ACTIVATION BARRIERS IN PHOTOSENSITIZED PYRIMIDINE DIMER SPLITTING

Pyrimidine dimers, which form by a symmetry allowed (π s2 + π s2) photocycloaddition reaction, are subject to photosensitized cycloreversion by electron donors, such as indoles. In a linked dimer–indole system, photoinitiated electron transfer occurs intramolecularly from indole to dimer to produce a charge‐separated species (dimer−.–indole+.). This species undergoes cycloreversion in competition with back electron transfer. Studies of the temperature dependence and solvent dependence of this competition have allowed the relative values of the activation parameters for the competing processes to be determined. In water (5–65°C) the free energy of activation of splitting minus that of back electron transfer (ΔΔG≠ = ΔG spl≠ ‐ ΔG bet≠) was found to be 1·3 kcal mol−1. The enthalpy of activation difference (ΔΔH≠) was found to be 1·1 kcal mol−1 and the entropy of activation difference (ΔΔS≠) was found to be −0·51 cal mol−1 K−1. In EPA (diethyl ether–isopentane–ethanol, 5:5:2; −85 to 25°C) the value of ΔΔG≠ remained the same, but the entropy and enthalpy contributions were different (ΔΔH≠ = 0·72 kcal mol−1; ΔΔS≠ = −1·8 cal mol−1 K−1). The results have been interpreted in terms of the effect of the polarity of the solvent on the transition states for the two competing processes. Enthalpy effects retard splitting more in water than in EPA, whereas entropy effects favor back electron transfer more in EPA than in water. Potential implications of these results for the mechanism of enzymatic photocycloreversion of pyrimidine dimers in DNA are considered. Copyright © 1990 John Wiley & Sons Ltd.