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

Abstract

Membrane osmometry provides a simple method to determine protein molecular weight but accuracy is limited 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, M sv/ M s = RT ϱ / A s × 1 / π + I , where M s = mass of solute, M sv = mass of solvent, R = The Universal gas constant, T= absulute temperature, ϱ = solvent density, A s = solute molecular mass, π = osmotic presute 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 nonideality parameter, I = 4.05 + 0.07, describes non-ideal curvature in the typical πV = nRT diagram better than the customary second power virial expansion requiring 3 fitting constant. Analysis of eight data sets on four proteins from the literature shows that molecular weight calculated from the slope of the new equation agress with chemical molecular weight within an RMS error of only 1.9%.