Photosynthetic antenna complexes capture and concentrate solar radiation by transferring the excitation to the reaction center that stores energy from the photon in chemical bonds. This process occurs with near-perfect quantum efficiency. Recent experiments at cryogenic temperatures have revealed that coherent energy transfer-a wave-like transfer mechanism-occurs in many photosynthetic pigment-protein complexes. Using the Fenna-MatthewsOlson antenna complex (FMO) as a model system, theoretical studies incorporating both incoherent and coherent transfer as well as thermal dephasing predict that environmentally assisted quantum transfer efficiency peaks near physiological temperature; these studies also show that this mechanism simultaneously improves the robustness of the energy transfer process. This theory requires long-lived quantum coherence at room temperature, which never has been observed in FMO. Here we present evidence that quantum coherence survives in FMO at physiological temperature for at least 300 fs, long enough to impact biological energy transport. These data prove that the wave-like energy transfer process discovered at 77 K is directly relevant to biological function. Microscopically, we attribute this long coherence lifetime to correlated motions within the protein matrix encapsulating the chromophores, and we find that the degree of protection afforded by the protein appears constant between 77 K and 277 K. The protein shapes the energy landscape and mediates an efficient energy transfer despite thermal fluctuations.biophysics | photosynthesis | quantum beating | ultrafast spectroscopy | quantum biology E nergy transfer through photosynthetic pigment-protein complexes operates with exceptionally high quantum efficiency (1). Recent studies have demonstrated that energy moves through antennae using not only a classical hopping mechanism but also a manifestly quantum mechanical wave-like mechanism at cryogenic temperatures (2-5). Theoretical studies of this process within the Fenna-Matthews-Olson antenna complex (FMO) show that this quantum transport mechanism requires a balance between unitary (oscillatory) and dissipative (dephasing) dynamics; further, this balance appears to be optimized near room temperature and contributes to the robustness of the process (6-9). This theory demands that quantum coherence persist long enough to affect transport, but quantum beating has never been observed in FMO at physiological temperature.The FMO pigment-protein complex from Chlorobium tepidum serves as a model system for photosynthetic energy transfer processes (2, 10-13). This complex conducts energy from the larger light-harvesting chlorosome to the reaction center in green sulfur bacteria (14, 15). Each noninteracting FMO monomer contains seven coupled bacteriochlorophyll-a chromophores arranged asymmetrically, yielding seven nondegenerate, delocalized molecular excited states called excitons (11,16). The small number of distinct states makes this particular complex attractive for theoretical studies o...

The photosynthetic light-harvesting apparatus moves energy from absorbed photons to the reaction center with remarkable quantum efficiency. Recently, long-lived quantum coherence has been proposed to influence efficiency and robustness of photosynthetic energy transfer in light-harvesting antennae. The quantum aspect of these dynamics has generated great interest both because of the possibility for efficient long-range energy transfer and because biology is typically considered to operate entirely in the classical regime. Yet, experiments to date show only that coherence persists long enough that it can influence dynamics, but they have not directly shown that coherence does influence energy transfer. Here, we provide experimental evidence that interaction between the bacteriochlorophyll chromophores and the protein environment surrounding them not only prolongs quantum coherence, but also spawns reversible, oscillatory energy transfer among excited states. Using two-dimensional electronic spectroscopy, we observe oscillatory excited-state populations demonstrating that quantum transport of energy occurs in biological systems. The observed population oscillation suggests that these light-harvesting antennae trade energy reversibly between the protein and the chromophores. Resolving design principles evident in this biological antenna could provide inspiration for new solar energy applications.energy transport | photosynthesis | quantum biology | ultrafast phenomena P hotosynthetic organisms employ light-harvesting antennae to capture and transport solar energy to the reaction center where charge separation occurs. This energy transport process proceeds through a complex network of coupled chromophores embedded in protein matrices of light-harvesting antenna complexes. Because of static Coulombic dipole couplings, the excitation typically delocalizes among two or more chromophoresthese delocalized excited states are known as "excitons." Though the excitonic states only delocalize across the chromophores, the protein bath is necessary for enabling energy transport by allowing the system to dissipate energy.The precise mechanism of dissipation and whether the protein helps to steer the transport remain interesting and open questions regarding optimal design of energy transport in disordered systems. In most electronic systems, coherences among states dephase far faster than the states themselves can relax thereby precluding contributions of coherence to relaxation processes. We define transport in such systems as "classical." Microscopically, classical transport arises from small, independent fluctuations within the protein that enable relaxation of excitonic populations through resonance energy transfer (1-4). This incoherent mechanism gives rise to exponential relaxation dynamics and ignores coherent dynamics.Recent studies on photosynthetic complexes reveal that quantum coherence persists on the same timescale as population transfer-long enough to impact transport dynamics (5-9). This experimental data implies t...

We present the first two-dimensional electronic spectra of photosynthetic antenna complexes bearing modifications to the protein and the chromophores. The vibronic structure of the Fenna-Matthews-Olson complex was altered by near-complete substitution of 13C for naturally abundant carbon and separately by randomly distributed partial deuteration. The structure and arrangement of the bacteriochlorophyll a chromophores were modified by deletion of the gene encoding the enzyme responsible for reducing the isoprenoid tail of the bacteriochlorophylls. Analysis of the time-dependent amplitude of the crosspeak corresponding to excitons 1 and 2 indicates that these modifications do not affect the frequency or dephasing of the beating observed in this particular peak. This result leads us to conclude that this beating indeed arises from electronic coherence and not vibrational wavepacket motion. We further conclude that the protection of zero-quantum coherences afforded by the protein matrix of this photosynthetic complex is not the result of a finely-tuned series of system-bath interactions perfected by billions of years of evolution but rather a simple downstream property of a close arrangement of chromophores within a phonon bath. We conclude with a brief discussion of the outstanding questions and possible applications of this phenomenon.

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