DESCRIPTION: The DNA within a chromosome is an important target in understanding the cellular effects of ionizing radiation. The types and quantities of radiation-induced damage that are produced in DNA result from th reactions of radicals generated from the direct ionization of the DNA (direct effect) and from the reactions of the DNA with free radicals formed in the surrounding environment. Within this environment, irradiation of bulk water an the water molecules that are tightly bound to DNA contribute to a large proportion of DNA damage. Evidence suggests that these tightly bound water molecules can, 1) comprise a large portion of the water that surrounds intracellular DNA, and 2) contribute to radiation-induced DNA damage primarily via the transfer of electron-deficient centers and electrons from this water layer to the DNA (quasi-direct effect); a mechanism that is different from the induction of DNA damage that is derived from the irradiation of bulk water (e.g. HO.,eaq-). The proportion of DNA damage that is derived from the quasi-direct effect and the irradiation of bulk water will vary depending on changes in DNA conformation, alteration of hydration water, and radical scavenging effects that can be mediated through the binding of metals, polyamines, and histone proteins to DNA. Since the mechanism for damaging DNA is different for the hydration water than for bulk water, interventional strategies that exclusively rely on scavenging hydroxyl radicals to modify DNA damage may have limited success in cells. It is the objective of this proposal to determine the mechanism and extent to which the water that is tightly bound to the DNA contributes to radiation-induced DNA damage. The hypothesis to be tested is that radiation-induced damage in these tightly bound water molecules can result in DNA damage that is indistinguishable from that caused by the direct ionization of DNA and modifiable by thiols. The types and quantities of radiation-induced DNA damage as a function of dose, hydration, oxygen concentration will be examined to ascertain the effect of that the binding of metals, polyamines, an histone proteins, in additional to the radioprotective effects of thiols, has on the contribution of the direct/quasi-direct effect and irradiated bilk wate on radiation-induced DNA damage. Information derived from these studies may be important for understanding the mechanisms by which ionizing radiation causes cancer, mutations, and cell lethality, and selecting or designing new radioprotectors that may reduce the deleterious effects of radiation or improv the outcome of radiotherapy.