Metal complexes are useful tools in molecular biology for recognizing, understanding, and modifying biomolecular structure, particularly DNA. Since many Cp metal complexes contain or are capable of generating species which cleave, alkylate, or crosslink oligonucleotides, behavior known to constitute the mode of action of many human cancer chemotherapeutic agents, we are developing these compounds as reagents for the modification of DNA.
Our initial work in this area has focused on the use of CpW(CO)3CH3, because its photochemical behavior has been well studied and has been reported to produce methyl radical via metal-alkyl bond homolysis at wavelengths greater than 300 nm (eq 1), conditions which do not degrade DNA. Additionally, it has been reported to be air- and water-stable and can be synthesized in one step from commercially available materials. We have shown that this complex cleaves DNA in concentration dependent manner at as few as 1.5 molecules/base pair, a surprisingly low ratio considering that the complex does not contain a DNA binding element and that most synthetic enediynes (which generate diradicals) without recognition devices exert this behavior at 100-1000 molecules/base pair. This result represents the first demonstration of DNA cleavage by homolysis of a metal-alkyl bond in a Cp-metal complex.
In investigating the mechanism of the cleavage, we have employed various radical trapping agents and ESR, studies which support the participation of methyl radical in the scission process. We have also synthesized compounds 1 - 3, which give increased amounts of double-strand scission, which is desired, because it is more difficult for cancer cells to repair.
We have also attached recognition elements to the complex (to furnish systems such as the netropsin derivative-substituted tungsten complexes shown below). Both compounds bind to AT-rich tracts of DNA, but only the complex on the right gives sequence-selective cleavage. It is also as effective at cleaving DNA as dynemicin A, one of the naturally-occurring enediyne anticancer antibiotics in clinical trials.
Our future work will include further mechanistic studies, investigations with other photoactive complexes, and the synthesis of new photonucleases.