Complexing and denaturation of DNA by methylmereuric hydroxide. II. Ultracentrifugation studies

Abstract When increasing concentrations of methylmercuric hydroxide are added to a Cs 2 SO 4 solution of native DNA, the buoyant density of DNA is unaltered until a critical concentration is reached above which there is a cooperative transition to denatured DNA which now binds so much CH 3 HgOH that it becomes very dense and nonbuoyant. As increasing concentrations of methylmercuric hydroxide are added to a Cs 2 So 4 solution of denatured DNA, the buoyant density gradually increases, indicating a gradual increase in the amount of methylmercury cation bound. The denatured DNA methylmercury complex becomes nonbuoyant at the same concentration of methylmercuric hydroxide as does the native DNA. These results support our previous interpretation that CH 3 HgOH reacts with the imino NH bonds of thymine and guanine in nucleic acids. The reaction occurs more or less independently at the different binding sites for denatured DNA, but it occurs cooperatively with simultaneous denaturation for native DNA. The nature of the transition of denatured DNA to the nonbuoyant state is not known, but it is probably due to an abrupt decrease in the degree of hydration of the DNA when its density and hydrophobic character are sufficiently increased by the binding of the methylmercury cation. Direct measurements of the amount of methylmercury bound by DNA, as observed by preparative ultracentrifugation, confirm approximately the buoyant density results as to the amount of methylmercury bound. The possibility of using methylmercuric hydroxide as a reagent for the separation of complementary strands, depending on then thymine of their thymine plus guanine content, is discussed.