A new paramagnetic hyperfine structure effect in manganese tetramers. The origin of “multiline” EPR signals from an S 2 state of a photosynthetic water-splitting enzyme

Abstract

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 S 2 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) p 01 (2 J ac−2 J ad+ J bd− J bc) cos 2θ=[ J cd− J ac− J ad+ 1 2 ( J bc+ J bd)] sin 2θ⪢0, where p 01=1.291 and θ, a polar angle of the spin—state vector in the x– y plane of S cd=0 and S cd=1 spin configurations, is ≈ 38°–39° in control samples.