From the analysis of the 55Mn HFI values, the oxidation state compositions of the OEC could be deduced: S0 3Mn(III) 1Mn(IV); S1 2Mn(III) 2Mn(IV); S2 1Mn(III) 3Mn(IV). Furthermore,

values for the exchange couplings were obtained and an assignment of the oxidation states to individual Mn ions in the cluster was proposed, see Fig. 6 (Kulik et al. 2007). Fig. 6 Top: Field-swept echo detected EPR spectrum at Q-band of the S2-state of the oxygen-evolving complex (OEC) in Photosystem II (BBY Angiogenesis inhibitor particles from spinach). The simulation has been obtained with four axial 55Mn HFI tensors and an anisotropic g-tensor (Kulik et al. 2005, 2007). Bottom: 55Mn ENDOR spectra both at Q-band and X-band (black) together with their simulations (red lines) using four different 55Mn hf tensors (colored lines). Note the better nuclear Zeeman resolution at Q-band. The inset in the upper panel shows the assignment of oxidation

states to the four Mn ions and the exchange coupling J among these ions Spin-polarized RP \( P_700^ \bullet + A_1^ \bullet – \) in plant Photosystem I In plant Photosystem I (PSI), the photosynthetic charge separation is triggered by the light absorption of the primary electron donor P 700. From its excited state P*, the electron is transferred through intermediate acceptors to the electron acceptor A1 (vitamin K1). As a result of the fast charge separation, the RP \( P_700^ \bullet + A_1^ \bullet – \) is created in a spin-correlated state that can be observed by EPR and ENDOR techniques. The system of two interacting electron spins has four AZD5582 purchase eigenstates, which can be ADAMTS5 described in terms of singlet and triplet states. Since spin multiplicity is conserved during fast electron transfer, the system is initially in the singlet state. In the course of spin learn more evolution also the triplet sublevels become populated. The general theory of ESE and ENDOR in polarized RPs is rather complicated (Fursmann et al. 2002; Poluektov et al. 2005). However, the situation is simplified in the weak coupling case, when the difference of the Larmor frequencies of two electron

spins Δω is much larger than the strength of the exchange and magnetic dipolar interactions between these spins. The system approaches this situation with increasing external magnetic field, since Δω increases due to the difference in g-factors of the radicals in the RP. This was utilized in pulse ENDOR studies of the laser flash generated spin-polarized RP \( P_700^ \bullet + A_1^ \bullet – \) (Fursmann et al. 2002; Epel et al. 2006). The Q-band transient EPR spectrum of this RP is shown in the top panel of Fig. 7. The numerical simulation shows that this spectrum is composed of the contributions of the signals of \( P_700^ \bullet + \) and \( A_1^ \bullet – \), each of which is spin polarized.