Density-contrast studies of macromolecules in solution. Cross verification of X-ray scattering and pycnometry experiments

The density-contrast method, commonly used in X-ray (and neutron) small-angle scattering studies of macromolecules in solution, can yield a wealth of information if; (a) the solute is monodisperse and the solution is ideal: (b) each macromolecule in solution has a volume associated with it inside of which the density distribution is independent of the density of the solvent. More specifically it has been pointed out that in this case an expression of the molecular weight can be obtained which does not involve the partial specific volume, provided that the X-ray experiments are performed on an absolute scale. On the other hand, it is well known that X-ray scattering experiments on an ideal solution of identical macromolecules can yield the molecular weight for any composition of the solvent, provided solute, solvent and partial specific volumes are defined and measured at constant chemical potential. Therefore a combination of X-ray scattering and densimetry experiments allows one to verify to what extent condition (b) holds true. Such a set of experiments has been performed on Rhesus monkey low-density serum lipoproteins, the solvent being water containing variable amounts of NaBr. It is concluded that in this case condition (b) is fulfilled. Moreover several important aspects of the analysis of the X-ray scattering curves can be verified.