Photosynthetic glow peaks and their relationship with the free energy changes

Abstract

This paper is concerned with relating thermoluminescence to the total free-energy change, ΣG, involved in detrapping a particular electron-hole pair as a photosynthetic sample is warmed from an initial low temperature. It extends a mathematical discussion of four possible mechanisms introduced in an earlier paper [DeVault, Govindjee and Arnold, Proc Nat'l Acad Sci USA 80: 983–987 (1983)]; here, particular attention is paid to the dependence of the absolute temperature of the maximum of a glow-peak, T _m , on the total free-energy change, ΣG. The conclusion from the cases studied is that T _m =ΣG/(k _B W) where ΣG is evaluated at T _m , W is a complicated function of temperature and of thermodynamic parameters in the steps of the mechanism, and k _B is the Boltzmann constant. If the rate limiting step in the mechanism of detrapping is not preceded by any step in which ΔG is appreciably negative, W is likely to have a value of about 33 and T _m is approximately proportional to ΣG. Otherwise W can become much smaller and more strongly dependent on temperature and T _m is no longer proportional to ΣG. These conclusions are of significance in lending theoretical support to the practice of inferring redox midpoint potential changes from shifts in T _m .