title: Regulation of electron transport in photosystem-II fragments by magnesium ions.
abstract: In Photosystem-II reaction-center particles (TSF-IIa) fractionated from spinach chloroplasts by Triton X-100 treatment, divalent cations appear to regulate electron-transport reactions. Oxidation of cytochrome b-559 after illumination of the particles was accelerated by the presence of Mg2+, whereas photoreduction of 2,6-dichlorophenolindophenol (DCIP) by diphenyl carbazide was inhibited, both at a half-effective concentration of Mg2+ of approx. 0.1 mM. The site of regulation was shown to be on the oxidizing side of Photosystem II, near P-680, based on the effects of actinin-light intensity and nature of the electron donors on DCIP photoreduction. Mg2+ was effective in quenching chlorophyll fluorescence in TSF-IIa particles, but the quenching was sensitive to the presence of 3(3,4-dichlorophenyl)-1,1-dimethylurea. In the reaction-center (core) complex of Photosystem II, where the light-harvesting chlorophyll-protein complex is absent, there seems to be no regulation by Mg2+ on excitation-energy distribution.
mesh_terms: D015086:2,6-Dichloroindophenol / Q000037:antagonists & inhibitors; D002734:Chlorophyll / Q000378:metabolism; D002736:Chloroplasts / Q000378:metabolism; D003573:Cytochrome b Group; D003580:Cytochromes; D004160:Diphenylcarbazide / Q000494:pharmacology; D004579:Electron Transport / Q000187:drug effects*; D005453:Fluorescence; D008274:Magnesium / Q000494:pharmacology*; D010084:Oxidation-Reduction; D010788:Photosynthesis / Q000187:drug effects*; D045332:Photosystem II Protein Complex; D014466:Ultraviolet Rays
chemical_list: D003573:Cytochrome b Group; D003580:Cytochromes; D045332:Photosystem II Protein Complex; D002734:Chlorophyll; D004160:Diphenylcarbazide; C019313:cytochrome b559; D015086:2,6-Dichloroindophenol; D008274:Magnesium