A NEW PARAMAGNETIC HYPERFINE-STRUCTURE EFFECT IN MANGANESE TETRAMERS - THE ORIGIN OF MULTILINE EPR SIGNALS FROM AN S2 STATE OF A PHOTOSYNTHETIC WATER-SPLITTING ENZYME

A new theory of EPR spectra for singlet states of Mn tetramers, allowing spin-hybridizations for component composite spins, is developed to elucidate the origin of the "multiline" EPR signals from the Mn clusters in an S2 State of photosystem II. By computer simulation it is shown that the Mn cluster most probably exists in the Mn(III, IV, IV, IV) oxidation state with a strong-antiferromagnetic exchange interaction (J(ab)S(a).S(b)) between a pair of Mn(III) and Mn(IV) ions and the other weak or intermediate ones satisfying the relations, (3/2)p01 (2J(ac)-2J(ad)+J(bd)-J(bc)) cos2-theta=[J(cd)-J(ac)-J(ad)+1/2 (J(bc)+J(bd))] sin2-theta> 0, where p01=1.291 and theta, a polar angle of the spin-state vector in the x-y plane of S(cd)=0 and S(cd)= 1 spin configurations, is almost-equal-to 38-degrees-39-degrees in control samples.