Monday, February 22, 2021

Are quantum processes relevant in photosynthesis?

What is "quantum biology"? It is the study of quantum effects in complex biological systems. These studies have a history stretching back to the early years of quantum mechanics. During photosynthesis, light-harvesting occurs on time scales of tens of microseconds. The couplings between the constituents and interaction with the bath will govern the dynamics of the system.

Photosynthetic pigments are held up by a protein scaffold. When interacting with light, the pigments do not act as independent units because the transition energies and oscillator strength are distributed within the scaffold. The resulting collective excitations are called "excitons." Coherence is a certain degree of spatial delocalization of the excitations in the system. Excited superpositions evolve in time as damped oscillations with a frequency corresponding to the constituent states' energy difference. It is widely accepted in science that the dephasing of any interexciton (the shared exitation of complexes) coherences results in sub–100-fs (femtosecond) decay times at room temperature. Energy transfer within the complex is impossible during such a short time. The reaction center of the photosystem shows similarly fast electronic decoherence rates.

The incoherent relaxation between exciton states forms energy transport with downhill in energy. Since these exciton states are partially localized, this relaxation corresponds to hopping between different regions of the complex. The variable site energies direct the energy flow, automatically providing a definite directionality for the transport.

The inhomogeneous charge distribution in the light-harvesting protein complex leads to varying site energies of the pigments. The effect directs the excitation energy toward the reaction center. Suppressed correlations in site energy fluctuations lead to an efficient dissipation of the excess energy of excitons. The absence of long-lived interexciton coherence on relevant time scales makes quantum occurrences impossible.

In conclusion, photosynthesis occurs on the fastest possible time scales. The dephasing of coherences in biological conditions is too instantaneous to contribute significantly to the light-harvesting function. Are quantum processes relevant in photosynthesis? Quantum coherence in photosynthetic light-harvesting is unlikely.

Read the whole article: Quantum biology revisited

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