PHOTOINDUCED ELECTRON-TRANSFER IN ETHIDIUM-MODIFIED DNA DUPLEXES - DEPENDENCE ON DISTANCE AND BASE STACKING

Long-range photoinduced electron transfer has been systematically exam ined in a series of small DNA duplexes covalently modified with ethidi um and Rh(phi)(2)bpy(3+) through time-resolved and steady-state measur ements of fluorescence quenching. Fast fluorescence quenching (k great er than or similar to 10(10) s(-1)) is observed for this donor/accepto r pair noncovalently bound to DNA, and transient absorption studies al low the assignment of this quenching to an electron-transfer mechanism . In the duplexes modified with tethered intercalators, intrahelix flu orescence quenching attributed to electron transfer occurs at distance s up to 30 Angstrom. Over a donor/acceptor separation of similar to 20 Angstrom, approximately 30% of the ethidium fluorescence is quenched, while at a separation of similar to 30 Angstrom, approximately 10% qu enching is observed. Time-resolved measurements indicate that this que nching is primarily static. Fluorescence polarization and melting stud ies indicate that the intercalators are rigidly bound, enhance helix s tability, and the duplex populations are structurally homogeneous. The distance dependence of the quenching yield observed in these duplexes is shallow, but the quenching reaction is highly sensitive to stackin g perturbations. Changes in the quenching yield with melting are direc tly correlated with hypochromicity associated with base stacking. More over, in duplexes containing a highly disruptive CA mismatch, large de creases in the quenching efficiency are observed, while with a well-st acked GA mismatch, electron transfer proceeds efficiently. Hence, in t hese covalently-modified assemblies, DNA-mediated electron transfer is found to depend only weakly on donor/acceptor separation, when compar ed to protein systems, but is highly sensitive to perturbations in bas e stacking.