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

doi:10.1073/pnas.0405157101

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

Department of Physics, Washington State University, 2710 University Drive, Richland, WA 99352; and W. R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99354

Edited by Harry B. Gray, California Institute of Technology, Pasadena, CA, and approved November 2, 2004 (received for review July 15, 2004)

Abstract

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 π 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.

Footnotes

↵ * To whom correspondence should be addressed. E-mail: ls.wang{at}pnl.gov.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: IP, ionization potential; PES, photoelectron spectroscopy; VDE, vertical electron-detachment energy; ADE, adiabatic electron-detachment energy; dAMP^–, 2′-deoxyadenosine 5′-monophosphate; dCMP^–, 2′-deoxycytidine 5′-monophosphate; dGMP^–, deoxyguanosine 5′-monophosphate; dTMP^–, deoxythymidine 5′-monophosphate; HOMO, highest occupied molecular orbital.