Ultracentrifuge studies with a synthetic boundary cell. I. General applications

This report presents some general applications of a new type of ultracentrifuge cell, called a synthetic boundary cell, in which a sharp, stable boundary is obtained by layering one solution over another more dense solution while the ultracentrifuge is in operation. Since such a boundary between a solvent and a dilute solution of a low molecular weight solute can be formed in the center of the cell, the complications encountered at the meniscus in conventional ultracentrifugation by the high diffusion and low sedimentation rate of the solute are obviated. By this technique the sedimentation velocity method has been extended to substances with molecular weights as low as 350. Sedimentation constants of a number of low molecular weight materials such as fraction A of insulin (molecular weight 3000), vitamin B 12 (molecular weight about 1500), β‐dextrin (molecular weight 1134), and sucrose (molecular weight 342) have been determined. The cell has also been used to form a boundary between solutions of two different concentrations of the same macromolecule and differential sedimentation constants have been obtained. The relationship between these differential sedimentation constants and those obtained for either of the two solutions when studied alone is discussed. Application of the cell to the determination of hydrodynamic volumes of sedimenting components is described. Measurements are made of the backward flow of solvent by the use of an indicator boundary formed by layering a virus solution over another virus solution containing an additional component, such as sucrose or a protein, which acts as an indicator. In addition, the cell was used in a preliminary investigation of some fundamental problems in ultracentrifugation such as the sedimentation of slowly sedimenting materials in the presence of more rapidly sedimenting components, the analysis of equilibrium systems such as insulin and chymotrypsin, and the role of convection.