March 8, 2021 feature
Do photosynthetic complexes use quantum coherence to increase their efficiency?
In a new report now published on Science Advances, Elinor Zerah Harush and Yonatan Dubi in the departments of chemistry and nanoscale science and technology, at the Ben-Gurion University of the Negev, Israel, discussed a direct evaluation of the effects of quantum coherence on the efficiency of three natural photosynthetic complexes. The open quantum systems approach allowed the researchers to simultaneously identify the quantum-nature and efficiency under natural physiological conditions. These systems resided in a mixed quantum-classical regime, which they characterized using dephasing-assisted transport. The efficiency was minimal at best therefore the presence of quantum coherence did not play a substantial role in the process. The efficiency was also independent of any structural parameters, suggesting the role of evolution during structural design for other uses.
Investigating quantum effects in biology
During photosynthesis, energy can be transferred from an antenna to a reaction center to collect light and convert it to chemical energy for use by the organism. Exciton-bound electron hole pairs formed the energy carriers in the photosynthetic process to carry harvested solar energy from the antenna to the reaction center via a network of bacteriochlorophylls (photosynthetic pigments that occur in bacteria), also known as the exciton-transfer complex (ETC). Interests on the ETC have expanded in the past decade where researchers used ultrafast nonlinear spectroscopy signals to demonstrate long-lived oscillations. The discovery of coherent oscillations in ETCs presented the hypothesis that quantum coherence occurred within natural photosynthetic complexes to assist energy transfer. Harush et al. sought to understand if quantum coherence could exist in the biological process of photosynthetic energy transfer. If so, was it used by the natural system for enhanced functional efficiency? While experimental and theoretical work have addressed these questions, they remain largely unanswered. In this work, the team addressed the questions using tools developed from the theory of open quantum systems. The findings suggest the unlikelihood for photosynthetic complexes to use quantum coherence to increase their efficiency.
The team considered three different photosynthetic ETCs (exciton-transfer complexes) during the experiments. These include the Fenna-Matthews-Olson (FMO) complex—which appears in green sulfur bacteria, the cryptophyte phycocyanin-645 (PC-645) protein—a part of the photosynthetic apparatus in cryptophyte algae, and light harvesting 2 (LH2) – a part of the purple photosynthetic bacterium Rhodopseudomonas acidophila. All three complexes showed coherent energy transfer oscillations in nonlinear two-dimensional spectroscopy measurements. The team plotted the exciton current as a function of the dephasing rate for the FMO complex and the PC-645 complex. The similarity between the plots indicted relative insensitivity of the current to the internal structure Hamiltonian. Using the bacterial populations Harush et al. tested the level of "quantumness" of the system. They recognized this using a connection between the exciton population and dephasing rate through the mechanism of environment-assisted quantum transport (ENAQT). The ENAQT effect was clearly visible in the results since the current showed a maximum in the dephasing rate. However, the current enhancement was minute at approximately 0.0015% increase to indicate the unlikely nature of the complex to impose a meaningful evolutionary driving force.
Effect of environment on photosynthetic transfer efficiency
The team next investigated the LH2 (light harvesting-2) complex to understand the connection between ENAQT (environment-assisted quantum transport) and the population. This was difficult due to the lack of spatial separation between the antenna and reaction center in the construct. The LH2 complex contained two rings of bacteriophyll pigments; B800 (yellow ring) and B850 (blue ring) named after their energy absorption resonance in nanometers and absorbing energy in the visible region of the spectrum. Each part of the complex could absorb light to excite an exciton, which transferred from one of the rings to the reaction center allowing many exciton transfer-paths to occur. However, a current versus dephasing curve for LH2 revealed the importance of coherence during transport. The team then plotted current as a function of dephasing rate of the LH2 system and noted a very small increase in current approximating 0.05 percent.
Current, coherence and classicality.
The results of the study established the absence of a substantial increase in the exciton current when comparing the fully quantum case with the physiologically realistic dephasing rates. They also took classical systems in to account, which were not defined by the lack of any coherence, although their coherences could be fully determined from the populations without additional information. Researchers had previously quantified the distinction between quantum and classical systems. In a classical system, the two currents will be the same, implying that quantum coherences do not carry additional information across the classical dynamics.
The outcome of this study indicated how the structures of interest relative to FMO, PC-645 and LH2 did not evolve to enhance the efficiency of the complexes. In the future, Elinor Zerah Harush and Yonatan Dubi intend to assess the origin of the observed dephasing time to acknowledge if the values calculated in the study are unique. The team also intend to understand other potential evolutionary advantages of the photosynthetic transfer complexes, which will guide biophysicists to broadly understand the possible role of quantum effects in photosynthetic complexes.
Engel G. S. et al. Evidence for wavelike energy transfer through quantum coherence in photosynthetic systems. Nature, doi: doi.org/10.1038/nature05678
Panitchayangkoon G. et al. Long-lived quantum coherence in photosynthetic complexes at physiological temperature. PNAS, doi: doi.org/10.1073/pnas.1005484107
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