| Summary: | 碩士 === 東吳大學 === 化學系 === 93 === Ammonia is an inhibitor of water oxidation and a structural analog for substrate water, making it a valuable probe for the structural properties of the possible substrate-binding site on the oxygen-evolving complex (OEC) in photosystem II (PSII). By using the NH3-induced upshift of the 1365 cm-1 IR mode in the S2QA-/S1QA spectrum and the NH3-modified S2 state EPR signals of PSII as spectral probes, we found that ethylene glycol has clear effects on the binding properties of the NH3 specific site on the OEC. Our results show that in PSII samples containing 30% (v/v) ethylene glycol, the affinity of the NH3-specific binding site on the OEC is estimated more than ten times lower than that in PSII samples containing 0.4 M sucrose. In addition, our results show that the NH3-induced upshift of the 1365 cm-1 IR mode in the S2QA-/S1QA spectrum is dependent on the concentration of ethylene glycol, but not dependent on the concentration of sucrose (up to 1.5 M) or methanol (up to 5.4 M). By comparing the concentration dependence of sucrose and ethylene glycol on NH3-induced spectral change and also by comparing the sucrose and ethylene glycol data at similar concentrations (~1 M), we conclude that ethylene glycol has a clear effect on the NH3-induced spectral changes. Furthermore, our results also show that ethylene glycol alters the steric requirement of the amine effect on the upshift of the 1365 cm-1 mode in the S2QA-/S1QA spectrum.
In PSII samples containing 30% ethylene glycol, only NH3, not other bulkier amines (e.g., Tris, AEPD and CH3NH2), has a clear effect on the upshift of the 1365 cm-1 mode in the S2QA-/S1QA spectrum; in contrast, in PSII samples containing 0.4 M sucrose, both NH3 and CH3NH2 have a clear effect. On the basis of the above results, we proposed that ethylene glycol might act directly or indirectly to decrease the affinity or limit the accessibility of NH3 and CH3NH2 to the NH3-specific binding site on the OEC in PSII. Finally, we also applied the same approach to test whether or not methanol is able to compete with ammonia on its binding site on the OEC. We found that 4% methanol does not show any significant effect on NH3-induced up-shift of the 1365 cm-1 mode in the S2QA-/S1QA spectrum and the NH3-modified S2 state g=2 multiline EPR signal. Our results suggest that methanol is unable to compete with NH3 on binding to the Mn site of the OEC that gives rise to the altered S2 state g=2 multiline EPR signal.
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