| Photosynthesis in oxygen-evolving organisms converts light energy into chemical energy in the forms of sugars, using carbon dioxide, water and some minerals as ingredients. Photosystem II(PSII), a cofactor-protein complex in the thylakoid membrane of higher plants, green algae, and cyanobacteria contains the oxygen evolving complex (OEC) that is responsible for the presence of the oxygen in the Earth's atmosphere. This complex consists of 4 Mn, a redox active tyrosine (YZ), a Ca 2+, a Cl- and an unknown number of HCO-3 . Despite the four recent X-ray crystal structures of PSII from cyanobacteria and many models of water oxidation mechanism, the physical and electronic structure of the Mn4Ca complex is not clear yet. It is widely accepted now that when the water oxidation in PSII occurs, the OEC cycles through five S states (arranged in a so-called Kok circle), with the dark stable state as S1. After the OEC reaches the S4 state, it spontaneously goes back to the S0 state with the concurrent release of dioxygen. In this dissertation, azide, an oxygen evolution inhibitor, was shown to occupy the chloride binding site near the manganese cluster in PSII. Exploring mercury chloride effects on the extrinsic proteins in PSII of spinach, we demonstrated that the 33 kDa protein can be removed without affecting the association of the 23 kDa and the 17 kDa proteins. Ammonia in its free base form, as a water-substrate analogue, has been shown to bind to the manganese cluster in the S2 state in PSII and thus modify the normal multiline signal to another form of multiline signal with a narrower hyperfine splitting. Using a rapid freeze quench apparatus coupled with a Nd-YAG laser, we characterized the kinetics of ammonia binding to the manganese cluster in PSII of spinach, and measured the upper limit of the ammonia-ligand exchange time to the manganese cluster to be around 25 milliseconds. We also explored the binding property of methylamine, and water to the manganese cluster in PSII. |
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