We have performed time-resolved fluorescence measurements on photosystem II (PSII) containing membranes (BBY particles) from spinach with open reaction centers. The decay kinetics can be fitted with two main decay components with an average decay time of 150 ps. Comparison with recent kinetic exciton annihilation data on the major light-harvesting complex of PSII (LHCII) suggests that excitation diffusion within the antenna contributes significantly to the overall charge separation time in PSII, which disagrees with previously proposed trap-limited models. To establish to which extent excitation diffusion contributes to the overall charge separation time, we propose a simple coarse-grained method, based on the supramolecular organization of PSII and LHCII in grana membranes, to model the energy migration and charge separation processes in PSII simultaneously in a transparent way. All simulations have in common that the charge separation is fast and nearly irreversible, corresponding to a significant drop in free energy upon primary charge separation, and that in PSII membranes energy migration imposes a larger kinetic barrier for the overall process than primary charge separation.

Singlet-singlet annihilation is used to study exciton delocalization in the light harvesting antenna complex LH2 (B800-B850) from the photosynthetic purple bacterium Rhodobacter sphaeroides. The characteristic femtosecond decay constants of the high intensity isotropic and the low intensity anisotropy kinetics of the B850 ring are related to the hopping time tau(h) and the coherence length N(coh) of the exciton. Our analysis yields N(coh) = 2.8+/-0.4 and tau(h) = 0.27+/-0.05 ps. This approach can be seen as an extension to the concept of the spectroscopic ruler.

High-spectral-resolution hole-burning and fluorescence line-narrowing spectra of excitons in LH2 complexes from the photosynthetic purple bacterium Rhodobacter sphaeroides have been investigated together with conventional broadband fluorescence spectra and their temperature dependence. The steady-state spectroscopy has been complemented by fluorescence lifetime measurements. The experimental results are discussed on the basis of the adiabatic Holstein exciton polaron model, modified by including diagonal disorder. As a result, a new interpretation for the LH2 antenna optical spectra is provided. The exciton when optically excited becomes localized after relaxation. The LH2 fluorescence is mainly due to self-trapped excitons not only at low temperature, as previously suggested (Timpmann, K.; Katiliene, Z.; Woodbury, N. W.; Freiberg, A. J. Phys. Chem. B 2001, 105, 12223), but also over the whole temperature range up to physiological temperatures because the self-trapped exciton binding energy is of the same order as the thermal excitation energy at ambient temperature. The conclusion is made that direct self-trapping relaxation dominates the common energy relaxation between exciton states and that the main factor limiting the relaxed exciton size is dynamic rather than static disorder. The coexistence of large and small exciton polarons at low temperatures has been confirmed. Exciton self-trapping also essentially modifies the long-wavelength tail of the absorption spectrum of LH2 complexes. The fraction of the absorption spectrum that is subject to hole burning is due to large-radius self-trapped excitons that are weakly coupled to the lattice. The rest of this spectrum that survives hole burning belongs to the strongly coupled self-trapped excitons/excimers. Implications of these results on the interpretation of Stark spectroscopy experiments as well as on photosynthetic energy transfer and trapping are discussed.

The nature of electronic excitations created by photon absorption in the cyclic B850 aggregates of 18 bacteriochlorophyll molecules of LH2 antenna complexes of photosynthetic bacteria is studied over a broad temperature range using absorption, fluorescence, and fluorescence anisotropy spectra. The latter technique has been proved to be suitable for revealing the hidden structure of excitons in inhomogeneously broadened spectra of cyclic aggregates. A theoretical model that accounts for differences of absorbing excitons in undeformed and emitting exciton polarons in deformed antenna lattices is also developed. Only a slight decrease of the exciton bandwidth and exciton coupling energy with temperature is observed. Survival of excitons in the whole temperature span from cryogenic to nearly ambient temperatures strongly suggests that collective, coherent electronic excitations might play a role in the functional light-harvesting process taking place at physiological temperatures.

A model for the spectral characteristics, the transition dipole moment orientations, and the energy transfer properties of chlorophyll (Chl) a and b molecules in the light-harvesting complex (LHC) II is proposed on the basis of the results from femtosecond transient measurements and other spectroscopic data. The model uses the structural data (Kühlbrandt; et al. Nature 1994, 367, 614) and is obtained using a genetic algorithm search of the large parameter space. Förster resonance transfer has been assumed as the mechanism of energy transfer. The spectral and orientational assignments of all twelve Chl molecules of a LHC II monomer are proposed. In the best fit model two of the seven Chl molecules that are proximal to the central luteins are Chl b. In contrast to prior assumptions, the basic feature of the model consists of an intermediately strong coupling (V < 100 cm-1) between the Chl a and b molecules in close pairs and the absence of substantial excitonic coupling between Chls a, thus indicating an overall limited influence of excitonic effects on spectra and kinetics. A theoretical estimation of exciton effects supports these model assumptions. Over most of the difference absorption spectrum good agreement between experimental and theoretical kinetics has been obtained. Energy transfer times in the symmetric LHC II trimer range from 90 fs to 5.1 ps. For the monomeric complexes only the longest lifetime is significantly affected and predicted to be just slightly longer (6.6 ps). The predicted transition dipole moment orientations result in weak coupling between the LHC II monomers. Several possible routes to improve both the data fitting and the reliability of the predictions in the future are discussed.

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The role of individual photosynthetic antenna complexes of Photosystem II (PSII) both in membrane organization and excitation energy transfer have been investigated. Thylakoid membranes from wild-type Arabidopsis thaliana, and three mutants lacking light-harvesting complexes CP24, CP26, or CP29, respectively, were studied by picosecond-fluorescence spectroscopy. By using different excitation/detection wavelength combinations it was possible for the first time, to our knowledge, to separate PSI and PSII fluorescence kinetics. The sub-100 ps component, previously ascribed entirely to PSI, turns out to be due partly to PSII. Moreover, the migration time of excitations from antenna to PSII reaction center (RC) was determined for the first time, to our knowledge, for thylakoid membranes. It is four times longer than for PSII-only membranes, due to additional antenna complexes, which are less well connected to the RC. The results in the absence of CP26 are very similar to those of wild-type, demonstrating that the PSII organization is not disturbed. However, the kinetics in the absence of CP29 and, especially, of CP24 show that a large fraction of the light-harvesting complexes becomes badly connected to the RCs. Interestingly, the excited-state lifetimes of the disconnected light-harvesting complexes seem to be substantially quenched.

One of the major players in oxygenic photosynthesis, photosystem II (PSII), exhibits complex multiexponential fluorescence decay kinetics that for decades has been ascribed to reversible charge separation taking place in the reaction center (RC). However, in this description the protein dynamics is not taken into consideration. The intrinsic dynamic disorder of the light-harvesting proteins along with their fluctuating dislocations within the antenna inevitably result in varying connectivity between pigment-protein complexes and therefore can also lead to nonexponential excitation decay kinetics. On the basis of this presumption, we propose a simple conceptual model describing excitation diffusion in a continuous medium and accounting for possible variations of the excitation transfer rates. Recently observed fluorescence kinetics of PSII of different sizes are perfectly reproduced with only two adjustable parameters instead of the many decay times and amplitudes required in standard analysis procedures; no charge recombination in the RC is required. The model is also able to provide valuable information about the structural and functional organization of the photosynthetic antenna and in a straightforward way solves various contradictions currently existing in the literature.