Direct evidence of quantum transport in photosynthetic light-harvesting complexes

Abstract

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...

Coherence and population dynamics of chlorophyll excitations in FCP complex: Two-dimensional spectroscopy study

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