Characterization of an S1-State Specific EPR Multiline Signal of the Photosystem II Manganese Cluster

Abstract

The oxygen evolving complex (OEC) of photosystem II (PS II) is composed of a four manganese cluster (1) which cycles through 5 different redox (S-) states, So to S4 (Kok model); S1 is the dark stable state. The best-characterized EPR signals of the OEC, recorded in the standard perpendicular-polarization mode (referring to the orientation of the static magnetic (H-) field relative to the H-component of the microwave) are the g=2 ‘multiline’ signal and the featureless g = 4.1 signal of the S2 state which are believed to originate from two half-integer spin configurations of the exchange-coupled Mn4 cluster (1). Since the Kok model predicts integer-spin states for S1 and S3 (given the half-integer spin of S2), parallel-polarization mode signals are expected from these states, unless they are diamagnetic. Indeed a broad featureless S1-specific signal in this EPR mode (geff = 4.8) was reported from plant PSII (2). We recently reported a S1-specific parallel-mode multiline signal (geff = 12) from both cyanobacterial PSII (3) and plant PSII depleted of the 17 and 23 kDa proteins (4) which shield the OEC at the lumenal peripherie of the plant PSII. On this congress we present further characterization of this signal such as temperature dependence, correlation with the S2 signals and modulation by ligand binding.