The multiline EPR signals arising from manganese in the S2 state of the oxygen-evolving system of spinach and the cyanobacterium Anacystis nidulans have very similar properties and are affected identically by NH3, suggesting that the system is highly conserved. The temperature dependence of the signal amplitude follows Curie behavior down to sub-helium temperatures. This is in contrast to previous reports, which were taken as evidence for a tetrameric manganese cluster. Thus, it seems that it is not yet possible from EPR data alone to distinguish between this model and a dimeric structure.
