Thermodynamics of Acidity and Photoacidity in trans-Resveratrol Analogues: The Role of Prototropic Tautomerism.

The thermodynamics of protonation equilibria and equilibria between enol and keto tautomers occurring in naturally found hydroxy derivatives of trans-resveratrol analogues were investigated. Reaction Gibbs free energies at room temperature were obtained using the B3LYP functional together with a continuum solvation model for water. From these data, acidity constants (pK a) and tautomerization constants (pK T) were calculated for the electronic ground state (S0) and the lowest excited singlet state (S1). The results reveal a pronounced dependence of both equilibria on the position of the hydroxy substituents. Upon photoexcitation, the compounds become considerably stronger acids and indicate, in many cases, a shift toward the keto tautomers rather than the parent enol forms. Moreover, our results support the feasibility of proton transfer from a hydroxy group on one aromatic ring to the adjacent ring via the central -CHCH- bridge. Such processes may be relevant for nonradiative de-excitation pathways and for the protective biochemical functions of these compounds in plants.