Energetics of photosynthetic glow peaks.
By postulating temperature-dependent equilibria between two or more electron carriers acting as traps for electrons or holes, it is possible to modify the Randall-Wilkins theory of thermoluminescence so as to explain the abnormally large apparent activation energies and apparent frequency factors observed in photosynthetic glow curves when fitted by unmodified Randall-Wilkins theory. The equilibria serve to inhibit the formation of the light-emitting excited state by withholding the needed precursor state. When the inhibition is released at higher temperature by shift of equilibrium with temperature, the rise of the glow peak can be much faster than would result from Arrhenius behavior based on the true activation energy and so appears to correspond to a higher activation energy accompanied by a larger frequency factor. From another viewpoint, the enthalpy changes, DeltaH, of the equilibria tend to add to the activation energy. Similarly the entropy changes, DeltaS, of the equilibria tend to add to the entropy of activation, giving the large apparent frequency factors. The positive values of DeltaS needed would correspond to entropy decreases in the forward early electron transport. A comparison of the glow peaks obtained by different workers is also presented.
Devault
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D
Department of Physiology and Biophysics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801.
Govindjee
Arnold
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W