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.

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