Abstract
Previously we have shown that p-aminobenzoic acid (PABA) interacts with OH radicals at rates three to six times those of various radioprotective cysteamine derivatives (1, 2). In spite of this fact, PABA is without radioprotective effect in bacteria (K. F. Nakken, unpublished observations). This lack of protective ability of an excellent OH scavenger might be due to an inability of PABA to interact with radicals of low reactivity (2). In attempts to throw light on this problem, the radiation chemistry of PABA in neutral and acid solution has been studied. Evidence will be presented to show that in neutral solution the main degradation product is 3-hydroxy-4-aminobenzoic acid (3-OH-PABA). The data indicate that the degradation of PABA is due exclusively to OH radicals and that the strong radiosensitizing effect of oxygen is due to interference with the back reactions that occur under anoxic conditions and neutral pH and that repair the initial radiation damage.
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