An E.S.R. Study of Free Radicals Formed by Reaction of Nucleotides and Their Constituents with Hydrogen Atoms

Abstract

Cytosine, pentoses, pentose phosphates, nucleosides, and nucleotides were suspended on quartz wool by f reeze-drying and exposed to H atoms at room temperature inside an ERS [electron spin resonance] cavity. Stable free radicals were observed in every case. Spectra of sugar radicals do not change shape during the course of the reaction. Those of cytosine radicals show a loss of resolution as the exposure progresses, suggesting that side reactions involving the amino group occur. The spectra of nucleo-side and nucleotide radicals change in shape as the exposure progresses. The initial spectra resemble those of the constituent bases, whereas late spectra contain a large component of a sugar radical spectrum. Comparison of spectra induced by H atoms with those induced by deuterium atoms suggests that atoms add to the bases, abstract from the sugars, and bring about both types of reactions with the nucleosides and nucleotides. The nucleosides and nucleotides ap pear to form H atom addition products on the base moiety initially; and on further exposure to give sugar radicals by a process which depends in part on the presence of base radicals. Thus H atom attack on bases is in effect transferred to the sugar. This process may be important in bringing about damage to the polymer backbone when nucleic acids are exposed to ionizing radiation.