Electron and hydrogen transfer reactions of nucleotides: from Stern-Volmer quenching to nucleoprotein structure

Abstract

The complex Pt 2(pop) 4 4− abstracts hydrogen atoms and electrons from DNA upon photolysis into the dσ ∗→ pσ excited state (pop = P 2O 5H 2 2−). In duplex DNA, the hydrogen atoms are abstracted from the 4′- and 5′- positions of the deoxynbose functionality, and electrons are abstracted by tunneling from the guanine nucleobase. At high Mg 2+ concentrations, the Pt 2(pop) 4 4− tetraanion can associate more intimately with the duplex, and both hydrogen atom and electron transfer are efficient; however, at low Mg 2+ concentrations, the complex is situated far from the duplex so that the only efficient pathway is electron tunneling. Therefore, the guanine/sugar ratio decreases with increasing Mg 2+. The electron transfer pathway can also be examined in the absence of the hydrogen transfer pathway in thermal reactions where the electron is abstracted from guanine by Ru(bpy) 3 3+ (bpy = 2,2′-bipyridine). These reactions can be initiated electrochemically by potentiation of the complex to the 3 + form, which produces catalytic enhancements in cyclic voltammograms. These enhancements show that guanine multiplets are more reactive than dispersed guanines. Finally, binding of DNA to the HhaI methyltransferase causes flipping of a cytosine into the active site of the enzyme, leaving behind an unpaired guanine residue that is more reactive towards electron transfer than the paired guanine, an effect visible in high-resolution electrophoresis gels after photolysis in the presence of Pt 2(pop) 4 4−.