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This paper reports the results of a systematic study of the effects of formamide and urea on the thermal stability and renaturation kinetics of DNA. Increasing concentrations of urea in the range 0 to 8 molar lower the Tm by 2.25 degrees C per molar, and decreases the renaturation rate by approximately 8 percent per molar. Increasing concentrations of formamide in the range from 0 to 50 percent lowers the Tm by 0.60 degrees C per percent formamide for sodium chloride concentrations ranging from 0.035M to 0.88M. At higher salt concentrations the dependence of Tm on percent formamide was found to be slightly greater. Increasing formamide concentration decreases the renaturation rate linearly by 1.1% per percent formamide such that the optimal rate in 50% formamide is 0.45 the optimal rate in an identical solution with no formamide. The effects of urea and formamide on the renaturation rates of DNA are explained by consideration of the viscosities of the solutions at the renaturation temperatures.
James R. Hutton (Sat,) studied this question.