Abstract
Sensitized photoreactions are frequently characterized by electron transfer processes. In this work electron transfer processes were observed and confirmed for two more photochemical reactions. 1. The fluorescence quenching of dyes by deoxyribonucleic acids (DNA) was identified as a reversible electron transfer from DNA bases in the ground state to sensitizer dyes in the first excited singlet state. The excited state potentials have been obtained by combination of electrochemical and spectroscopic data. The electron-donating properties of nucleic acid bases increase in the order: Gua > Ade > Thy > Cyt. Quenching preferentially occurs by Gua. 2. Irreversible photodynamic degradation of DNA by visible light in the presence of a sensitizer dye can be described by a redox mechanism where the oxidized sensitizer radical is an important intermediate. The photodynamic efficiency of the dye-DNA system can be predicted from the linear free energy relationship between the redox potentials of dyes and nucleic acid bases.
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