The effects of ionising radiation on frozen aqueous RNA: an electron paramagnetic resonance study

Abstract

Exposure of frozen aqueous solutions of various RNA samples to ⁶⁰Co γ-rays at 77 K gives well defined parallel components (g_z= 2.077) which are assigned to alkoxyl radicals (RO˙) formed from the ribose units. The formation of these radicals, presumably by electron loss from ROH units followed by very rapid proton transfer to water molecules hydrogen bonded to the –OH group, establishes that electron loss from the sugar moieties is significant. No primary sugar radicals have been detected for DNA irradiated under the same conditions. This has been explained in terms of rapid electron-donation from proximal bases (i.e., positive ‘hole’ transfer to a neighbouring base) which must also occur for RNA. For the latter, however, proton-transfer is able to compete With electron-transfer.

The major electron-gain and -loss centres have combined EPR spectra that closely resemble that for DNA. Loss is confined to the purine bases and gain to the pyrimidine bases. Unfortunately, addition of an electron to cytosine, thymine and uracil in these polymeric systems generates remarkably similar EPR doublet spectra, and it is, in our view, extremely difficult to make an accurate estimate of the relative yields of these centres by computer synthesis. In the case of DNA, the one-electron adduct of thymine is converted into the 5,6-dihydrothymine radical (˙TH) on warming to ca. 200 K. i.e., protonation at the C6 site of the pyrimidine ring. Spectra for the analogous 5,6-dihydrouracil radical (˙UH) have been recorded, but these features are completely absent on annealing irradiated frozen aqueous RNA systems. Possible reasons for this contrast are discussed.