Determination of the partial specific volume of conjugated proteins

Abstract

Specific volume quantities of macromolecules are of importance in connection with a variety of physico-chemical and biochemical techniques, such as analytical ultracentrifugation and small-angle scattering, e.g., for the determination of molecular weights in two- and multicomponent solutions. Specific volumes may be obtained from density experiments, calculations, or reliable approximations. Experimental values for the partial specific volume (¯v) of native conjugated proteins in aqueous solution vary in a wide range (0.59-1.05 cm3/g), while the values for nonconjugated proteins cluster around 0.735 cm3/g, as follows from a thorough investigation of the relevant literature. Volumes of conjugated proteins are substantially influenced by the nature and content of the nonprotein moiety. Typical glyco-, nucleo-, and metalloproteins show ¯v- values lower than those found for nonconjugated proteins, while lipoproteins exhibit high values. The ¯v-values of conjugated proteins in dilute salt or buffer solution (≤0.2 M) may be assumed to be composed additively from compositional increments. Based on the composition and assumed average values for the partial specific volumes of the main constituents of conjugated proteins (¯va=0.54, 0.61, 0.735, and 1.02 cm3/g for the nucleic acid, carbohydrate, protein, and lipid moieties, respectively), reliable calculations can be performed if some precautions are taken into account. In this context the theoretical background will be discussed, especially with respect to ultracentrifugal experiments in multicomponent solutions.