Intermediates of the 3 state of the oxygen evolving complex of photosystem II. (1) Decay products

Abstract

Near-IR light excitation of the S3 state produces, in addition to other signals that appear to be associated with excited state configurations of S3, a derivative-shaped EPR signal at g ~ 5.1,2 This signal bears unexpected similarities to the one observed earlier by Nugent et al.3, in samples that had undergone multiple turnovers above S1 and subsequently stored at 77 K for a week or longer. We have confirmed the observations of Nugent et al.3 and furthermore shown that, the g ~ 5 signal evolves in apparent correlation with the decay of the S3 state. The g ~ 5 signal that evolves slowly at 77 K is super-imposable and shares similar EPR properties with the one induced by NIR excitation of S3. At temperatures > -80 0C the S2 multiline and g = 4.1 signals grow in parallel during the decay of the g ~ 5 or the freshly prepared S3. It is suggested that, the broad g ~ 2 radical that forms immediately after excitation of the S3 state by NIR light2 arises from centers where the positive hole was transferred from the Mn cluster to YZ. These centers rapidly recombine to yield an excited S2 state configuration identical to the one that is produced during the slow decay of S3 at 77 K. Theoretical simulations of the g ~ 5 signal will be presented.