Department of Chemistry, Faculty of Science
Australian National University
EPR spectroscopy has been used to measure the relaxation time of signal IIvf in the thylakoids.
Kok and co-workers proposed a model in which the five successive oxidation states (S0 to S4) are involved in the oxygen evolving complex. Each of the Si -> Si+1 transitions is driven by a photochemical turnover.
A tyrosine residue (Yz+) which gives rise to signal IIvf (very fast) acts as an electron donor to P680+. The rates of the Yz+ re-reduction by the water oxidising Mn cluster are S-state dependent. Single saturating flashes have been used to turnover the states. If each flash produced 100% single S state turnover, the measured signal would be corresponded to one Si -> Si+1 transition only. However, the misses due to the absence of the charge separation in some centers and double hits due to the flash duration cause a mixed population of the S states. So, each signal corresponds to a mixture of S states depending on the magnitude of the miss ([[alpha]]) and double hit ([[beta]]) probabilities. The oxygen releasing event occurs on the S3 to S0 transition through the transient S4 state. An oxygen sensitive spin label has been used to measure the flash induced oxygen release pattern, which allows determination of the [[alpha]] and [[beta]] parameters.
S0 and S1 are the two dark-stable states and the S2 and S3 states decay to S1 in the dark within minutes. Using the [[alpha]] and [[beta]] parameters, the EPR signals have been deconvoluted and the re-reduction decay times of YZ+ in all the S states have been obtained.