PARENT SESSION

Symposium S6A Type II reaction centres: Excited state dynamics and donor side
Thursday September 2nd, 2004 10:20 AM-12:20 PM Room 511D
Chair: Bill Rutherford
Co-Chair: Marilyn Gunner

The lowest-energy excited-state of P680 in oxygen-evolving Photosystem II extends to 700 nm. Joseph Hughes^*,1, Elmars Krausz¹, Paul Smith², Ron Pace², Hans Riesen³, ¹ Research School of Chemistry, ANU, Canberra, ACT, Australia² Department of Chemistry, Faculty of Science, ANU, Canberra, ACT, Australia³ School of Physical, Environmental and Mathematical Sciences, Canberra, ACT, Australia

ABSTRACT- Photosystem II (PSII) undergoes efficient charge separation upon photo-excitation, rapidly forming P680⁺ and Pheo_D1^-. The charge separating entity is an excited state of P680. The nature of P680, whether it be a monomeric, dimeric, or multimeric system of reaction centre chlorophyll a molecules has been a subject of intense debate for decades. In active oxygen-evolving PSII preparations, Pheo_D1^- formation is followed by reduction of the secondary acceptor Q_A. The amount of Q_A^- formation can be established via characteristic Pheo_D1 electrochromic shifts in either the Q_y or Q_x region [1]. We have studied the wavelength dependence of Q_A^- formation at 1.7 K, and hence primary charge separation, via low-fluence (1-100 mJ/cm²) selective illumination (2.3 nm bandwidth) in the range 680-700 nm. Since uphill energy transfer is forbidden at 1.7 K, this provides an action spectrum which establishes that the lowest-energy excited-state of P680 in oxygen-evolving PSII cores extends to 700 nm. The absorption in this region is dominated by CP47, indicating the underlying P680 absorption is very weak. We also present complementary data that shows efficient (up to 1%) persistent spectral hole-burning 1.7 K in oxygen-evolving PSII preparations extends to 695 nm. Hole-burning activity near 684 nm and 690 nm is attributed to slow excitation transfer to the reaction centre from a CP43 chlorophyll (40-210 ps) and CP47 chlorophyll (600 ps), respectively. These results provide new insights into the function of PSII and the nature of P680. In particular, the weakly absorbing lowest-energy state presents a paradigm shift for P680, with respect to its analogy with the bacterial reaction centre primary donor. [1] S. Peterson Arsköld, V. M. Masters, B. J. Prince, P. J. Smith, R. J. Pace, E. Krausz, J. Am. Chem. Soc. 125 (2003) 13063.

KEY WORDS: p680, charge-separation, photosystem ii, hole-burning