling kinetics stimulated by the excitation pulses. IntroductionEstablishing a relationship between the structural build-up and spectral properties of pigment-protein complexes is one of the goals of spectroscopic research of photosynthetic systems. Despite numerous investigations, it has turned out to be a nontrivial task to correlate the structure and spectra. 1 The best understood photosynthetic pigment-protein complex confirming this statement is a bacteriochlorophyll a (Bchl a) containing pigment-protein complex from green sulfur bacteria, known as the FMO protein after Fenna, Matthews, and Olson. 2 The structure of this complex has been determined to 1.9 Å resolution, 3,4 and the amino acid sequence of the protein is known as well. 5 The FMO complex is a trimer (C 3 symmetry) composed of identical protein subunits of 47 kDa, each of which hosts seven Bchl a molecules arranged in a highly nonregular fashion. A number of spectroscopic studies 6-8 have been performed on FMO complexes, and some theoretical efforts 9,10 have been applied. Despite the absence of the exact coincidence of the theoretical calculations and the spectroscopic results, some qualitative conclusions were formulated. By analyzing lowtemperature absorption and CD spectra on the basis of the exciton theory, it was concluded 9 that the longest wavelength absorption spectrum subband was determined by the absorption of Bchl a No. 7 (according to the numbering scheme of Matthews and Fenna 3 ) on which the excitation resides for most of the excited state lifetime even at room temperature. The hole-burning experiments 6 as well as the singlet-triplet difference absorption measurements 7,8 supported the assumption of the exciton interaction between the pigment molecules in the monomer and between Bchl a ...

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