PARENT SESSION
Posters P6A Type II reaction centres: Excited state dynamics and donor side. Abstracts (313-346)

Identifying spectral signatures from key cofactors of Photosystem II and the Cytochrome b₆f complex using low-temperature polarization spectroscopies. Sindra Peterson Årsköld^{*,1, 6}, Jörgen Ström², Rafael Picorel³, Michael Seibert³, Jian-Ren Shen⁴, Paul Smith⁵, Ronald Pace⁵, Elmars Krausz⁶, ¹ Dept. of Biochemistry, Lund, Sweden⁶ Research School of Chemistry, Canberra, ACT, Australia² Dept. of Plant Cell Biology, Lund, Sweden³ National Renewable Energy Laboratories, Golden, Co, USA⁴ RIKEN Harima Institute, Hyogo, Japan⁵ Faculties of Science, Dept. of Chemistry, Canberra, ACT, Australia

ABSTRACT- We are addressing the nature of the pigments of the intact Photosystem II (PSII) reaction center. To this end, we have developed a fully active PSII core preparation (Smith et al, Biochemistry 41 (2002) 1981-1989), showing unprecedented spectral detail. We are studying the pigments of this complex through high-precision, low-temperature absorption, circular dichroism (CD), linear dichroism (LD), and magnetic CD (MCD) spectroscopy. We apply these techniques to a range of well-defined PSII samples: Functional PSII cores from four different organisms, and isolated subfragments of the spinach PSII core (CP43, CP47, RC5, RC6, CP47-RC). From this multidimensional study we can draw several conclusions: 1) Functional PSII cores from different species show considerable spectral variance, although the features are of similar width and in the same regions in all samples. 2) In spinach, accidental degeneracy appears at 683.5 nm, where both P680 and a CP43-Chlorophyll (Chl) absorb. Both display negative CD. 3) The PheoD1 resonance can be identified in all species studied from its electrochromic reaction to reduction of the nearby quinone, QA (Peterson rsköld et al J. Am. Chem. Soc. 125 (2003) 13063-13074). 4) The spectra of the isolated spinach subunits do not add up to the spectrum of the native, assembled core. Thus, the inner antenna is needed for the reaction center to be in its naturally functional state. Isolated "RC" material is not representative of the functional PSII reaction center. 5) The P680 pigments show an intrinsic lack of MCD intensity, both in the intact reaction center and in the isolated "RC" preparations. This serves as a useful signature for this assembly (Krausz and Peterson rsköld, subm. (2004), Ed. A. Collings, Wiley). We have also used MCD spectroscopy to investigate the Cytochrome (Cyt) b6f Complex. This has given qualitative and quantitative information on the four hemes, visible in MCD in different oxidation and spin states. We have also studied the single Chl present in this complex - a very useful reference, as it is the only known single Chl in a protein environment. Spectroscopic data from this pigment can be used to analyze data from larger Chl assemblies such as the light-harvesting complexes and the photosystems.

KEY WORDS: b6f, optical spectroscopy, P680, PSII