PARENT SESSION
Posters P8A Cytochrome b-c complexes. Abstracts (382-393)

Unusual optical properties of the monomeric chlorophyll a in the cytochrome b₆ f complex of oxygenic photosynthesis. William Cramer¹, Naranbaatar Dashdorj^*,2, Huamin Zhang¹, Hanyoup Kim², Jiusheng Yan¹, Sergei Savikhin², ¹ Department of Biological Sciences, West Lafayette, IN, USA² Department of Physics, West Lafayette, IN, USA

ABSTRACT- The cytochrome b₆f(Cytb₆f) complex mediates electron transfer between the reaction centers of photosystem I and photosystem II, and facilitates proton translocation across the membrane that powers the Q-cycle mechanism in the oxygenic photosynthesis. High-resolution X-ray crystallographic structures (Kurisu et al., Science, 302, 1009-1014, 2003; Stroebel et al., Nature, 426, 413-418, 2003) of the Cyt b₆f complex unambiguously confirmed earlier findings (Huang et al., Biochemistry, 33, 4401-4409, 1994) of a chlorophyll a (Chl a) molecule as an intrinsic component of the Cyt b₆f complex. In this work, we studied the monomeric Chl a molecule in the highly active dimeric Cyt b₆f complexes by means of ultrafast time-resolved pump-probe spectroscopy. The purity of the samples was ensured by using X-ray ready crystals diluted in a buffer and was found to be crucial for obtaining reliable results. The experimental data revealed unusually short excited state decay lifetime (∼200 ps), which is a factor of ∼25 shorter than natural fluorescence lifetime reported for monomeric Chl a molecules in organic solution. A structure-based mechanism of fluorescence quenching is discussed as well as its implication to the stability of the complex.

KEY WORDS: Chlorophyll a, Photosynthesis, Cytochrome b6f, Flourescence lifetime