Triplets of aromatic dyes bound to DNA as probes of the mobility of DNA-bound metal ions

Abstract

The interaction of various metal ions with DNA in aqueous solutions has been studied by a variety of different techniques [l] , Metal ions bound to DNA can influence its biological activity and affect its physico-chemic~ properties in solution. In this letter we describe a novel application of spectroscopic techniques to demonstrate that Mn2+ and Ag’ ions bound to DNA in aqueous solutions at room temperature diffuse from base pair to base pair along the DNA helix. The technique involves the optical excitation of triplet excited states of polycyclic aromatic molecules or dyes which are physically intercalated between the base pairs of DNA. The lifetimes of these triplets, in the absence of dissolved oxygen, may be as long as 150 ms [2] but typically are in the range of 20-30 ms. These triplets are readily quenched by metal ions which are also bound to DNA. Using the aromatic dye acridine orange (phosphate/dye binding ratio > 100) we have observed that even small amounts of bound metal ions give rise to a uniform decrease in the lifetime of the triplets of all the DNAbound aromatic molecules. This conclusion is based on the fact that the triplet decay curves are exponential in the absence as well as in the presence of low concentrations of metal ions, showing that all of the dye molecules are equally accessible to the mobile metal ion quenchers. The base pair to base pair hopping frequency is estimated to be 2 IO5 set-’ for Mn*+ and 2 5 X 10’ set-’ for Ag’ions bound to DNA in aqueous solution at 29C. 2. Experimental