Time-resolved photoelectron imaging of the isolated deprotonated nucleotides

Abstract

Using time-resolved photoelectron spectroscopy, the excited state dynamics of gas-phase mass-selected nucleotide anions have been monitored following UV excitation at 4.66 eV. The spectra reveal that the dynamics of the 2′-deoxyguanosine 5′-monophosphate anion (dGMP⁻) are very similar to those of the adenosine nucleotide (dAMP⁻) and are insensitive to solvent. Comparison of our results with other literature suggests that nucleotides of the two purine bases share a common relaxation pathway, whereby the initially populated ¹ππ* states relax to the ground electronic state without involvement of any other intermediary electronic states. In the analogous pyrimidine nucleotides of thymine and cytosine, dTMP⁻ and dCMP⁻, no such unified mechanism is observed. Photoexcited dTMP⁻ behaves much like the isolated nucleobase thymine, exhibiting rapid relaxation to the ground electronic state, although with a minor long-lived channel. On the other hand, isolated dCMP⁻ is longer lived than its cytosine nucleobase, and hence it appears that the presence of the sugar and phosphate in the nucleotide arrangement leads to a modification of the available relaxation pathways. Nucleotides are the basic monomer building blocks of DNA and our results present important new benchmark data to develop an understanding of the molecular mechanism by which photodamage can be mediated when DNA is exposed to UV light.