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

Spectral properties of the PS I reaction center from T. elongatus: structure-based modelling of optical spectra. Eberhard Schlodder^*,1, Enrica Bordignon², Thomas Renger³, ¹ Max-Volmer-Laboratorium für Biophysikalische Chemie, Berlin, Germany² Universität Osnabrück, Osnabrück, Germany³ Institut für Chemie (Kristallographie), Berlin, Germany

ABSTRACT- Transient optical spectroscopy has been applied to study the spectral properties of the PS I reaction center from the thermophilic cyanobacterium Thermosynechococcus elongatus. Information on the optical properties have been obtained by monitoring the (³P700 - P700) and (P700⁺A₁^- - P700A₁) absorbance and linear dichroism difference spectra with high spectral resolution at 5 K. Remarkably, the large homogeneous broadening of about 350 cm^-1 and the position of the main bleaching band at about 703 nm are only slightly dependent on the temperature. Evidence is presented that the difference spectra induced by oxidation or triplet formation of P700 cannot be described if only the heterodimer of Chl a and Chl a ′constituting the special pair is taken into account. On the basis of the 2.5 X-ray crystal structure of Photosystem I [1], the 5 K difference spectra were calculated applying a theory for linear absorption of aggregates with strongly coupled pigments [2]. From the calculations, we obtained the site energies of the chlorophyll molecules in the reaction center of PS I. The very different shapes of the experimental (³P700-P700) and (P700⁺A₁^--P700A₁) spectra are best described by the assumption that the triplet and the cation are not localized on the same pigment of the special pair. [1] Jordan, P., Fromme, P., Witt, H. T., Klukas, O., Saenger, W., and Krauss, N. (2001) Nature 411, 909-917 [2] Renger, T., and Marcus, R.A. (2002) J. Chem. Phys. A 116, 9997-10019

KEY WORDS: P700, Photosystem I, optical spectroscopy, excitonic interaction