In inside-out thylakoids, the damping of the oscillation pattern of o2yields evolved from a flash sequence is highly pH-dependent. The oscillation pattern is much more damped at pH 7.6 than at acidic pH (pH 6.5). The light saturation behaviour of the transition S2→ S3also shows a pH dependence. At pH 7.6, the light saturation behaviour of the S2→ S3transitions has the same shape as the light saturation behaviour of the other S-state transitions. In contrast, at pH 6.5, it exhibits two different light saturation processes. At pH 6.5, pH-dependent centers contribute to about 30% of the S2→ S3transitions. These centers, inactive at pH 7.6, would be responsible for the decrease of the miss on the transition S2→ S3, explaining the less damped O2yield pattern at acidic pH. The results are interpreted in terms of the possible existence of an additional donor in the S2state, which needs to be protonated in order to oxidize the state S2to S3.
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