Ultracentrifuge studies of interactions and equilibria: impact of interactive computer modelling.

molecular weight. Combination of heterogeneity and nonideality functions has proved a very recalcitrant problem (Williams, 1972). Although no easy solution to this problem emerges, the values of M and u (log M), always regarded as underestimates, do have comparative value. There is, however, an alternative representation of the heterogeneity: in terms of an isodesmic association (Kim et al., 1977). Although the concentrationdependence of s or M for these materials does not immediately indicate such an association, the extreme curvature near the base in sedimentation equilibrium (Creeth & Knight, 1968), together with the absence of a plateau in sedimentation velocity (Creeth & Knight, 1967), are suggestive features, and make the possibility worth examining. A striking property of isodesmic association in the ultracentrifuge (in certain concentration ranges) is that, as the total concentration rises, the distribution of monomer and low polymers may be little affected, whereas the concentration of the higher polymers, detectable only near the cell-base, is greatly increased (Table 1). Clearly, if observation is restricted to, say, 95% of the total column length, the very steep rise near the base would be undetected, and the characteristic increase in M t could well remain undetected. Experimental verification of the association has so far been obtained only with two samples, but this is sufficient [together with the results reported by Hill et al. (1977) for a glycoprotein of simpler structure1 to suggest that the gel-forming properties of these materials are a simple consequence of well-defined thermodynamic properties observable at all concentrations. Such a conclusion would have important physiological implications, for these glycoproteins are intimately concerned with protection of all mucous membranes.