Method to improve the accuracy of membrane osmometry measures of protein molecular weight

Membrane osmometry provides a simple method to determine protein molecular weight but accuracy is limited by nonideal behavior. Recent studies (Fullerton et al., Biochem. Cell Biol., in press) show that non-ideal osmotic response of protein solutions is described by the empirical equation, Msv/M(s) = RT rho/A(s) x 1/II+I, where M(s) = mass of solute, Msv = mass of solvent, R = the Universal gas constant, T = absolute temperature, rho = solvent density, A(s) = solute molecular mass, II = osmotic pressure, and I = the nonideality parameter. This linear relation is used in this paper to demonstrate that measurement of molecular weight from the slope simplifies such measures and improves the accuracy relative to classical methods. The molecular weight of bovine serum albumin is measured with error less than 0.9%. The single dimensionless non-ideality parameter, I = 4.05 + 0.07, describes non-ideal curvature in the typical IIV = nRT diagram better than the customary second power viral expansion requiring 3 fitting constants. Analysis of eight data sets on four proteins from the literature shows that molecular weight calculated from the slope of the new equation agrees with chemical molecular weight within an RMS error of only 1.9%.