INHOMOGENEITY OF THE EPR MULTILINE SIGNAL FROM THE S-2-STATE OF THE PHOTOSYSTEM-II OXYGEN-EVOLVING ENZYME

The manganese complex (Mn-4) which is responsible for water oxidation in photosystem II is EPR detectable in the S-2-state, one of the five redox states of the enzyme cycle, The S-2-state is observable at 10 K either as an EPR multiline signal (spin S = 1/2) or as a signal at g = 4.1 (spin S = 3/2 or 5/2). It has recently been shown that the state responsible for the multiline signal is converted to that responsible for the g = 4.1 signal upon the absorption of near-infrared light [Bou ssac A, Girerd J-J, Rutherford AW (1996) Biochemistry 35:6983-6989]. I t is shown here that the yield of the spin interconversion may be vari able and depends on the photosystem II (PSII) preparations. The EPR mu ltiline signal detected after near-infrared illumination, and which or iginates from PSII centers not susceptible to the near-infrared light, is shown to be different from that which originates from infrared-sus ceptible PSII centers, The total St-multiline signal results from the superposition of the two multiline signals which originate from these two PSII populations. One S-2 population gives rise to a ''narrow'' mu ltiline signal characterized by strong central lines and weak outer li nes. The second population gives rise to a ''broad'' multiline signal in which the intensity of the outer lines, at low and high field, are proportionally larger than those in the narrow multiline signal. The l arger the relative amplitude of the outer lines at low and high field, the higher is the proportion of the near-infrared-susceptible PSII ce nters and the yield of the multiline to g = 4.1 signal conversion. Thi s inhomogeneity of the EPR multiline signal is briefly discussed in te rms of the structural properties of the Mn-4 complex.