V. Shafirovich et al., Oxidation of guanine at a distance in oligonucleotides induced by two-photon photoionization of 2-aminopurine, J PHYS CH B, 103(49), 1999, pp. 10924-10933
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, less than or equal to 100 mJ/cm(2)/pul
se) 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 hydr
ated 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 ox
idize 2'-deoxyguanosine and guanosine 5'-monophosphate but do not react wit
h the other three DNA nucleosides or nucleotides. The efficiency of photoio
nization of 2AP decreases in the following order: free 2AP > 2AP incorporat
ed into single-stranded oligonucleotide > 2AP in double-stranded oligonucle
otides. Photoionization of single 2AP residues incorporated into 18-mer sin
gle- and double-stranded oligonucleotides results in the oxidation of guani
ne residues in GG doublets. Employing gel electrophoresis methods, strand c
leavage at these GG sites and at the site of the 2AP residue is observed af
ter treatment of the irradiated oligonucleotides with hot piperidine. Using
nanosecond transient absorption techniques, it is shown that the unimolecu
lar oxidation of guanine by 2AP radicals at a distance can also be monitore
d directly in single-stranded oligonucleotides containing GG-doublets, It i
s found that increasing the number of nucleic acid bases between the 2AP ra
dicals and the GG-doublet from 0 to 5 results in a decrease in the rate con
stant of guanine radical formation by a factor greater than 10(3).