Nature of the Mn Centers in Photosystem II. Modeling and Behavior of the g = 4 Resonances and Related Signals

Abstract

The two forms of the g ≈ 4.1 signal recently identified in photosystem II (Smith, P. J.; Pace, R. J. Biochim. Biophys. Acta 1996, 1275, 213) have been simulated at several frequencies as near-axial spin 3/2 centers. In both cases, an explicit spin coupling model is assumed, involving two magnetically isolated Mn pairs, one for each signal type. For that pair assumed to give rise to the spin 1/2 multiline signal as the ground state, the modeling of the first-excited-state 4.1 signal gives estimates of the fine structure parameters for the individual Mn centers and the exchange coupling constant for the pair. The fine structure terms suggest that one Mn ion is a conventional MnIII ion in a highly axially distorted environment. The other Mn center, which is formally spin 3/2, is unlikely to be a conventional MnIV ion, but rather a MnIII−radical ligand pair, strongly antiferromagnetically coupled to give a net spin 3/2 state. The coupling between this Mn−radical center and the other MnIII is weak (J = −2.3 cm...