The Influence of Added H₂ O and D₂ O on Anoxic Radiation Sensitivity in Bacterial Spores

With careful control of water vapor pressure for equilibrating dried spores of Bacillus megaterium, changes were measured in the <tex-math>${\rm O}_{2}\text{-independent}$</tex-math> component (<tex-math>$k_{{\rm I}}$</tex-math>) of radiation damage over the narrow range in which water increase causes a rise in the inactivation constant. The measures were made for both H₂ O and D₂ O as added water. For added H₂ O, <tex-math>$k_{{\rm I}}$</tex-math> begins to increase at a water content below and is complete at a content above that which represents a monolayer in the cell (1.2 molecules <tex-math>${\rm H}_{2}{\rm O}/{\rm DNA}$</tex-math> phosphate); for D₂ O the behavior is the same but quantitatively different (1.3 molecules <tex-math>${\rm D}_{2}{\rm O}/\text{phosphate}$</tex-math>). This suggests that sensitization is caused by water radiolysis in a water shell immediately external to the target molecule, and the fact that in D₂ O the spores are slightly more sensitive than in H₂ O points to the hydroxyl radical as the important species. This supports other studies that indicate that ·OH species is important in anoxic radiation-induced damage to these cells.