Sedimentation and viscosity of bacteriophage T7 DNA in presence of CH3HgOH

The effects of increasing concentartions of methylmercuric hydroxide (CH₃HgOH) on the rate of sedimentation, S⁰, and intrinsic viscosity, η], of T7 DNA have been studied at 20°C in 0.005, 0.05, and 0.5M Na₂SO₄, respectively, whereby each salt solvent conatined, in addition, 0.005M sodium borate, pH 9.18, as a buffer. Both S⁰ and η] are independent of organomercurial concentration as long as DNA remains native. Denaturation, brought about by the complexing of CH₃HgOH with the polymer, produces large changes in S⁰ as wll as η]. The sedimentation coefficient increases strongly with increasing oragnomercurial concentration once strand separation has occured. Experimental difficulties prevented measuring of η] in the posttransition region. The data on S⁰ have been used, in combination with available information on the so-called density increment (?ρ/?c₂), to obtain information on the frictional properties of single-stranded and methylmercurated T7 DNA. The frictional coefficient, defined as f′₂ = M₂(?p/?c₂)/S⁰ηN_A, where M₂ is the molecular wieght of T7 DNA, c₂ is the concentration of DNA in g/ml of solution, η_r the realtive viscosity of the salt solvents, and N_A is Avogadro's number, was evaluated for all three salt media as a function of organomercurial concentration. f′₂ of native T7 DNA was found not to be sensitive to changes in ionic strngth; but f′₂ of single-stranded and methylmercurated T7 DNA varied strongly with salt concentration. Since f′₂ of single-stranded T7 DNA was barely affected by organomercurial concentration at a given ionic strength, it is concluded that the dramatic variations of S⁰ with pM (pM ≡ -logCH₃HgOH]) observed in the posttransition zone reflect only changes in the thermodynamic interactions (“preferential interactions”) existing between DNA and the vatious other solution components, but not changes in the coil dimensions of the polymer.