The interaction of low-molecular-weight thiols with sonicated DNA was examined using spin filtration to concentrate the DNA. Cationic thiols (WR 1065 and cysteamine) behaved as counterions and were found to have increased concentrations in the DNA retentate relative to the filtrate. Anionic thiols (GSH, 2-mercaptoethanesulfonate, mercaptosuccinate) behaved as coions and were decreased in concentration in the DNA fraction. Concentrations of the uncharged thiol 2-mercaptoethanol were little influenced by DNA. The results demonstrate the importance of counterion condensation and coion depletion in determining the concentrations of charged species near DNA. They provide a rationale for enhanced effectiveness of WR 1065 and cysteamine as radioprotectors compared to neutral and anionic thiols and suggest that anionic thiols such as GSH should be poor radioprotectors of DNA.

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