Oxygen-Evolution Patterns from Spinach Photosystem II Preparations

Patterns of O₂ evolution resulting from sequences of short flashes are reported for Photosystem (PS) II preparations isolated from spinach and containing an active, O₂-evolving system. The results can be interpreted in terms of the S-state model developed to explain the process of photosynthetic water splitting in chloroplasts and algae. The PS II samples display damped, oscillating patterns of O₂ evolution with a period of four flashes. Unlike chloroplasts, the flash yields of the preparations decay with increasing flash number due to the limited plastoquinone acceptor pool on the reducing side of PS II. The optimal pH for O₂ evolution in this system (pH 5.5-6.5) is more acidic than in chloroplasts (pH 6.5-8.0). The O₂-evolution, inactivation half-time of dark-adapted preparations was 91 min (on the rate electrode) at room temperature. Dark-inactivation half-times of 14 h were observed if the samples were aged off the electrode at room temperature. Under our conditions (experimental conditions can influence flash-sequence results), deactivation of S₃ was first order with a half-time of 105 s while that of S₂ was biphasic. The half-times for the first-order rapid phase were 17 s (one preflash) and 23 s (two preflashes). The longer S₂ phase deactivated very slowly (the minimum half-time observed was 265 s). These results indicate that deactivation from S₃ → S₂ → S₁, thought to be the dominant pathway in chloroplasts, is not the case for PS II preparations. Finally, it was demonstrated that the ratio of S₁ to S₀ can be set by previously developed techniques, that S₀ is formed mostly from activated S₃ (S₄), and that both S₀ and S₁ are stable in the dark.