Small-angle X-ray and neutron scattering by biological macromolecules.

be 35.1 kJ/mol (8.4 kcal/mol) for formation of the complex and 115.9 kJ/mol(27.7 kcal/mol) for dissociation of the complex. The kinetic results, as well as helping to describe and validate the model of hyaluronate-proteoglycan interaction, are also of value in the design of experiments. In the experiment shown in Fig. 1 equilibrium is only attained after approx. 1-lfh, as monitored by light-scattering (which will not be very sensitive to the final stages of equilibration), and clearly, as formation of the complex is, presumably, a diffusion-controlled process, the rate constants determined in this study are themselves a function of the sizes of the hyaluronate and proteoglycan molecules used. Thus in many experiments in which the size of hyaluronateproteoglycan complexes or the degree of aggregatability of a proteoglycan preparation is being measured, or in which aggregatable proteoglycans are being fractionated from nonaggregatable proteoglycans on the basis of hyaluronate-binding alone, both the equilibrium and kinetic parameters discussed in the present communication will need to be taken into consideration if correct results are to be obtained. However, the major value of such equilibrium and kinetic data will, undoubtedly, lie in their ultimate contribution to our understanding of the functions of the various cartilage macromolecules. For if, for example, the dissociation constant for hyaluronate-proteoglycan in solution at physiological ionic strength, pH and temperature may be extrapolated to the context in vivo, there is little need for additional stabilization by a link protein, which re-opens the issue of the true function of link protein.