Molecular weights of homogeneous coliphage DNA's from density-gradient sedimentation equilibrium

Abstract

The problem of obtaining meaningful molecular weights for high molecular weight DNA samples by sedimentation equilibrium in a density gradient has been solved by correcting the theory for thermodynamic non-ideality. Several different methods of virial corrections are suggested and examined to determine the best method of obtaining molecular weights at infinite dilution. A pronounced virial effect is experimentally demonstrated for high molecular weight DNA's. The concentration distributions at sedimentation equilibrium are studied as a function of DNA concentration for T7, T5 and T4 coliphage DNA's and the results extrapolated to infinite dilution to obtain molecular weights for these DNA's. The values obtained by this technique are 23.2 × 10 6 for T7, 65.7 × 10 6 for T5 and 104 × 10 6 daltons for T4 DNA. These results are in agreement with recent hydrodynamic predictions but are in disagreement with many literature values. A brief review of the literature is presented to elucidate these discrepancies.