PHOTOCHEMICAL-REACTIONS OF PHOTOSYSTEM-II IN ETHYLENE-GLYCOL

The behavior of photosystem II (PSII) reactions was investigated under conditions of decreasing water content by the addition of increasing concentrations of ethylene glycol (EG). The photosynthetic activities were measured for PSII samples either directly in aqueous solutions of EG or in the standard buffer medium following EG treatment, Several e ffects on PSII arise upon exposure to EC. Below 50% EG there are no si gnificant Irreversible changes, although there is a slowing of the Q(A )(-) reoxidation kinetics in the presence of EG. At concentrations of 50-70% EG, protein structural changes occur that include the release o f the 16, 23, and 33 kDa extrinsic proteins and two of the catalytic M n ions. For these samples, the capacity for O-2 evolution is considera bly reduced and the formation of donor side H2O2 is enhanced. In 60% E C, the nanosecond components in the rate of P680(+) reduction are conv erted entirely to microsecond kinetics which upon return of the sample to the standard buffer medium are partially restored, indicating that EG has a reversible, solvent effect on the PSII donor side. At concen trations of EG >70% chlorophyll fluorescence measurements reveal rever sible increases in the F-0 level concomitant with the generation and d isappearance of a 5 mu s decay component in the P680(+) reduction kine tics. This result may indicate a solvent-induced uncoupling of the lig ht harvesting pigment bed from the reaction center complex. As the EG concentration is increased to 80-100%, there is an irreversible loss o f the primary charge separation. The use of EC as a cryoprotectant and as a water-miscible organic solvent for PSII is discussed.