PARENT SESSION
Posters P5A Type II reaction centres : Structure. Abstracts (289-312)

The native reaction centre of PSII in oxygen-evolving preparations. Elmars Krausz^*,1, Sindra Peterson Arskold^{*,1, 2}, Joseph Hughes¹, Paul Smith³, Ron Pace³, ¹ Research School of Chemistry, Canberra, ACT, Australia² Department of Biochemistry, Lund, Sweden³ Faculties Chemistry, Canberra, ACT, Australia

ABSTRACT- Electronic assignment of the reaction centre of PSII remains the subject of considerable interest and debate. The charge-separating assembly has been variously considered by workers to involve a special pair analogous to the bacterial reaction centre, a more delocalised excitation involving a number of reaction-centre pigments, or even just a single chl a. We have performed a range of low-temperature optical studies to elucidate the native reaction centre, present in active, oxygen-evolving preparations and compared these to results from inactive, reaction centre only PSII preparations. Intact PSII shows superior spectral detail in absorption, CD, MCD and electroabsorption spectroscopies. This helps overcome the spectral congestion arising from light-harvesting assemblies present in intact systems. From MCD and CD data, we assign a prominent feature at 683.5 nm in spinach core complexes as due to the reaction centre and only partly due to CP43. Low-temperature illumination of active PSII leads to stable QA- formation. Subsequent electrochromic shifts locate the Qy transition of D1 pheoa to be at 685 nm. This pheoa does not appear to be to be strongly coupled to other pigments. Surprisingly efficient QA- formation, along with spectral hole-burning, occurs with excitation wavelengths as low as 695 nm. Hole-burning linewidths show excitation transfer to the charge-separating state from trap states in CP47 and CP43 (40-400 ps). This is not consistent with the shallow trap model. The presence of a weak, relatively broad, low-energy charge-separating state does imply at least one significant coupling amongst the chlas in the reaction centre but is inconsistent with the special pair paradigm. Here the interacting transition dipoles are strongly anti-parallel. The observed phenomenology in active PSII indicates a parallel dipole interaction, leading to a weak low-energy excitation.

KEY WORDS: Charge Separation, Low Temperature Spectroscopy, Photosystem II, Electronic Structure