PARENT SESSION
Posters P2A Type I reaction centres. Abstracts (181-218)

The energy gap between P₇₀₀⁺-A₀^- and P₇₀₀^* in Photosystem I determined by delayed fluorescence at 270-77 K. Yutaka Shibata^*,1, Takashi Kasahara¹, Sinpei Akai¹, Shigeru Itoh¹, Isamu Ikegami², ¹ School of Material Science (Physics), Nagoya, Japan² Teikyo University, School of Pharmaceutical Sciences, Tsukui-gun, Kanagawa, Japan

ABSTRACT- The treatment of freeze dried photosystem I (PSI) with water-containing ether leads to extraction of most of the antenna chlorophylls (Chl's) leaving most of the electron transfer component except phylloquinone, the secondary acceptor A₁ (ether-PSI). In the ether-PSI, a fluorescence component with a long lifetime of about 40 ns is observed around 700 nm at 77 K. The delayed fluorescence is emitted from the excited state of the special pair, P700*, formed by the enhanced charge recombination between P700⁺ and A₀^- due to the lack of A₁. The delayed fluorescence seems to originate via the thermal activation against the energy gap between P700* and P700⁺-A₀^- state. We estimated this energy gap by analyzing the temperature dependence of the intensity of delayed-fluorescence. The time-resolved fluorescence spectra were measured with a streak scope (Hamamatsu) at 77-270 K. The spectra were fitted to several skewed Gaussian bands. The band around 700 nm that showed very long decay time of about 40 ns was assigned to the delayed fluorescence. The relative intensity of this component showed a very weak temperature dependence, suggesting a very low enthalpy change between P700* and P700⁺-A₀^-. The energy gap was estimated to be about 180 meV at 270 K and it decreased at lower temperatures. Simple thermodynamic analysis demonstrated that the entropic factor dominates the energy gap as estimated in the experiments for photosystem II, purple bacterial reaction centers. Another interpretation of the results in PSI is that the energy gap is not dominated by the entropy factor, and the very weak temperature dependence of the intensity of the delayed fluorescence represents the inhomogeneity of the enthalpy change.

KEY WORDS: Ether Treatment, Delayed Fluorescence, Energy Gap