Isomer-Selected Photoelectron Spectroscopy of Isolated DNA Oligonucleotides: Phosphate and Nucleobase Deprotonation at High Negative Charge States

Fractionation according to ion mobility and mass-to-charge ratio has been used to select individual isomers of deprotonated DNA oligonucleotide multianions for subsequent isomer-resolved photoelectron spectroscopy (PES) in the gas phase. Isomer-resolved PE spectra have been recorded for tetranucleotides, pentanucleotides, and hexanucleotides. These were studied primarily in their highest accessible negative charge states (3–, 4–, and 5–, respectively), as provided by electrospraying from room temperature solutions. In particular, the PE spectra obtained for pentanucleotide tetraanions show evidence for two coexisting classes of gas-phase isomeric structures. We suggest that these two classes comprise: (i) species with excess electrons localized exclusively at deprotonated phosphate backbone sites and (ii) species with at least one deprotonated base (in addition to several deprotonated phosphates). By permuting the sequence of bases in various [A_5–<i>x</i>T_<i>x</i>]^4– and [GT₄]^4– pentanucleotides, we have established that the second type of isomer is most likely to occur if the deprotonated base is located at the first or last position in the sequence. We have used a combination of molecular mechanics and semiempirical calculations together with a simple electrostatic model to explore the photodetachment mechanism underlying our photoelectron spectra. Comparison of predicted to measured photoelectron spectra suggests that a significant fraction of the detected electrons originates from the DNA bases (both deprotonated and neutral)

Probing the Influence of Size and Composition on the Photoelectron Spectra of Cadmium Chalcogenide Cluster Dianions

We have synthesized a series of compounds comprising the halide-free cadmium chalcogenide cluster dianions [Cd₈Se­(SePh)₁₆]^2–, [Cd₁₇Se₄(SePh)₂₈]^2–, [Cd₁₇S₄(SPh)₂₈]^2–, and [Cd₁₇Se₄(SPh)₂₈]^2– to study their size- and composition-dependent electronic properties free of matrix effects. Toward this end, photoelectron spectra of the isolated dianions electrosprayed from solution were recorded at several detachment photon energies. Together with quantum chemical calculations, these data reveal a systematic correlation between electronic properties such as electron affinities or repulsive Coulomb barriers and the composition/size of these semiconducting cluster complexes. We infer that the excess negative charges are localized at the apical sites of these near-tetrahedral molecules