Reaction kinetics for positive charge accumulation on the water side of choloplast photosystem II

Recent experiments in our laboratory [l-3] have identified an e.p.r. component, Signal llvf, as arising from the oxidized donor to P680, Z’. Following a saturating flash the formation of Signal llvf, corresponding to the oxidation of Z, occurs within 100 psec. The decay, which is attributed to the rereduction of Z’ by the S states [4], occurs with a halftime of approx. 700 ps. However, these parameters were determined under steady state conditions, and therefore this 700 psec decay for Signal llvf reflects the average halftime for the reaction Z’ + S, + Z + Sri+++ where n = 0, 1, 2, 3, and So = S, = S2 = S3 prior to each flash. The paired-flash oxygen evolution experiments of Joliot et al. [S] , Kok et al. [4], Bouges-Bocquet ‘,6] and Diner [7] have measured the overall relaxation time for Photosystem Il. The results of these experiments, coupled with the fluorescence data of Zankel [8] and of Joliot et al. [9], show that the rate limiting step for the recovery of photoactivity in Photosystem II units is a function of the oxidation state of the S enzyme. Photosystem II units in states So, S1 , or Sz prior to a flash regain photoactivity as Qis reoxidized, indicating that reactions occurring on the reducing side of Photosystem II are ratelimiting. However, for Photosystem II units in S3 prior to a flash, the rate limitation shifts to the