Direct experimental observation of the low ionization potentials of guanine in free oligonucleotides by using photoelectron spectroscopy.

Photodetachment photoelectron spectroscopy is used to probe the electronic structure of mono-, di-, and trinucleotide anions in the gas phase. A weak and well defined threshold band was observed in the photoelectron spectrum of 2'-deoxyguanosine 5'-monophosphate at a much lower ionization energy than the other three mononucleotides. Density function theory calculations revealed that this unique spectral feature is caused by electron-detachment from a pi orbital of the guanine base on 2'-deoxyguanosine 5'-monophosphate, whereas the lowest ionization channel for the other three mononucleotides takes place from the phosphate group. This low-energy feature was shown to be a "fingerprint" in all the spectra of dinucleotides and trinucleotides that contain the guanine base. The current experiment provides direct spectroscopic evidence that the guanine base is the site with the lowest ionization potential in oligonucleotides and DNA and is consistent with the fact that guanine is most susceptible to oxidation to give the guanine cation in DNA damage.