Structure-function relations in photosystem II. Effects of temperature and chaotropic agents on the period four...

The characteristic period four oscillation patterns of oxygen evolution induced by a train of single-turnover flashes were measured in dark-adapted samples as a function of temperature and upon addition of chaotropic agents. The following results were obtained: (a) Within the range of 0 < theta < 35 degrees C, the ratio of the oxygen yield induced by the 4th and 3rd flashes of the train, Y4/Y3, and the oxygen yield induced by the 2nd flash, Y2, exhibit similar dependencies on the temperature in isolated thylakoids, PS II membrane fragments, and inside-out vesicles. (b) Below a characteristic temperature theta c of 20-25 degrees C, the values of Y4/Y3 and Y2, which reflect (at constant S0 dark population) the probabilities of misses and double hits, respectively, remain virtually independent of temperature, whereas above theta c these parameters increase. (c) The dark decays of S2 and S3 via fast and slow kinetics due to reduction of the water oxidase by YD and other endogenous electron donor(s), respectively, exhibit comparatively strong temperature dependencies in thylakoids with the following activation energies: EA(S2fast) = 55 kJ/mol, EA(S3fast) = 50 kJ/mol, EA(S2slow) = 85 kJ/mol, and EA(S3slow) = 75 kJ/mol. The activation energy of S0 oxidation to S1 by YDox was found to be markedly smaller with a value of EA(S0) = 30 kJ/mol. (d) Incubation with chaotropic agents at concentrations which do not significantly impair the oxygen evolution capacity leads to modifications of the oscillation pattern with remarkable differences for various types of agents: Tris and urea are practically without effect; guanidine hydrochloride affects Y4/Y3 in a similar way as elevated temperature but without significant changes of Y2 and the decay kinetics of S2 and S3; and anions of the Hofmeister series (SCN-, ClO4-, I-) cause a drastic destabilization of YDox. Possible structure-function relations of the PS II complex are discussed on the basis of these findings.