TRANSIENT ELECTRON-PARAMAGNETIC-RESONANCE OF THE TRIPLET-STATE OF P(700) IN PHOTOSYSTEM .1. EVIDENCE FOR TRIPLET DELOCALIZATION AT ROOM-TEMPERATURE

Spin-polarized EPR spectra of the triplet state of P700, the primary e lectron donor in photosystem I (PS I), have been measured for the firs t time at room temperature. The measurements were performed on intact PS I from Synechococcus sp. after prereduction of all iron-sulfur cent ers and on vitamin K1 depleted PS I from Synechocystis 6803. The two p reparations give similar spectra with a polarization pattern which ind icates that the triplet state is formed via recombination of a radical pair. The axial and nonaxial zero-field splitting (zfs) parameters ar e found to be Absolute value of D = (284 +/- 15) X 10(-4) cm-1 and Abs olute value of E = (22 +/- 3) X 10(-4) cm-1, respectively. The E-value is 42% smaller than in monomeric chlorophyll a, while the D-value is nearly the same. Measurements of the Synechocystis 6803 sample at 4.5 K yielded zfs parameters which are identical with those of the chlorop hyll monomer, in agreement with previous results. In order to explain this behavior, it is proposed that the triplet excitation is delocaliz ed over the two halves of a chlorophyll dimer at room temperature but appears localized on one half at low temperature. The observed zfs par ameters are obtained if (1) the magnetic z-axes of the two chlorophyll s are collinear, (2) the magnetic y-axes (and x-axes) of the two chlor ophylls make an angle of approximately 55-degrees with each other, and (3) the admixture of charge-transfer states to P-3(700) is negligible It is suggested that the orientation within the membrane of the two c hlorophylls of P700 is similar to that of the special pair in purple b acteria, 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 expl anations for the reduced E-value at room temperature are considered.