Intermediates of the S3 to S2 transition in the Oxygen Evolving Complex of Photosystem II.

Abstract

We extend a recent EPR study of the S3 state1 and examine the transients formed following near-IR (NIR) light excitation at 4.2K. A series of signals evolve at the expense of the low-field signals at g ~ 10 (// mode) and g ~ 17 (- mode) of the untreated S3 state. A broad radical signal at g ~ 2 is followed at 4.2 K by the evolution of a signal at 2.9 and at 77 K by a signal at g = 5. At ¿80 0C a g = 21/3.7 signal with a // mode component at g ~ 23 evolves. The overall phenomenology indicates that, following the NIR excitation some of the centers decay to the initial S3 state via the g = 21/3.7 intermediate, while others recombine to form an S2' state (g = 5 signal).2 Of interest is the comparison of the light sensitivity of the S2' and S3 states. They both give very similar broad radical signals at g ~ 2 but the signal in S3 is assigned to the transfer of the positive hole from the Mn cluster to YZ,2 while the g ~ 2 signal in S2' is assigned to charge separation.3 These observations, offer new insights into the electronic configuration of the OEC in its higher oxidation states. 1. Ioannidis, N. and Petrouleas, V. (2000) Biochemistry 39, 5246-5254. 2. Petrouleas, V., Ioannidis, N., Sanakis, Y., Sioros, G. Abstract S10. 3. Nugent, J.H.A., Turconi, S., and Evans, M.C.W. (1997) Biochemistry 36, 7086-7096.