No abstract

Two oxidation products and one reduction product were identified .in single crystals of tyrosine Hel irradiated at 4.2"K. Both oxidation products are associated with the loss of an electron from the phenol ring of tyrosine. The reduction product is the result of electron addition to the carboxyl group. The identification of these species followed from an analysis of proton hyperfine couplings measured by the ENDOR technique.

Previously, double lesions in which two adjacent bases are modified were identified in DNA oligomers exposed in solution to ionizing radiation. However, the formation of such lesions in polymer DNA had not been demonstrated. Using reference oligomer containing a specific double lesion and employing liquid chromatography-mass spectrometry (LC-MS), it was possible to show directly that double lesions are formed in irradiated calf thymus DNA. The double lesion in which a pyrimidine base is degraded to a formamido remnant and an adjacent guanine base is oxidized to 8-oxoguanine was detected in DNA X-irradiated in oxygenated aqueous solution. The double lesion in which the methyl carbon atom of a thymine base is covalently linked to carbon at the 8-position of an adjacent guanine base was detected in DNA irradiated in a deoxygenated environment.

Evidence is presented for the formation of products in irradiated dinucleoside monophosphates in which both bases are damaged. The dinucleoside monophosphates d(GpT), d(GpC), d(TpG) and d(CpG) were X-irradiated in oxygenated aqueous solution. Product identification was by NMR spectroscopy. In products containing double base lesions, guanine is converted to 8-hydroxyguanine and the pyrimidine base is degraded to a formamido remnant.

The electrons trapped in single crystals of rhamnose X-irradiated at low temperature were studied by ENDOR spectroscopy. Hyperfine couplings of protons in the environs of the electron have been determined from ENDOR measurements, including those of some of the more remote carbon-bound hydrogen atoms. The likely site of electron trapping in the crystal structure of rhamnose was inferred from calculations of the electric potential generated by the dipoles of hydroxy groups about preexisting void spaces. Electron-proton distances for nonexchangeable hydrogen atoms from points within the void were calculated from the crystal structure and compared with distances obtained from hyperfine couplings. Good agreement was obtained between experimental and calculated values.