A temperature-dependent conformational change in photosystem-II thylakoid membrane

Honoring Bacon Ke at 100: a legend among the many luminaries and a highly collaborative scientist in photosynthesis research

Bacon Ke, who did pioneering research on the primary photochemistry of photosynthesis, was born in China on July 26, 1920, and currently, he is living in a senior home in San Francisco, California, and is a centenarian. To us, this is a very happy and unique occasion to honor him. After providing a brief account of his life, and a glimpse of his research in photosynthesis, we present here "messages" for Bacon Ke@ 100 from: Robert Alfano (USA), Charles Arntzen (USA), Sandor Demeter (Hungary), Richard A. Dilley (USA), John Golbeck (USA), Isamu Ikegami (Japan), Ting-Yun Kuang (China), Richard Malkin (USA), Hualing Mi (China), Teruo Ogawa (Japan), Yasusi Yamamoto (Japan), and Xin-Guang Zhu (China).

Determination of membrane lipid phase transition temperature from 13C-NMR intensities

A new and simple approach for the determination of the temperature of gel-to-liquid crystalline phase transitions (Tc) of biological (chloroplast) membrane lipids from 13C-NMR resonance intensities is proposed. The variation of intensity of a temperature-sensitive NMR resonance is monitored by recording the spectra of the sample at a range of temperatures. From such a series of spectra recorded at different temperatures, a temperature-insensitive resonance is located. Then the ratio of the intensity of the temperature-sensitive to the intensity of the temperature-insensitive resonance is calculated from each spectrum to even out the procedural error, if any. The values of this ratio at different temperatures, when plotted against sample temperature, shows a break at Tc as confirmed by spin label ESR studies.

Membrane-surface electric properties of triton-fractionated spinach subchloroplast fragments

Surface charge density of subchloroplast fragments fractionated from spinach by Triton X-100 treatment was estimated from cation-induced quenching of chlorophyll fluorescence, with the premise that the fluorescence yield is dependent on the surface electric potential of the preparations. Application of the Gouy-Chapman theory of diffuse double layer to the subchloroplast preparations, or treating the surface of the preparations under electric charge regulation conditions yielded a result suggesting the Photosystem II reaction-center preparation (TSF-IIa) to be more negatively charged than the Photosystem I reaction-center preparation (TSF-I). Isoelectric points of the subchloroplast fragments were determined by measuring 90 degrees light scattering and more directly by gel isoelectric focusing. Isoelectric points of TSF-I and -IIa were estimated to be 4.8 and 4.0 from light-scattering experiments, and 4.5 and 4.1 from gel electrophoresis, respectively. The TSF-II preparation that contains both a light-harvesting complex and the reaction-center (core) complex showed a small cation-induced quenching of chlorophyll fluorescence. This fluorescence quenching may be ascribed mostly to the regulation of energy transfer in the preparation (Yamamoto, Y. and Ke, B. (1980) Biochim, Biophys. Acta 592, 296-302). Furthermore, the TSF-II preparation showed a broad and indefinite peak in light scattering in the pH range 3-8, suggesting that the complex probably carries a small amount of charge in this pH range. The physiological role of the membrane surface charge of the subchloroplast preparations in membrane structure and cation regulated processes in chloroplast is discussed.

Regulation of electron transport in photosystem-II fragments by magnesium ions

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.

Regulation of excitation energy distribution in photosystem-II fragments by magnesium ions

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.

Flash-induced charge separation and dark recombination in a photosystem-II subchloroplast particle

The decay time of flash-induced absorption changes in a Photosystem-II subchloroplast fragment is very temperature sensitive down to 210 K, below which it remains constant at 1.25 +/- 0.05 ms. The difference spectrum from the near-infra-red to the ultraviolet regions indicates that the monophasic decay represents charge recombination between P-680+ and the reduced primary acceptor. The charge recombination proceeds by electron tunneling. The P-680 concentration in the TSF-IIa fragment was estimated to be one in 30 +/- 5 total chlorophyll molecules.