The photosynthetic unit (PSU) of purple photosynthetic bacteria consists of a network of bacteriochlorophyll-protein complexes that absorb solar energy for eventual conversion to ATP. Because of its remarkable simplicity, the PSU can serve as a prototype for studies of cellular organelles. In the purple bacterium Rhodobacter sphaeroides the PSU forms spherical invaginations of the inner membrane, Ϸ70 nm in diameter, composed mostly of lightharvesting complexes, LH1 and LH2, and reaction centers (RCs). Atomic force microscopy studies of the intracytoplasmic membrane have revealed the overall spatial organization of the PSU. In the present study these atomic force microscopy data were used to construct three-dimensional models of an entire membrane vesicle at the atomic level by using the known structure of the LH2 complex and a structural model of the dimeric RC-LH1 complex. Two models depict vesicles consisting of 9 or 18 dimeric RC-LH1 complexes and 144 or 101 LH2 complexes, representing a total of 3,879 or 4,464 bacteriochlorophylls, respectively. The in silico reconstructions permit a detailed description of light absorption and electronic excitation migration, including computation of a 50-ps excitation lifetime and a 95% quantum efficiency for one of the model membranes, and demonstration of excitation sharing within the closely packed RC-LH1 dimer arrays.excitation transfer ͉ network kinetics ͉ photosynthetic light harvesting ͉ quantum efficiency ͉ systems biology P hotosynthesis, the main source of energy for the biosphere (1, 2), is initiated when the thousands of pigments that cooperate to form an interconnected photosynthetic unit (PSU) harvest and transfer solar energy before its conversion to a charge separation. Peripheral pigment-protein complexes deliver energy to a reaction center (RC) (3-6), where it is used for the transmembrane electron transfers (7, 8) that eventually drive ATP synthesis. The structures of the key protein complexes involved in this process have been solved both for oxygenic photosynthetic organisms, such as cyanobacteria, algae, and plants (9-14), and for anoxygenic photosynthetic bacteria (15)(16)(17)(18)(19)(20)(21)(22)(23)(24). This recent progress makes it possible to study the biophysical processes involved in photosynthesis in atomic detail all the way down to the quantum mechanical level (25-31). However, it remains a challenge to understand how a biological membrane comprising hundreds of photosynthetic complexes functions with great efficiency. To address this challenge we embarked on the in silico construction of an entire photosynthetic membrane, namely the purple bacterial PSU (32), based on a combination of cryo-EM (24, 33-35), NMR (22), x-ray crystallography (18,19,23,36), and atomic force microscopy (AFM) (37-41) data.The purple bacterial PSU displays remarkable simplicity compared with its eukaryotic, oxygenic analogues, being evolutionarily more primitive (42). It contains six different kinds of proteins that work cooperatively: LH2 antenna complexes (18,19,2...

This perspective examines how hundreds of pigment molecules in purple bacteria cooperate through quantum coherence to achieve remarkable light harvesting efficiency. Quantum coherent sharing of excitation, which modifies excited state energy levels and combines transition dipole moments, enables rapid transfer of excitation over large distances. Purple bacteria exploit the resulting excitation transfer to engage many antenna proteins in light harvesting, thereby increasing the rate of photon absorption and energy conversion. We highlight here how quantum coherence comes about and plays a key role in the photosynthetic apparatus of purple bacteria.

As most biological species, photosynthetic lifeforms have evolved to function optimally, despite thermal disorder and with fault tolerance. It remains a challenge to understand how this is achieved. To address this challenge the function of the protein-pigment complex photosystem I (PSI) of the cyanobacterium Synechococcus elongatus is investigated theoretically. The recently obtained high resolution structure of this complex exhibits an aggregate of 96 chlorophylls that are electronically coupled to function as a light-harvesting antenna complex. This paper constructs an effective Hamiltonian for the chlorophyll aggregate to describe excitation transfer dynamics and spectral properties of PSI. For this purpose, a new kinetic expansion method, the sojourn expansion, is introduced. Our study shows that at room temperature fluctuations of site energies have little effect on the calculated excitation lifetime and quantum yield, which compare favorably with experimental results. The efficiency of the system is found to be robust against 'pruning' of individual chlorophylls. An optimality of the arrangement of chlorophylls is identified through the quantum yield in comparison with an ensemble of randomly oriented chlorophylls, though, the quantum yield is seen to change only within a narrow interval in such an ensemble.