Transient electron paramagnetic resonance of the triplet state of P700 in photosystem. I: Evidence for triplet delocalization at room temperature

Spin-polarized EPR spectra of the triplet state of P 700 , 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 K 1 depleted PSI from Synechocystis 6803. The two preparations give similar spectra with a polarization pattern which indicates that the triplet state is forms via recombination of a radical pair. The axial and nonaxial zero-field splitting (zfs) parameters are found to be |D|=(284±15)×10 -4 cm -1 and |E|=(22±3)×10 -4 cm -1 , respectively