Alternative procedures for equilibrium determinations in the ultracentrifuge

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Abstract
Procedures have been described which enable solute concentrations in ultracentrifugal equilibrium to be evaluated more directly than hitherto. The first two variations, quite general in application, involve allowing a layer of solvent to flow on to solution and then following the subsequent rise in concentration of solute in the region just below the meniscus. The observations are made with Rayleigh interference optics. These methods have been tested with substances ranging in size from sucrose to γ‐globulin, and good agreement obtained with accepted molecular weights. Alternative procedures, which depend on increasing the rotor speed after equilibrium is reached, and observing the fall in concentration at the meniscus, are more critically dependent on base‐line reproducibility, but a satisfactory method has been found which is applicable to proteins of molecular weight down to 10,000 or even lower. The validity of this method has been confirmed by means of albumin and γ‐globulin. Not only have procedures for obtaining molecular weights been simplified, but the new techniques can be operated so as to reduce the time taken to reach equilibrium.

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