title: Regulation of excitation energy distribution in photosystem-II fragments by magnesium ions.
abstract: Fluorescence characteristics of Photosystem-II subchloroplasts (TSF-II and TSF-IIa) fractionated by Triton X-100 treatment were studied in relation to cation-induced regulation of excitation-energy distribution within subchloroplast fragments. Absorption spectra and fluorescence-emission spectra at 77 K showed that TSF-II contains the light-harvesting chlorophyll-protein complex in addition to the reaction-center complex, which is present alone in TSF-IIa. Mg2+ increased the ratio of F695nm to F685nm in the fluorescence-emission spectrum of TSF-II particles at 77 K, but had no effect on TSF-IIa particles. Mg2+ also induced a quenching of chlorophyll fluorescence at room temperature in TSF-II, an effect that was insensitive to the presence of DCMU. The DCMU-insensitive fluorescence quenching was not observed in the TSF-IIa preparation. These results suggest an existence of cation-induced regulation of excitation-energy transfer in TSF-II preparations. Presence of antenna chlorophyll molecules alone does not seem to be sufficient for observing energy-transfer regulation by cations in Photosystem-II preparations.
mesh_terms: D002734:Chlorophyll / Q000378:metabolism; D002736:Chloroplasts / Q000187:drug effects* / Q000648:ultrastructure; D004734:Energy Metabolism; D005453:Fluorescence; D008274:Magnesium / Q000494:pharmacology*; D010788:Photosynthesis / Q000187:drug effects
chemical_list: D002734:Chlorophyll; D008274:Magnesium