Base-Specific Ionization of Deprotonated Nucleotides by Resonance Enhanced Two-Photon Detachment

Abstract

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Time-resolved photoelectron imaging of the isolated deprotonated nucleotides

“…The k 1 high energy cutoff of 3.1 eV agrees well with the adiabatic detachment energy for the nucleobase within dGMP -, which was found to be 4.65 eV. 28 The negative intensity of k 1 < 1 eV is indicative of a population transfer from the higher eKE region to the lower eKE region. Note that the extracted τ 1 lifetime is within our temporal resolution.…”

“…A key feature of this experimental scheme is that, although in principle the total photon energy (7.76 eV) is sufficient to detach the electron from many parts of the anion, including the charge carrying phosphate group, by exploiting the 1 ππ* ← S 0 resonance, we can selectively detach electrons only from the nucleobase moiety. 28 This allows us to think of the experiment as photoelectron spectroscopy on a neutral subgroup, whilst the charged phosphate group remains a spectator. In our previous dAMP -experiments, we found that the excited state dynamics of the nucleotide could be modelled using biexponential kinetics.…”

“…A second concern of the experimental scheme is that detachment leads to a zwitterionic final state, where an electron resides on the phosphate, and a hole is on the nucleobase. Resonance enhanced two-photon photodetachment showed that this zwitterion is only the lowest lying neutral state for dGMP -, 28 and in the other nucleotides the neutral species without a charged phosphate is lower in energy. These considerations will have no effect on comparisons between nucleotides, as the same class of final state is accessed in all of them.…”

Recent advances in experimental techniques to probe fast excited-state dynamics in biological molecules in the gas phase: dynamics in nucleotides, amino acids and beyond

“…In their experiment, Yang et al  [117] were able to one-photon ionize from the sugar and the phosphate as well as the base, whereas Chatterley et al could only resonantly two-photon ionize the base. As such, in the work by Yang et al , the lowest electron binding energy in dAMP − , dTMP − and dCMP − was attributed to photodetachment from the phosphate moiety, whereas for dGMP − , the lowest binding energy site corresponds to photodetachment from the base moiety, in agreement with Chatterley et al  [109].

Figure 11.( a ) Two-photon resonant photodetachment spectra of (top–bottom) dAMP − , dTMP − , dCMP − and dGMP − , ( b ) time-resolved total electron yield of dGMP − fit to two exponential decays yielding lifetimes of less than 50 fs (red) and approximately 700 fs (blue) and ( c ) time-resolved photoelectron spectrum of dGMP − .

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“…Chatterley et al  [109] have performed experiments on the deprotonated nucleotide anions of all four DNA bases by coupling ESI with TR-PES using an experimental set-up described in detail separately [116]. In this work, dXMP − (where X is a DNA base, for example, dGMP − would be 2′deoxy-guanosine 5′-monophosphate, see figure 1 b ) was generated via ESI from an approximately 0.5 mM solution of either dXMP or its sodium salt in methanol.…”