| Popis: |
The DNA double helix provides a well-characterized molecular pi stack in which charge transfer rates and efficiencies may be examined. We investigated the phenomenon in a series of DNA duplexes modified with various photo- and redox-active species. Charge transfer dynamics through DNA were distance independent for a series of duplexes modified with 7-deazaguanine and a covalently attached ethidium chromophore. The decay times were grouped into two components: (1) injection of charge into the DNA base stack and (2) a correlated charge transfer corresponding to the reorientation of ethidium within the duplex. Using the modified bases 2-aminopurine and 7-deazaguanine, intrastrand charge transfer through DNA was observed on the picosecond timescale. Charge transfer rates and quenching yields were also dependent on the reaction?s driving force and the composition of the intervening base stack. The efficiency of photooxidation of 7-deazaguanine by a ruthenium(II) intercalator in DNA over 7?14 angstroms was shallow and dependent on the chirality of the covalently attached metallointercalator. Nanosecond to subnanosecond decay rates were measured, and the spectroscopic signature of a charge transfer intermediate was observed. A series of ruthenium(II) intercalators with high oxidation potentials was created. Redox reactivity of the compounds with DNA did not correlate directly with oxidation potential and was dependent on DNA binding and luminescence quenching abilities. Thus, redox potential may not be used as the sole predictor of reactivity with the base stack of DNA. Finally, the binding of ruthenium(II) and rhodium(III) intercalators to DNA was investigated with CD and NMR spectroscopies. Data confirmed that metallointercalators do not preferentially bind next to each other along the double helix. Hence, direct contact of reactants is not responsible for fast and efficient charge transfer between metallointercalators bound noncovalently to DNA. These studies have provided direct measurements of the dynamics of DNA-mediated charge transfer and proven, once again, that the DNA pi stack facilitates fast and efficient charge transfer. Most importantly, the stacking and dynamics of the reactants and DNA bases were found to affect the charge transfer behavior. |
