Volume changes and electrostriction in the primary photoreactions of various photosynthetic systems

Abstract

Photoacoustics allows the determination of enthalpy and volume changes on the microsecond time scale of photoexcited reaction centers from Rhodobacter sphaeroides, PSI and PSII centers from Synechocystis and of both centers in whole cells of that cyanobacterium. Both the volume decrease, -25 A3 and the enthalpy, -0.4 eV, are the same for the bacterial centers and PSI. However the volume change for PSII is near zero and the enthalpy far larger, -1 eV. Assigning the volume changes to electrostriction allows a coherent explanation of these observations. One can explain the large volume decrease in the bacterial centers with an effective dielectric coefficient of ~4. This is a unique approach to this parameter so important in estimation of protein energetics. The value of the volume contraction for PSI can only be obtained if the acceptor is the super-complex (Fe4S4)(SR4) with charge change from ¿1 to ¿2. The simple complex Fe4S4 with charge change from +3 to +2 would produce twice as large a positive expansion of the volume. The null result with PSII is explained by sub-microsecond electron transfer from the neighboring tyrosine Z anion to the P680 cation. The result is that charge is only moved from the tyrosine anion to the quinone with no separation of charge and thus very little change in electrostriction.Thus the tyrosine is anionic or the proton transfer to a polar region occurs in sub-microsecond time.