Oxidation of Guanine at a Distance in Oligonucleotides Induced by Two-Photon Photoionization of 2-Aminopurine

Photoionization of free 2-aminopurine (2AP) in aqueous solutions, or of 2AP residues in oligonucleotides, is observed upon excitation with 308 nm XeCl excimer laser pulses (fwhm = 12 ns, ≤ 100 mJ/cm²/pulse) and involves the consecutive absorption of two photons. The absorption of light by the normal DNA bases at this wavelength is negligible under the same conditions. The kinetics and transient absorption spectra of the hydrated electrons and of 2AP radicals resulting from the deprotonation of the 2AP radical cations formed initially, have been investigated using standard spectroscopic methods. The 2AP radicals in aqueous solutions reversibly oxidize 2′-deoxyguanosine and guanosine 5′-monophosphate but do not react with the other three DNA nucleosides or nucleotides. The efficiency of photoionization of 2AP decreases in the following order: free 2AP > 2AP incorporated into single-stranded oligonucleotide > 2AP in double-stranded oligonucleotides. Photoionization of single 2AP residues incorporated into 18-mer single- and double-stranded oligonucleotides results in the oxidation of guanine residues in GG doublets. Employing gel electrophoresis methods, strand cleavage at these GG sites and at the site of the 2AP residue is observed after treatment of the irradiated oligonucleotides with hot piperidine. Using nanosecond transient absorption techniques, it is shown that the unimolecular oxidation of guanine by 2AP radicals at a distance can also be monitored directly in single-stranded oligonucleotides containing GG-doublets. It is found that increasing the number of nucleic acid bases between the 2AP radicals and the GG-doublet from 0 to 5 results in a decrease in the rate constant of guanine radical formation by a factor greater than 10³.