Attosecond Electron Delocalization in the Conduction Band through the Phosphate Backbone of Genomic DNA

Partial density of states in the empty conduction band of the phosphate backbone sites in DNA was probed using energy-dependent resonant Auger spectroscopy. Results show that genomic DNA with periodic backbones exhibits an extended state despite separation of each phosphate group by an insulating sugar group. In antisense DNA with an aperiodic backbone, the equivalent state is localized. Remarkably rapid electron delocalization occurs at ca. 740 attoseconds for wet DNA, as estimated using the core-hole clock method. Such delocalization is comparable to the Fermi velocity of carbon nanotubes.

Figure 1

(a) A phosphorus $1s$ core electron is excited into an unoccupied state in the conduction band. (b) Spectator Auger final state ($2h1e$) caused by localization of the electron to the core-hole site. (c) Normal Auger final state ($2h$) caused by delocalization of the electron to the conduction band coupling to the continuum.

Figure 2

Schematic representation of phosphodiester and phosphorothioate linkages.

Figure 3

(a) A typical P $KLL$ resonant Auger spectrum of dry DNA at 2152.9 eV. Spectator Auger ($2h1e$, red line) and normal Auger ($2h$, blue line) features are visible. The dots and the solid lines, respectively, represent the experimental data and a least-squares fit. A $\mathrm{C}1s$ photoemission peak is also visible. (b) Integrated intensities of spectator (open circles) and normal (open squares) Auger components near the P $K$ edge. Results of curve fitting for $2h1e$ and $2h$ spectra are shown, respectively, as red and blue lines. For dry and wet DNA, the $2h$ cross sections were decomposed into a Gaussian function (green line) for the conduction band and a Gaussian error function (yellow line) for the continuum. In PS-oligo, the $2h$ cross section was fitted by a Gaussian error function (blue line). The sum of $2h1e$ and $2h$ cross sections is shown as closed circles, which is similar to $\mathrm{P}1s$ XAS (black line).

Figure 4

Excitation energy dependence of electron-delocalization time for dry (circles) and wet (squares) DNA. The ionization time for PS-oligo is shown as green triangles. Error bars show $\pm 1\sigma$. Each solid line is obtained by an exponential fit.