Organic Physical Chemistry Laboratory

Physical Chemistry of Photosynthesis

Figure 1 shows zoom-up images of the site of plant photosynthesis, where the photosynthetic proteins are working. They are integrated in the thylakoid membrane within the cellular organelle called “chloroplast” within the plant cells that compose the plant leaf. In the plant photosynthesis, the two proteins called photosystem II (PS II) and photosystem I (PS I) play a role as dual solar cells connected in series; the electrons drawn from water molecules by PS II flow through quinone molecules in the thylakoid membrane, cytochrome b6f protein, and then toward PS I. Finally, the electrons are donated to NADP+ to synthesize NADPH. The light energy is utilized as the driving force of the electron flow.

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Figure 1.

Figure 2 shows the crystal structure of PS II revealed on 2011 with a spatial resolution of 1.9 Ǻ. One molecule of PS II protein contains 35 chlorophylls (Chl’s), 2 pheophytin, and several carotenoid molecules. Several pigments at the center compose the reaction center taking the central role in the primary photo-induced charge separation. The other Chl’s play a role of antennae, which transfer the absorbed photon energy to the reaction center.

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Figure 2.

Figure 3 shows schematically the energy levels of the Chl’s bound to the photosystem. Due to the extremely dense packing of Chl’s, there are strong electronic interactions (called “excitonic coupling”) among the Chl molecules. It has been revealed that the strong excitonic couplings result in the quantum mechanical de-localization of the excited states over multiple Chl molecules. Because of the de-localization of the excited state, the simple picture that a molecule transfer the energy to the other (Fig. 3 middle) no longer holds. Instead, we need a modified picture of light-harvesting that the light energy is transferred to the reaction center through the relaxation among the de-localized excited states composed of a group of the strongly coupled Chl’s.

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Figure 3.

Nowadays, we know the detailed molecular structures of photosynthetic proteins thanks to the recent progress in the structural biology. We have obtained a framework based on which we can understand the photo-induced reaction in the photosynthetic proteins appropriately considering the quantum mechanical effects. Such an understanding will shed light on the unresolved mysteries in photosynthesis research field listed below.

・ It is known that photosynthetic organisms have mechanisms to regulate the quantum yield of photosynthetic reaction in response to the change in the light condition of the surroundings. The molecular mechanisms for the regulation have not been clarified yet.

・ There has been a model assuming that antennae proteins migrate within the biological membrane for regulative functions. We are developing a novel optical microscope to verify the model through the direct observation of the protein migration.

・ How the delicate structures of the photosynthetic proteins shown in Figs. 1 and 2 are formed in the plant cells? We are trying to answer this important question by using a newly developed cryogenic optical microscope.