Physics Colloquium - Very low-energy electron-induced damage of DNA

We have examined theoretically and experimentally the low energy (1-25 eV) electron-induced damage of DNA oligomers.  Specifically,  we have calculated the elastic scattering of 5-30 eV electrons within the B-DNA 5'-CCGGCGCCGG-3' and A-DNA 5'-CGCGAATTCGCG-3' sequences using the separable representation of a free-space electron propagator and a curved wave multiple scattering formalism.  The disorder brought about by the surrounding water and helical base stacking leads to featureless amplitude build-up of elastically scattered electrons on the sugars and phosphate groups for all energies between 5-30 eV.  However, some constructive interference features arising from diffraction were revealed when examining the structural waters within the major groove.  These appear at 5-10, 12-18 and 22-28 eV for the B-DNA target and at 7-11, 12-18 and 18-25 eV for the A-DNA target.  Though the diffraction depends upon the base-pair sequence, the energy dependent elastic scattering features are primarily associated with the structural water molecules localized within 8-10 Å spheres surrounding the bases and/or the sugar-phosphate backbone.  The electron density build-up occurs in regions of electron attachment resonances, direct electronic excitation and dissociative ionization.  We correlated these scattering features with our measured DNA single and double strand breaks and suggested that states involving major groove waters may be important in low-energy electron induced damage of  DNA.  Compound resonance states involving interfacial water and excitation energies > 5 eV seem to be required for lethal double strand breaks,

We have also recently extended this work to excitation energies below 5 eV by examining the damage using Raman-microscopy and scanning electrostatic force microscopy.  Very efficient damage via single strand breaks is observed below 5 eV excitation energies.  This involves π* negative ion resonances that are initially localized on the bases but transferred to the σ* states of the sugar-phosphate bond.  The efficacies of these channels depend upon the base-pair sequences as well as the presence of water.  This is revealed using a novel graphene-based platform.