Biochemistry 1993, 32,4842-4847
4842
Transient Electron Paramagnetic Resonance of the Triplet State of P700 in Photosystem I: Evidence for Triplet Delocalization at Room Temperature+ Ina Sieckmann,* Klaus Brettel,*-~J Christian Bock,* Arthur van der Est,* and Dietmar Stehlik’J Fachbereich Physik, Freie Universitiit Berlin, Arnimallee 14, 1000 Berlin 33, Germany, and Max- Volmer-Institut, Technische Universitiit Berlin, Strasse des 17, Juni 135, 1000 Berlin 12, Germany Received December IO, 1992; Revised Manuscript Received February 16, 1993
ABSTRACT:
Spin-polarized EPR spectra of the triplet state of P700, the primary electron donor in photosystem
I (PS I), have been measured for the first time at room temperature. The measurements were performed on intact PS I from Synechococcus sp. after prereduction of all iron-sulfur centers and on vitamin Kl depleted PS I from Synechocystis 6803. The two preparations give similar spectra with a polarization pattern which indicates that the triplet state is formed via recombination of a radical pair. The axial and nonaxial zero-field splitting (zfs) parameters are found to be ID1 = (284 f 15) X 10-4 cm-I and [El = (22 f 3) X 10-4 cm-l, respectively. The E-value is 42% smaller than in monomeric chlorophyll a, while the D-value is nearly the same. Measurements of the Synechacystis 6803 sample at 4.5 K yielded zfs parameters which are identical with those of the chlorophyll monomer, in agreement with previous results. In order to explain this behavior, it is proposed that the triplet excitation is delocalized over the two halves of a chlorophyll dimer at room temperature but appears localized on one half at low temperature. The observed zfs parameters are obtained if (1) the magnetic z-axes of the two chlorophylls are collinear, (2)the magnetic y-axes (and x-axes) of the two chlorophylls make an angle of approximately 55’ with each other, and (3) the admixture of charge-transfer states to 3 P 7is~negligible. ~ It is suggested that the orientation within the membrane of the two chlorophylls of P7w is similar to that of the special pair in purple bacteria, but that the electronic coupling between the two chlorophylls is weaker in P700. Possible origins of the temperature dependence of the triplet delocalization in P700 are discussed, and alternative explanations for the reduced E-value at room temperature are considered.
The primary electron donor of photosystem I, P700,l is made up of chlorophyll a, and substantial evidence is available that two excitonically coupled molecules form a ’special pair” in analogy to the primary electron donor of purple bacteria [for a recent review, see SCtif (1992)]. However, low-temperature EPR studies of the triplet state of P700 yielded zero-field splitting parameters that agree well with those of monomeric Chl u (Frank et al., 1979;Rutherford & Mullet, 1981). This is in contrast to purple bacteria, where both zfs parameters, D and E, of the triplet state of the special pair are considerably smaller than those of the corresponding monomeric BChl’s [e.g., Thurnauer (1979) and Levanon and Norris (1982)l. The axial and nonaxial zfs parameters, D and E, are related to the spatial distribution of the two unpaired electron spins by the following relationships:
This work was supported by grants from the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 3 12, Teilprojekt A1 and A5. * To whom correspondence should be addressed. f Fachbereich Physik. 8 Max-Volmer-Institut. 11 Present address: Section de Bidnergdtique, Ddpartement de Biologie Cellulaire et Molbulaire, CE Saclay, URA CNRS 1290, 91 191 Gifsur-Yvette, Cedex, France.
where g@ is the gyromagnetic ratio of the spins, and r12 = ( x I z , Y ~ ~ , is z Ithe ~ ) vector joining them. The angular brackets imply an average over the electronic wave function. For planar aromatic molecules such as the chlorophylls, the z-axis is normal to the molecular plane so that ( zi2>
