Photoinduced short-range electron transfer in DNA with fluorescent DNA bases: lessons from ethidium and thiazole orange as charge donors.

Charge transfer processes through the double helix of DNA cover a broad range of mechanistic models ranging from superexchange to hopping mechanisms. Over the last decade, these processes were studied by our group in a photoinduced fashion since (i) the starting time for the charge transfer is clearly defined by the absorption of the photon and (ii) photoexcitation delivers the necessary driving force to the DNA system. It is a prerequisite to modify oligonucleotides synthetically with suitable organic fluorophores that serve as photoinducable charge donors. In the first part of this perspective article we summarize our recent advances in the area of DNA-mediated reductive electron transfer processes over short ranges using synthetic DNA-donor-acceptor systems. The second part of this article focuses on ethidium as the photoinducable charge donor. Ethidium-modified DNA can be used to compare oxidative hole transfer with reductive electron transfer since the type of charge transfer can be controlled by choosing the right charge acceptor. Recent results showed that an efficient charge transfer through DNA using covalently bound ethidium is strongly influenced mainly by DNA dynamics but also by several other parameters that affect the electronic coupling between charge donor and acceptor.