Pheophytin

Pheophytin or phaeophytin (abbreviated Pheo) is a chemical compound that serves as the first electron carrier intermediate in the electron transfer pathway of Photosystem II (PS II) in plants, and the photosynthetic reaction center (RC P870) found in purple bacteria. In both PS II and RC P870, light drives electrons from the reaction center through pheophytin, which then passes the electrons to a quinone (Q_A) in RC P870 and RC P680. The overall mechanisms, roles, and purposes of the pheophytin molecules in the two transport chains are analogous to each other.

In biochemical terms, pheophytin is a chlorophyll molecule lacking a central Mg²⁺ ion. It can be produced from chlorophyll by treatment with a weak acid, producing a dark bluish waxy pigment. The probable etymology comes from this description, with pheo meaning dusky and phyt meaning vegetation.

In the 1977s, scientists Klevanik, Klimov, Shuvalov performed a series of experiments to demonstrate that it is pheophytin and not plastoquinone that serves as the primary electron acceptor in photosystem II. Using several experiments, including electron paramagnetic resonance (EPR), they were able to show that pheophytin was reducible and, therefore, the primary electron acceptor between P680 and plastoquinone (Klimov, Allakhverdiev, Klevanik, Shuvalov). This discovery was met with fierce opposition, since many believed pheophytin to be a byproduct of chlorophyll degradation. Therefore, more experiments ensued to prove that pheophytin is indeed the primary electron acceptor of PSII, occurring between P680 and plastoquinone (Klimov, Allakhverdiev, Shuvalov). The data that was obtained is as follows:

These observations are all characteristic of photo-conversions of reaction center components.

Pheophytin is the first electron carrier intermediate in the photoreaction center (RC P870) of purple bacteria. Its involvement in this system can be broken down into 5 basic steps. The first step is excitation of the bacteriochlorophylls (Chl)₂ or the special pair of chlorophylls. This can be seen in the following reaction.

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