55Mn ENDOR of the S2-state multiline EPR signal of photosystem II:: Implications on the structure of the tetranuclear Mn cluster

We have performed continuous-wave electron paramagnetic resonance (CW-EPR) and electron spin echo electron nuclear double resonance (ESE-ENDOR) experiments on the multiline form of the St-state of untreated, MeOH-treated, and ammonia-treated spinach photosystem LI (PS LI) centers. Through simultaneously constrained simulations of the CW-EPR and ESE-ENDOR data, we conclude that four effective Mn-55 hyperfine tensors (A(X), A(Y), A(Z)) are required to properly simulate the experimental data [untreated and MeOH-treated PS II centers (MHz): -232, -232, -270; 200, 200, 250; -311, -311, -270; 180, 180, 240; ammonia-treated PS II centers (MHz): 208, 208, 158; -150, -150, -112; 222, 222, 172; -295, -315, -390]. We further show that these effective hyperfine tensors are best supported by a trimer/monomer arrangement of three Mn(IV) ions and one Mn(III) ion. In this topology, Mn-A, Mn-B, and Mn-C form a strongly exchange coupled core (JAB and J(BC) < -100 cm(-1)) while Mn-D is weakly exchange coupled (J(CD)) to one end of the trinuclear core. For untreated and MeOH-treated PS LI centers, the Mn(III) ion is either MnA or Mnc, with a zero-field-splitting of D = -1.25 to -2.25 cm(-1). For ammonia-treated PS II centers, the Mn(III) ion is Mno, with a zero-field-splitting of D = +0.75 to +1.75 cm(-1). The binding of the ammonia ligand results in a shift of the Mn(III) ion from the trinuclear con to the monomer Mn ion. This structural model can also account for the higher spin of the g = 4.1 signal and the magnetic properties of the S-0-state.