Electron spin resonance and intermediate neglect of differential overlap molecular orbital study of the .pi. cations of (hydroxymethly)uracil and (hydroxymethyl)cytosine. Evidence for internal hydrogen bonding

Abstract

The ..pi..-cation radicals of 5-(hydroxymethyl)uracil (HOMeU) and 5-(hydroxymethyl)cytosine (HOMeC) have been produced by Cl/sub 2//sup -/ attack in ..gamma..-irradiated basic 12 M LiCl and photoionization in basic 8 M NaClO/sub 4/ glasses at low temperatures. Analysis of the ESR spectra found for these radicals shows that each of the ..pi..-cation radicals converts to another species probably by a change in a nitrogen protonation state as the temperature is raised. For example, 5-(hydroxymethyl)uracil cation shows a 28.5-G average splitting for the two hydroxymethyl-group ..beta.. protons which convert upon annealing to a 36-G splitting. The splittings and the narrowness of the line widths found after annealing are suggestive of a configuration which is intramolecularly rigid and stabilized by a intramolecular hydrogen bond from the hydroxyl proton to the 4-position oxygen. The ..pi.. cation of 5-(hydroxymethyl)cytosine converts to a radical with substantial spin density on the exocyclic nitrogen which again shows strong evidence for intramolecular hydrogen bonding. Final radicals are found to be produced from the ..pi..-cation radicals of (hydroxymethyl)uracil and (hydroxymethyl)cytosine by deprotonation of a methylene proton. INDO calculations for the ..pi.. cation of (hydroxymethyl)uracil as a function of orientation of the hydroxyl group show that the hydrogen bond to the 4-positionmore » oxygen increases in strength by a factor of 3 upon deprotonation at the 3-position nitrogen. The hydrogen bond is therefore predicted to substantially stabilize the ..pi..-cation radical.« less