DEOXYRIBONUCLEATE SOLUTIONS - SEDIMENTATION IN A DENSITY GRADIENT PARTIAL SPECIFIC VOLUMES DENSITY AND REFRACTIVE INDEX INCREMENTS AND PREFERENTIAL INTERACTIONS

Density increments (∂ρ/∂c2)°μ in solutions of NaDNA in NaCl and CsDNA in CsCl were determined over a wide range of salt concentrations; calf thymus DNA, fragmented by sonic irradiation to a molecular weight of 4–6 × 105 was used. The partial specific volume v̄2° of NaDNA at 25°C was found to ho 0.500 ml/g in water, and that of CsDNA 0.440 ml/g. Both values increase with increasing NaCl and CsCl concentration. Refractive index increments under various experimental conditions were also determined. The relevance of the density increments (at constant, chemical potential of diffusible solutes) to equilibrium sedimentation in a density gradient and the evaluation of molecular weights is discussed. Distribution coefficients of diffusible components, sometimes referred to as preferential solvation or net hydration, were derived from the density increments and partial volumes and compared with direct experimental results, whenever available, from membrane distribution and isopiestic distillation. The thermo‐dynamic significance of the distribution coefficients as well as possible interpretations in terms of specific molecular mechanisms are considered. Copyright © 1968 John Wiley & Sons, Inc.