DISTANCE-DEPENDENT PHOTOINDUCED ELECTRON-TRANSFER IN SYNTHETIC SINGLE-STRAND AND HAIRPIN DNA

The singlet state of stilbene-4,4'-dicarboxamide can serve as a fluore scent probe of both DNA conformation and electron transfer. Covalent i ncorporation of the stilbene-dicarboxamide into DNA structures with re stricted conformational mobility results in inhibition of stilbene iso merization and an increase in its fluorescence quantum yield and lifet ime. The fluorescence of stilbenedicarboxamide is selectively quenched by proximate guanine, but not by the three other DNA nucleobases. Sel ective quenching occurs via an electron transfer mechanism in which st ilbene serves as the electron acceptor and guanine as the electron don or. Kinetic analysis of the distance dependence of electron transfer i n stilbene-bridged hairpins suggests that duplex DNA is more effective than proteins as a medium for electron transfer, but that it does not function as a molecular wire.