Determination of Absolute Molecular Weights Using Sedimentation Equilibrium Analytical Ultracentrifugation

One of the most fundamental parameters describing a biological macromolecule is its mol wt, M (unit g/mol), or equivalently the dimensionless “relative molecular mass,” M _r . Despite this, it is not always easy to determine or, indeed, for a heterogeneous system, define. With a homogeneous system, the simplest procedure is to calculate it directly from the chemical formula-for example, for a protein whose amino acid composition or sequence is known. In most cases, however, this simple route is not possible; the protein may be glycosylated to an extent that may be difficult to establish precisely, for example, or it may self-associate in solution. Also, the macromolecular system itself may be polydisperse , i.e., contain a range of macromolecular species of different mol wt, and in these systems, it is of value to determine the various types of average mol wt (usually the number, weight, or z average) or the mol-wt distribution . If the system is self-associating, one may also be interested in evaluating the association constant(s).