Anomalies in sedimentation. V. Chains at high fields, practical consequences

The theory of a preceding paper [B.H. Zimm, Biophys, Chem. 1 (1974) 279] is used to calculate a numerical table for the change of sedimentation coefficient with centrifugal field for chain molecules. A simple formula is found to fit the results within 1.3% up to the centrifugal field at which S/So = 0.377; this formula is S/So=(1+0.1155y2)-1/4, where y is proportional to M2/So times the centripetal acceleration, M being the molecular weight and So the sedimentation coefficient at zero acceleration. Applying this formula to DNA, we conclude that at a given centrifuge speed the sedimentation coefficient must reach a maximum at a particular molecular weight and be smaller at higher molecular weights. The value of the maximum depends on the conditions, but can come at less than 150 S for DNA under typical conditions. When a maximum is present, the profile of a sedimenting non-homogeneous band is also severely distorted.