Proteindependent Charge Transfer in DNA

From spectroscopic and biochemical studies it has become clear that DNAmediated CT is extremely sensitive to the re-stacking of the intervening DNA bases and to disruption and perturbation of the DNA structure or conformation. This indicates that sensing of DNA damage could be accomplished, at least in part, on the basis of CT chemistry. In considering these possibilities, it is important to discover whether DNA-mediated CT does occur within the cell. Charge transfer in HeLa cell nuclei has recently been probed by use of a rhodium photooxidant [15]. After incubation and irradiation the genomic DNA was isolated and analyzed. This revealed that base damage occurs preferentially at the 5'-G of GG sites. More importantly, oxidative G damage was found at protein-bound sites that were inaccessible to the rhodium photooxidant, as examined by footprinting. This clearly indicates that CT processes could occur in cells.

Specific DNA-protein interactions which either promote or inhibit CT processes through the protein-DNA interface would be the most crucial part of a biological system sensing DNA damage. Recent experiments have shown clearly that DNAmediated CT processes are modulated both negatively and positively by DNA-binding proteins. Most importantly, each of the observed influences of the proteins can be explained by special structural features of the corresponding DNA-protein complexes. Thus, special DNA-protein interactions result in a characteristic modulation of the DNA-mediated CT.

Barton et al. developed an assay enabling the study of the influence of DNA-binding proteins simply by gel electrophoretic analysis of oxidative G damage

Fig. 4.6.3. Examples of photooxidants which have been used in biochemical studies of HT in DNA.

Hole Injection

Gox proximal

Gox distal

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