Formation spectra of the EPR split signals from the S0, S1, and S3 states in photosystem II induced by monochromatic light at 5 K

The interaction EPR split signals from photosystem 11 (PSII) have been reported from the S-0, S-1, and S-3 states. The signals are induced by illumination at cryogenic temperatures and are proposed to reflect the magnetic interaction between Yz(center dot) and the Mn4Ca cluster. We have investigated the formation spectra of these split EPR signals induced in PSII enriched membranes at 5 K using monochromatic laser light from 400 to 900 nm. We found that the formation spectra of the split S-0, split S-1, and Split S-3 EPR signals were quite similar, but not identical, between 400 and 690 nm, with maximum formation at 550 nm. The major deviations were found between 440 and 480 nm and between 580 and 680 nm. In the regions around 460 and 680 nm the amplitudes of the formation spectra were 25-50% of that at 550 nm. A similar formation spectrum was found for the S-2-State multiline EPR signal induced at 0 degrees C. In general, the formation spectra of these signals in the visible region resemble the reciprocal of the absorption spectra of our PSII membranes. This reflects the high chlorophyll concentration necessary for the EPR measurements which mask the spectral properties of other absorbing species. No split signal formation was found by the application of infrared laser illumination between 730 and 900 nm from PSII in the So and S, states. However, when such illumination was applied to PSII membranes poised in the S-3 state, formation of the Split S-3 EPR signal was observed with maximum formation at 740 nm. The quantum yield was much less than in the visible region, but the application of intensive illumination at 830 nm resulted in accumulation of the signal to an amplitude comparable to that obtained with illumination with visible light. The Split S-3 EPR signal induced by NIR light was much more stable at 5 K (no observable decay within 60 min) than the Split S-3 signal induced by visible light (50% of the signal decayed within 30 min). The Split S-3 signals induced by each of these light regimes showed the same EPR spectral features and microwave power saturation properties, indicating that illumination of PSII in the S-3 state by visible light or by NIR light produces a similar configuration of Yz(center dot) and the Mn4Ca cluster.