Photosynthetic water splitting is catalyzed by a Mn4CaO5 cluster in photosystem II, whose structure was recently determined at a resolution of 1.9 angstrom [Umena, Y. et al. 2011, Nature, 473:55-60]. To determine the electronic structure of the Mn4CaO5 cluster, pulsed electron-electron double resonance (PELDOR) measurements were performed for the tyrosine residue Y-D(center dot) and S-2 state signals with non-oriented and oriented photosystem II (PS H) samples. Based on these measurements, the spin density distributions were calculated by comparing with the experimental results. The best fitting parameters were obtained with a model in which Mn1 has a large positive projection, Mn3 has a small positive projection, and Mn2 and Mn4 have negative projections (the numbering of Mni (i=1-4) is based on the crystal structure at a 1.9 angstrom resolution), which yielded spin projections of 1.97, -120, 1.19 and -0.96 for Mn1-4 ions. The results show that the Mn1 ion, which is coordinated by H332, 0342 and E189, has a valence of Mn(III) in the S-2 state. The sign of the exchange interactions J(13) is positive, and the other signs are negative. 2(C) 013 Els,evier B.V. All rights reserved.