Some effects of X-irradiation on the melting temperature of DNA bound with normal aliphatic diamines, cystamine, and β,β′-diaminodiethylsulfide have been examined. The Tm decreased linearly with dose for doses below 180 krads, the gradient being used as a measure of the protective capacities of the various binding agents. Some protection of the DNA against denaturation arose from the physical presence of the bound diamines, since the aliphatic diamines diaminohexane through diaminodecane gave significant protection. A chain-length dependence of this protection was observed, the maximum effect occurring with diaminononane. In situ radical scavenging was also effective. Cystamine was not found to be a better protector than β,β′-diaminodiethylsulfide or diaminononane. Thus these effects are not sufficient to account for the differences in the capacities of the compounds to protect in vivo systems against ionizing radiation, and this implies that further factors, possibly more biological in nature, are involved in in vivo radioprotection.

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