Some aspects of the mechanism of protection of irradiated DNA by cysteamine were investigated by using ESR spectroscopy. Solutions of nucleic acids and their constituents with 10% by weight of cysteamine hydrochloride were dried, evacuated, and irradiated with X-rays at 100°K. The ESR spectra of the samples were studied as a function of increasing annealing temperature. In the native DNA system the intensity of the organosulfur radical at about room temperature reached thirteen times that expected from the cysteamine present. Similarly, high transfer ratios were observed for denatured DNA, dTMP, dUMP, ribose-5′-phosphate, and deoxyribose-5′-phosphate. About one quarter of this transfer ratio was found for RNA and UMP. Very little or no transfer was found for thymidine, thymine, dAMP, dGMP, dCMP, 5-methyl dCMP, and deoxyribose. The organo-sulfur spectrum disappeared at the melting point of cysteamine hydrochloride in all samples in which it developed except for native DNA, where the signal persisted at least to 373°K. It is postulated that the cysteamine binds through its amino group to the phosphate group in the nucleotide and that transfer of spins to the sulfur atom is facilitated by a 2,4-diketo structure in the base as present in thymine and uracil and by a deoxyribose sugar structure.

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