Electron spin resonance studies have been performed on systems of DNA and β-mercaptoethylamine (cysteamine). The samples were studied after lyophilization and were exposed to x-rays and observed in the spectrometer at near liquid nitrogen temperatures. Spectra were recorded at this temperature and after the samples had been annealed. The transfer of free radicals from the DNA to cysteamine as a function of cysteamine content, pH, and pyrimidine content of the DNA was studied. The following general observations were made. (1) The number of spins transferred per cysteamine molecule remained constant in the DNA-cysteamine system when 8.5% or less cysteamine was added. (2) The transfer was also observed to be a linear function of the thymine content of partially depyrimidinated DNA. These results permitted an estimate to be made of the migration range of those DNA free radicals which ultimately were transferred to the thiol. (3) Lowering the pH of the system reduced the transfer of spins to cysteamine. The effect of pH on the intramolecular migration of spins in DNA samples was studied. It was observed that as the pH was lowered the annealed spectrum became increasingly similar to the spectrum of irradiated thymine. At pH ≦ 2.0 the samples carbonized during lyophilization and stable free radicals were observed in the unirradiated samples.

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