Studies on depurination of DNA by heat

Depurination by heat of DNA and DNA constituents, in solution and in the dry state, has been further investigated. The extent of depurination in solution varies inversely with the ionic strength and pH. Depurination at elevated temperatures in solution appears to be mainly an acid-catalysed hydrolysis. Studies of depurination in the dry state, in the same conditions of heating used to induce mutations in dry cells and spores, indicated that approximately 30 molecules of purine are liberated per molecule of DNA, probably by the pyrolytic breakage of the N-glycosidic bond and/or destruction of the sugar. In solution and in the dry state, the nature and the extent of depurination of DNA by heat differs from that of the constituent deoxymononucleotides. Depurination, which appears to occur throughout the entire molecule of DNA, results in most conditions in a slightly greater liberation of guanine compared with adenine. The energies of activation of depurination of DNA in solution and in the dry state were calculated. The extent of depurination was unaffected by incorporation of 5-bromouracil into the DNA. Correlations between depurination and changes in the viscosity and transforming activity of DNA and the nature and extent of heat-induced mutational sites are discussed.