Comparative study of the g=4.1 EPR signals in the S2 state of photosystem II

The Mn-4 complex which is involved in water oxidation in photosystem II is known to exhibit three types of EPR signals in the S-2 state, one of the five redox states of the enzyme cycle: a multiline signal (spin 1/2), signals at g > 5 (spin 5/2) and a signal at g = 4.1 (or g = 4.25), The g = 4.1 signal could be generated under two distinct sets of conditions: either by illumination at room temperature or at 200 K in certain experimental conditions (g4(S) signal) or by near-infrared illumination between approximate to 77 and approximate to 160 K of the S-2-multiline state (g4(IR) signal). The two g = 4.1 signals arise from states which have quite different stability in terms of temperature. In the present work we have compared these two signals in order to test if they originate from the same or from different chemical origins. The microwave power saturation properties of the two signals measured at 4.2 K were found to be virtually identical, Their temperature dependencies measured at non-saturating powers were also identical. The presence of Curie law behavior for the g4(S) and g4(IR) signals indicates that the states responsible for both signals are ground states. The orientation dependence, anisotropy and resolved hyperfine structure of the two g4 signals were also found to be virtually indistinguishable. We have been unable to confirm the behavior reported earlier indicating that the g4(S) signal is an excited state, nor were we able to confirm the presence of signal from a higher excited state in samples containing the g4(S), nor a radical signal in samples containing the g4(IR). These findings are best interpreted assuming that the two signals have a common origin i.e. a spin 5/2 ground state arising from a magnetically coupled Mn-cluster of 4 Mn ions. (C) 2000 Elsevier Science B.V. All rights reserved.