Relationships between the Transition of the Physical Phase of Membrane Lipids and Photosynthetic Parameters in Anacystis nidulans and Lettuce and Spinach Chloroplasts.
The transition of the physical phase of lipids in membrane fragments of a blue-green alga Anacystis nidulans was studied by a spin labeling technique. The maximum hyperfine splitting of the electron spin resonance spectrum of the N-oxyl-4', 4'-dimethyloxazolidine derivative of 5-ketostearic acid plotted against the reciprocal of the absolute temperature gave a discontinuity point that was characteristic of a transition of the physical phase of the hydrocarbon region of membrane lipids. The phase transition appeared at approximately 13 or 24 C in the organisms grown at 28 or 38 C, respectively.The temperature dependence curve of chlorophyll a fluorescence in intact cells, membrane fragments, and extracted lipids of Anacystis cells suspended in a buffer solution showed that the fluorescence yield became maximum near the phase transition temperatures. These findings suggest that chlorophyll a in the thylakoid membrane works as a native fluorescence probe for the detection of phase transition.The temperature dependence of photosynthetic electron transport reactions was studied by measuring the oxidoreductive reactions of P700 and by measuring O(2) evolution. Each of the Arrhenius plots of the reaction rates was composed of two straight lines with a break near the phase transition temperatures. The activation energy was always lower above than below the transition temperatures. It is proposed to explain these phenomena that a reaction involving plastoquinone is influenced by the physical state of membrane lipids.The shift between the pigment state 1 and state 2 measured by fluorescence transients also showed a characteristic break in the Arrhenius plots near the phase transition temperatures; below the transition temperatures the shift almost disappeared. This suggests that the configurational change of the thylakoid membrane related to the state 1 and state 2 shift is dependent on the physical state of membrane lipids. In the chloroplasts of lettuce and spinach, on the other hand, there was no break in the Arrhenius plot of the electron transport reactions or of Mg(2+)-induced changes of chlorophyll a fluorescence.It is suggested that the transitions of the hyperfine splitting of the ESR signal, electron transport, and the configurational change, as well as the appearance of the maximum of chlorophyll a fluorescence, in the thylakoid membranes of Anacystis nidulans are all related to the transition of the physical phase of membrane lipids between the liquid crystalline state and the mixed liquid crystal-solid state.
Murata
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Faculty of Science, Department of Biophysics and Biochemistry, University of Tokyo, Hongo-Tokyo, Japan.