The Mechanism Of Photoinhibition Of Isolated Photosystem Ⅱ In Water Splitting Reaction

Photosystem Ⅱ(PSⅡ) is a pigment-protein complex of thylakoid membrance of higher plant, algae and cyanobacterias where light energy is used for oxidation of water with the formation of molecular oxygen, protons and electrons. PSⅡ is the most sensitive photosynthetic machinery in higher plant, which can catalyze water oxidation reaction driven by solar energy, but the excess energy will cause the damage of PSⅡ structure, resulting in the decreasing of photosynthetic efficiency. Recent studies have shown that PSⅡ can be used as important resources for the realization of the solar photocatalytic water splitting into hydrogen. Therefore, it is of great significance to analyze the mechanism of photoinhibition of PSⅡ and enhance the PSⅡ photosynthetic efficiency under strong light conditions.We prepared PSⅡ membrane fragments from spinach and measured the oxygen evolution activity with Clark-type electrode. Then we modified the PSⅡ sample, obtained the PSⅡ samples with Mn-cluster depleted and examined its electron transfer activity with UV-visible spectrophotometer. By studing the PSⅡ membrane fragments and the Mn cluster depleted PSⅡ sample and analyzing the experiments results, we explain the mechanism of photoinhibition of PSⅡ:(1) We compared the activity of the intact PSⅡ with the PSⅡ with Mn-cluster depleted and explained the two-step mechanism of photoinhibition of PSⅡ: Light first damages the the institution of manganese cluster resulting the inactivity of the oxygen-evolving complex, then P680+· is accumulated in the photochemical reaction center, causing the damage to the photochemical reaction center.(2) We analyzed the activity of the PSⅡ with Mn-cluster depleted base on electron donor and explained the electron donor mechanism of photoinhibition of PSⅡ: Chlorophyll P680 occurs charge separation by optical excitation, but P680+· can not get electrons from the electron donor side, the existence of time of P680+· is extended. P680+· has strong oxidation, and will destroy the D1 protein and pigment molecules of photochemical reaction center, causing the electron transfer activity decreased.(3) We analyzed the activity of the PSⅡ with Mn cluster depleted base on oxygen and explained the electron accept side mechanism of photoinhibition of PSⅡ: Chlorophyll P680 absorbs excess energy, and exacerbates the charge separation, resulting in the excessive accumulation of charged groups(P680+·, Pheo-·, QA-·). This not only causes electron transport congestion in the acceptor side, but also causes 3[P680+· Pheo-·] charge recombination formatting the triplet excited state of P680(3P680). Then, the triplet excited P680 reacts with oxygen to generate singlet oxyge. However, the singlet oxygen can lead to the degradation of the D1 protein of the photochemical reaction center, causing electron transfer activity decreased.Based on the mechanism of photoinhibition of PSⅡ, we have also proposed a method of photoprotection of PSⅡ. The corresponding experimental results have shown that this is an effective way to realization of the photoprotection of PSⅡ...