The theory of the singlet-singlet annihilation in quasi-homogeneous photosynthetic antenna systems is developed further. In the new model, the following important contributions are taken into account: 1) the finite excitation pulse duration, 2) the occupation of higher excited states during the annihilation, 3) excitation correlation effects, and 4) the effect of local heating. The main emphasis is concentrated on the analysis of pump-probe kinetic measurements demonstrating the first two above possible contributions. The difference with the results obtained from low-intensity fluorescence kinetic measurements is highlighted. The experimental data with picosecond time resolution obtained for the photosynthetic bacterium Rhodospirillum rubrum at room temperature are discussed on the basis of this theory.

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 ...

The aim of this paper is to review and discuss the results obtained by fluorescence and absorption spectroscopy of bacterial chromatophores excited with picosecond pulses of varying power and intensity. It was inferred that spectral and kinetic characteristics depend essentially on the intensity, the repetition rate of the picosecond excitation pulses as well as on the optical density of the samples used. Taking the different experimental conditions properly into account, most of the discrepancies between the fluorescence and absorption measurements can be solved. At high pulse repetition rate (>10(6) Hz), even at moderate excitation intensities (10(10)-10(11) photons/cm(2) per pulse), relatively long-lived triplet states start accumulating in the system. These are efficient (as compared to the reaction centers) quenchers of mobile singlet excitations due to singlet-triplet annihilation. The singlet-triplet annihilation rate constant in Rhodospirillum rubrum was determined to be equal to 10(-9) cm(3) s(-1). At fluences >10(12) photons/cm(2) per pulse singlet-singlet annihilation must be taken into account. Furthermore, in the case of high pulse repetition rates, triplet-triplet annihilation must be considered as well. From an analysis of experimental data it was inferred that the singlet-singlet annihilation process is probably migration-limited. If this is the case, one has to conclude that the rate of excitation decay in light-harvesting antenna at low pumping intensities is limited by the efficiency of excitation trapping by the reaction center. The influence of annihilation processes on spectral changes is also discussed as is the potential of a local heating caused by annihilation processes. The manifestation of spectral inhomogeneity of light-harvesting antenna in picosecond fluorescence and absorption kinetics is analyzed.

Excitons in circular aggregates of dimers are discussed with the aim to understand the possible spectral and energy transfer properties of the ringlike peripheral complexes of photosynthetic bacteria. The system is explicitly heterogeneous (i.e., the difference in transition energies of the molecules within the dimer as, well as the difference in the intra-and interdimer resonance interactions, is accounted for). It is demonstrated that the energy spectrum of such a system exhibits many of the features as observed in spectrally inhomogeneous circular aggregates. The model is used to illustrate the changes in absorption and circular dichroism spectra that take place upon incorporating a dimer into a circular chain. The exciton dynamics in the aggregate is considered in the Haken-Strobl-Reineker approach. When terms are neglected describing the phase relaxation between nonnearest neighbors, the equations for the diagonal density matrix elements are obtained containing both coherent exciton motion within the dimersthe building block of the aggregatesand an incoherent hopping of the excitation between dimers. It is demonstrated that these equations contain a wavelike soliton solution (if dephasing is absent) as well as a diffusion-like solution (for large dephasing rates).

The energy transfer and trapping kinetics in the core antenna of Photosystem I are described in a new model in which the distance between the core antenna chlorophylls and P700 is proposed to be considerably longer than the distance between the chlorophylls within the antenna. Structurally, the model describes the Photosystem I core antenna as a regular sphere around P700, while energetically it consists of three levels representing the bulk antenna, P700 and the red-shifted antenna pigments absorbing at longer wavelength than P700, respectively. It is shown that the model explains experimental results obtained from the Photosystem I complex of the cyanobacterium Synechococcus sp. (A.R. Holzwarth, G. Schatz, H Brock, and E. Bittersman (1993) Biophys. J. 64: 1813-1826) quite well, and that no unrealistic charge separation rate and organization of the long-wavelength pigments has to be assumed. We suggest that excitation energy transfer and trapping in Photosystem I should be described as a 'transfer-to-the-trap'-limited process.