STRUCTURE-FUNCTION RELATIONS IN PHOTOSYSTEM-II - EFFECTS OF TEMPERATURE AND CHAOTROPIC AGENTS ON THE PERIOD 4 OSCILLATION OF FLASH-INDUCED OXYGEN EVOLUTION

The characteristic period four oscillation patterns of oxygen evolutio n induced by a train of single-turnover flashes were measured in dark- adapted samples as a function of temperature and upon addition of chao tropic agents. The following results were obtained: (a) Within the ran ge 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 i nduced by the 2nd flash, Y2, exhibit similar dependencies on the tempe rature in isolated thylakoids, PS II membrane fragments, and inside-ou t vesicles. (b) Below a characteristic temperature theta(c) of 20-25-d egrees-C, the values of Y4/Y3 and Y2, which reflect (at constant S0 da rk population) the probabilities of misses and double hits, respective ly, 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 Y(D) and o ther endogenous electron donor(s), respectively, exhibit comparatively strong temperature dependencies in thylakoids with the following acti vation energies: E(A)(S2fast) = 55 kJ/mol, E(A)(S3fast) = 50 kJ/mol, E (A)(S2slow) = 85 kJ/mol, and E(A)(S3slow) = 75 kJ/mol. The activation energy of S0 oxidation to S1 by Y(D)ox was found to be markedly smalle r with a value of E(A)(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 chan ges of Y2 and the decay kinetics of S2 and S3; and anions of the Hofme ister series (SCN-, ClO4-, I-) cause a drastic destabilization of Y(D) ox. Possible structure-function relations of the PS II complex are dis cussed on the basis of these findings.