DNA-MEDIATED ELECTRON TRANSFER REACTIONS

本方案的总体目标是，利用金属核酸

The overall objective of this proposal, using metal-nucleic acid chemistry, is to explore the role of DNA as a polymer in mediating electron transfer reactions. Oligonucleotides will be constructed which contain metal complexes covalently linked to the double helix so as to carry out studies of photoinduced electron transfer between the donor- acceptor pairs on a DNA duplex as a function of DNA length, sequence, and structure. The DNA helix, as a synthetically amenable and structurally well characterized polymer, may be useful (i) fundamentally, in delineating aspects of long range electron transfer processes through pi- stacks, (ii) biologically, towards understanding how radical reactions may damage DNA, and (iii) practically, in the development of biosensors. Oligonucleotides will be constructed which contain a metallointercalating donor or acceptor tethered to the 5'-terminus; dipyridophenazine (dppz) complexes of ruthenium(II) will serve as donors and phenanthrenequinone (phi) complexes of rhodium(III) will serve as electron acceptors. Intercalation of the complexes within the helix will be established through luminescence studies of the ruthenium-modified oligonucleotide base-paired with unmodified complement. The position of intercalation and hence the metal-metal distance on the polymer will be established through photocleavage experiments using the rhodium-modified polymer bound to unlabeled complement. Photoelectron transfer rates between the metal sites on a double strand formed by annealing the rhodium-modified polymer to the ruthenium-modified polymer will be determined through time-resolved luminescence and transient absorption spectroscopy. Preliminary results indicate fast photoelectron transfer over 41 angstrome through the DNA duplex. Oligonucleotides which differ in length intervening between the metal donor and acceptor will be prepared to establish the distance-dependence in electron transfer rate, and sequences will be synthesized which can adopt preferentially the A- or Z-conformations to determine the contribution of base stacking to the electron transfer process. To compare through-bond versus a pi-stack mechanism, electron transfer rates will be compared between double helices of the form (5'-Ru- oligonucleotide-Rh-3'). (3'complement-5') and (5'Ru-oligonucleotide- 3').(3'-complement'Rh-5'). Agents which bind site-specifically to the intervening duplex, such as cis-platin, a restriction enzyme, or an alternate, site-specifically bound rhodium complex, will be employed to examine perturbations in the long-range electron transfer process. A potential DNA-"diode" will also be constructed to examine whether the incorporation of electron-donating or accepting groups on the helix can impart a "sequence-dependent" directionality to the electron transfer process.