The McKoy group's research focuses on the interactions of slow electrons with large molecules, and especially biomolecules such as the bases and other constituents of DNA. Low-energy electrons are known to produce single and double strand-break lesions in DNA, but the mechanisms behind this damage are not yet clear. Experimental evidence indicates that the electrons are trapped to form temporary negative ions, but major questions remain about the sites of trapping, the nature of the anion states formed, and the bond-breaking process. Are the electrons initially captured on the bases and subsequently transferred to the phosphate-sugar backbone, or do they attach directly to the backbone? Does the trapping happen in the ground electronic state, or must the electron first excite the molecule to a higher state? And which bonds actually break? Our goal is to use high-level computational methods to help answer these and related questions in electron-molecule dynamics.
To address such questions, one must solve the many-electron Schrödinger equation for the electron-molecule collision system. In this sense, what we do is very similar to the electronic structure studies of bound states that are carried out by many chemists using standard software packages like Gaussian. However, because we have a free electron, the boundary conditions on the solutions are different. To deal with the many complications that arise in dealing with an unbound electron, special-purpose programs are required. Moreover, because we are interested in large molecules such as nucleotides, we must design programs that run efficiently on parallel computer systems consisting of hundreds of processors.
For the past few years, we have doing initial studies of electron collisions with the DNA and RNA bases and with the related nucleosides and nucleotides, as well as with components of the DNA backbone. Although much remains to be done, these studies have provided useful insight into electron-capture mechanisms important at low collision energies. One surprising result was that some of the temporary ions formed by attaching electrons to the ground electronic state appear to be able to decay readily into triplet excited states, which may be one way that electrons promote disruption of DNA. Through continued program development and application, we intend to carry forward these studies to make closer connection to conditions in the living cell by considering larger moieties such as nucleotide pairs and by incorporating waters of hydration.