QBP2: Quantum coherence and photosynthesis

If we describe particles by wave function; a mathematical representation of the quantum state of a system, probabilistic interpretation of wave function can explain various quantum effects, and as long as there exsits a definite phase relation between different states, the system is coherent.

 

"Coherence or decoherence represents a very fast process for macroscopic objects, Since these are interacting with many microscopic objects, with an enormous number of degrees of freedom. The decoherence time relies on the short-time behaviour of the fidelity between the initial and the time dependent state."

 

 

Different types of quantum processes that operate at the same time scale can interact strongly either to assist or to impede one another. Evolved biological systems exhibit the quantum Goldilocks effects (choose the most suitable, like the tale), natural selection pushes together time scales to allow quantum processes to help each other.

In photosynthetic enargy transfer, the convergence of quantum time scales may be gives rise to move efficient and robust transport. As we know, sunlight is converted into chemical energy in the form of sugars. Chlorophyll within the chloroplasts in plant cells is used to synthesise glucose from water and carbon dioxide using the energy from sunlight.

The light-harvesting complex is the location of chlorophyll and carotenoid pigment molecules that take part in photosynthesis is found on the thylakoid membrane. They absorb incoming photons, and the excitation energy is transferred to the reaction centre, which here comprises a pair of chlorophyll molecules. The energy is transferred out through an electron acceptor  and used in further reactions of photosynthesis.

Previously we knew that energy transport in photosynthesis involves interactions between the electronic states of chromophores, energy is passed on to a neighbouring chromophore with a slightly lower energy level, it is a random hopping process between chromophores, with an overall energy gradient.

 

But in 2007, Graham Fleming of the University of California at Berkeley used a technique called photon-echo spectroscopy to study the energy-transfer process in the so-called Fenna–Matthews–Olson (FMO) pigment–protein complex – the part of the photosynthetic apparatus that mediates energy transfer in the tharmophilic bacterium Chlorobium tepidum." In this experiment using an ultrashort “pump” pulse from a laser,they tried to excite coherent excitons  and then probing the way the excitation evolved over time using subsequent ultrashort laser pulses to look for signs of beating."

(Fig:Graham Fleming, Hohjai Lee and Yuan-Chung)

Fleming concluded that "the results suggested that correlated protein environments preserve electronic coherence in photosynthetic complexes and allow the excitation to move coherently in space, enabling highly efficient energy harvesting and trapping in photosynthesis.Rather than simply serving as a static structure that holds the pigments in the proper geometry for efficient energy transfer to the reaction centers, as was anticipated, we find that the protein environment in the reaction centers plays a dynamic role in optimizing the efficiency of the energy transfer.”

In 2010,  Fleming’s former postdocs, photochemist Gregory Scholes at the University of Toronto in Canada, collaborated with chemist Elisabetta Collini and co-workers to look for such effects under ambient conditions in yet another type of organism: photosynthetic cryptophyte algae, which can harvest light efficiently enough to carry out photosynthesis even in dim conditions. They too saw beats lasting for longer than a tenth of a picosecond at room temperature. This putative electronic coherence, they said, was an order of magnitude longer-lived than had been previously thought – and long enough for the energy to get transferred from the chromophores to the photosynthetic reaction centre".

The same idea was tested by Dwayne Miller of the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, Germany, and his team using dye molecules joined together in pairs by a more flexible linker, they came to a different conclusion. They argued that the previously observed coherence was too small in amplitude to originate in excitons, and was indeed a kind of classical resonance effect involving molecular vibrations. Besides, they said, any electronic coherence decays too rapidly to have any meaningful biological consequences.

Miller said “We went back and redid the original work at room temperature and actual physiological conditions, and it is absolute certain there are no long-lived electronic coherences that direct energy transfer.The beats are comparable in amplitude, frequency and decay rate to trivial Raman vibrations of the electronic ground state excited in the process. It is Raman that they saw, not long-lived electronic coherence.”

Elisabet Romero of the Free University of Amsterdam and her co-workers have carried out spectroscopic experiments that they interpret as evidence that vibrations in the reaction centre of the photosystem from spinach support quantum coherence between excitonic states, by strengthening the interactions between them, and promoting coherent energy transfer .

Romero says "In my view, the relevant issue is to understand how plants or other photosynthetic organisms are able to transfer energy and electrons on an ultra-fast timescale in the right direction with high quantum efficiency. If we understand that, we might be able to do something useful with it."

In my point of view, light revealed many truth through out our history. Before the idea about quantum coherence came out, we thought the energy transport in photosynthesis is a random hopping process, but then we observed different efficiency of energy transport in different photosynthetic organisms, so there must be a reason.

Photosynthesis is the source of energy in our food chain, it produces the most stable form of energy required for living beings.

Quantum coherence may be an explanation how photosynthetic organisms gains quantum efficiency when harvesting light energy. I am really exited about it, and waiting for new experimental evidences.