Accuracy of molecular mass determination of proteins in solution by small-angle X-ray scattering

One of the most important overall parameters, which can be derived from smallangle X-ray scattering (SAXS) experiments on macromolecular solutions is the molecular mass (MM) of the solute. In particular, for a monodisperse protein solution, MM of the solute is calculated from the extrapolated scattering intensity at zero angle I(0). Assessing MM by SAXS provides valuable information about the oligomeric state and absence of unspecific aggregation in solution. The value of MM can either be estimated by comparison with a protein standard with a known MM or by determining the absolute scattering intensity using, e.g., water scattering. In both cases, knowledge about the solute concentration and about the partial specific volume of the protein is required. By measuring 13 well characterized globular proteins with MMs ranging from 13.7 to 669 kDa we analyze the sources of possible systematic deviations and assess the accuracy of MM determination using SAXS. The data indicate that all these proteins have approximately the same ‘effective’ value of the partial specific volume of about 0.7425 cm g . It is shown that both inter-protein and water calibration can be used for molecular mass determination by SAXS and in most cases the errors do not exceed 10%.