A STUDY OF AQUEOUS SOLUTIONS OF NUCLEIC ACID CONSTITUENTS EXPOSED TO MONOCHROMATIC 160 nm VACUUM-UV LIGHT BY SPIN-TRAPPING METHOD

Spin-trapping technique was employed to detect and identify free radical intermediates produced in aqueous solutions of nucleic acid constituents (pyrimidine bases and pyrimidine nucleosides) after irradiation by monochromatic 160 nm vacuum-UV light from the electron storage ring. Short-lived free radicals produced in the molecules were converted into relatively long-lived free radicals (spin-adducts) by the reaction with MNP used as a spin trap. The resulting nitroxide radicals were subsequently analysed by esr.
The esr measurements were made after irradiating H₂O solution containing sample and the spin trap and once freeze-drying it and re-dissolving the residual powder in D₂O to get the spectrum with a well-resolved hyperfine structure. Thus, clear evidence that most of the radicals were not formed by H-addition but formed by OH-addition at the C5 position of the 5, 6 double bond were obtained for pyrimidine bases. For pyrimidine nucleosides, although the effect of H₂O-D₂O exchange was not recognized on resolution improvement of the hyperfine structure of the esr spectra, careful analysis of the hyperfine structure made it possible to identify the radical structures; OH-addition radicals at the C6 of the double bond of the base moiety in addition to the OH-addition radicals at the C5 position for all except for 2-deoxycytidine. Evidence for the formation of free radicals at the sugar moiety was not clear.