Research Interests:

Current Research Activities:  

 

Direct Ionization Effects in DNA:  Cellular consequences of exposure to ionizing radiations include lethality, mutation, and carcinogenesis. It is generally agreed that cell nuclei, and primarily the DNA within them, are a cell’s most radiation-sensitive target.  Initially, in cells as with any material, the energy deposited by radiation creates ionized species at random.  Within the medium of cell nuclei, ionization away from the DNA produces radical ions which may diffuse to and react with it; ionization directly within the DNA may lead immediately to molecular modifications of the bases or sugar-phosphate units.  Evidently, through their effect on DNA, these energy-deposition events create DNA modifications which trigger the cellular responses. Therefore, radiation-initiated molecular modifications to DNA, through the biochemical reactions triggered by them, are central to the biological consequences of ionizing radiation.

Cells possess a wide range of enzymes which function as damage monitors and repair mediators for DNA.  However, complex damages, consisting of clustered lesions spaced by about one helical turn or less, are difficult for the repair systems to detect and manage.  If clustered damage occurs on the sugar-phosphate backbone, the result can be a double-strand break either directly or by attempted repair.  If clusters occur within the bases, the result can be misrepair leading to the possibility of mutation.  Importantly, it is now known that clustered damages, within the bases and / or sugar, are created with with very low doses of low LET radiations.  At these doses (< 1 Gy), it is highly probable that at least one  lesion in a cluster  forms by an ionization event directly within the DNA.

This work focuses on the initial modifications to DNA from energy deposited directly within it; i.e., the immediate physical and chemical modifications to the bases and the sugar-phosphate strands.  This is an important health-related issue since direct ionization of DNA and its hydration shell is estimated to cause about half the lesions critical for lethality. In addition, the probability that clustered damages depend on at least one direct ionization event makes this mechanism important for mutagenesis and carcinogenesis. 

Since the initial products of ionization are free radicals, due to a one-electron loss or gain, free-radical-initiated mechanisms must play a key role.  Therefore, DNA-centered molecular free-radical intermediates are an important link between the initial event of energy deposition and the lesions determining biological endpoints.   Consequently, it is necessary to arrive at unambiguous identification of DNA radicals whose lifetimes are sufficiently long to undergo subsequent chemical reactions, and to understand the possible reactions in which these radicals may participate as influenced by their immediate surroundings.

The molecular complexity of cellular media presents an immediate challenge for achieving the necessary molecular-level understanding directly from study of cells. For that reason, study of radiation-initiated DNA chemistry within cells is best approached by careful use of model systems.  Our approach, is to use selected, well-defined molecular systems as models and to apply the high-resolution, radical-specific methods of electron paramagnetic resonance spectroscopy (EPR) with its companion, electron-nuclear double resonance (ENDOR) spectroscopy.  These methods emphasize the use of crystals where possible, since doing so permits taking full advantage of the atomic-coordinate-level characterization of these systems provided by diffraction studies.  (The use of crystals also avoids the biochemical activity of cells and permits a clear focus on the molecular identity and chemical behavior of the initial free-radical species.)