Comparative study of the g=4.1 EPR signals in the S-2 state of photosystemII

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 fi ve 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 sig nal could be generated under two distinct sets of conditions: either by ill umination at room temperature or at 200 K in certain experimental condition s (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 t wo g = 4.1 signals arise from states which have quite different stability i n terms of temperature. In the present work we have compared these two sign als in order to test if they originate from the same or from different chem ical origins. The microwave power saturation properties of the two signals measured at 4.2 K were found to be virtually identical, Their temperature d ependencies measured at non-saturating powers were also identical. The pres ence 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 signa ls were also found to be virtually indistinguishable. We have been unable t o 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 signa l in samples containing the g4(IR). These findings are best interpreted ass uming that the two signals have a common origin i.e. a spin 5/2 ground stat e arising from a magnetically coupled Mn-cluster of 4 Mn ions. (C) 2000 Els evier Science B.V. All rights reserved.