Smudging Genetic Blueprints

High-energy radiation is notorious for damaging DNA, primarily by breaking chemical bonds. Damage to DNA can cause mutations, cancer, or even death. Much of this damage is inflicted by secondary, or low-energy, electrons knocked out of atoms in the DNA molecules by radiation. The low-energy electrons get captured by the DNA bases (which make up the letters of the genetic code), temporarily forming a negatively charged molecule (anion). The anion lasts just long enough to transfer its excess energy to the weakest nearby chemical bond, often breaking it.

In DNA, the weakest link is the carbon-oxygen bond between the sugar and phosphate groups that form its backbone. Low-energy electrons can cause breaks in one or both strands of DNA. Although the letters of the code remain undamaged, these structural breaks increase the likelihood that the code will be misread and permanent, and potentially catastrophic, biological changes will ensue.

Recently, Graduate Student Stefano Tonzani and Fellow Chris Greene decided they'd like to better understand the physics of the capture of low-energy electrons by DNA bases. According to Tonzani, the bases capture the electrons because their carbon-nitrogen ring structures include empty orbitals that energetically favor low-energy electrons. Orbitals are atomic energy states that can hold up to two electrons. The ring structure of one DNA base, adenine, is shown at right.

The researchers performed a theoretical analysis of the electron resonances within the bases at the moment of electron capture. They discovered that the electrons mostly stay inside the ring structure of the bases, as shown at right. The darkest blue and darkest red areas indicate the locations where the extra electron is most likely to be. From the diagram, it's clear the electrons don't pile up on the hydrogen atoms. If they did, then a negatively charged hydrogen atom could break off the ring (or the sugar backbone), and this doesn't appear to happen.

The new analysis also clarified that the electron resonance created during electron capture is a narrow type of resonance that easily lasts long enough in living cells to lead to bond breaking. This type of resonance, called a Π resonance, is shaped like two identical cocoons, one above and one below the (flat) plane of each atom in a DNA base.

Even with this new understanding of low-energy electron capture, there's still much to be learned about radiation damage to DNA. Physicists still must determine how the captured electrons couple to another type of resonance, called a Σ resonance. Σ resonances allow captured electrons to leave the ring structure and transfer their energy to the DNA backbone.

The research reported here is scheduled for publication in the Journal of Chemical Physics in January of 2006.   - Julie Phillips