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Relationship Between Areas Under Ultracentrifugal Sedimentation Patterns and Concentration for Milk Proteins

Dairy Products Laboratory Eastern Utilization Research and Development Division, USDA, Washington, D. C.

ABSTRACT

Interpretation of ultracentrifugal results often requires estimation of concentrations from areas defined by sedimentation patterns. This is especially true in studies of unknown mixtures, such as may be produced by heating milk, and of known mixtures in which interactions may occur. Although proteins are basically similar in structure, and presumably similar in refractive index, there is need of assurance as to the limits within which constant proportionality between area and concentration may be assumed. Accordingly, many sedimentation patterns on known systems were accumulated and remeasured. Essential results are shown in Figures 1 and 3. The conclusion is that for all small molecules, and even for moderately large ones (S = 100 x 10^–13), the concentration/area factor has a single definite value within narrow limits; but that above S = 100 x 10^–13, approximately, the factor may be expected to increase with particle size. The breakdown in constant proportionality occurs at particle diameters of the order of 500Å, roughly coincident with onset of light scattering.

³ Present address: U. S. Naval Research Laboratory, Washington, D. C.