Jm. Peloquin et al., Mn-55 ENDOR of the S-2-state multiline EPR signal of photosystem II: Implications on the structure of the tetranuclear Mn cluster, J AM CHEM S, 122(44), 2000, pp. 10926-10942
We have performed continuous-wave electron paramagnetic resonance (CW-EPR)
and electron spin echo electron nuclear double resonance (ESE-ENDOR) experi
ments on the multiline form of the St-state of untreated, MeOH-treated, and
ammonia-treated spinach photosystem LI (PS LI) centers. Through simultaneo
usly 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, 1
80, 240; ammonia-treated PS II centers (MHz): 208, 208, 158; -150, -150, -1
12; 222, 222, 172; -295, -315, -390]. We further show that these effective
hyperfine tensors are best supported by a trimer/monomer arrangement of thr
ee 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 M
n-D is weakly exchange coupled (J(CD)) to one end of the trinuclear core. F
or 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 ammon
ia-treated PS II centers, the Mn(III) ion is Mno, with a zero-field-splitti
ng 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.