Hidden self‐association of proteins

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Abstract
Sedimentation equilibrium measurements were carried out on solution bovine serum albumin, aldolase, and ovalbumin in phosphte-buffered saline, pH7.2, at 10°C. The data obtain for each protein were analyzed to yield the dependence of apparent weight-average molecular weight upon protein concentration, over a concentration range of ca 1–200 g/L. Using the approximate theory of Chatelier and Minton ((1987) Biopolyymers 26, 507–524), models are formulated for the dependence of apparent weight-average molecular weight upon cocentration in non-ideal solutions containing proteins which may self-associate accroding to a monomer/n-mer or a monomwer/dimer/tetramer scheme. The concentration dependence data for serum albumin may be accounted for, assuming either no self-asociation or weak monomer/dimer/association. The data for aldolase may be accounted for assuming either weak monomer/dimer or weak monomer/timer association. The data for ovalbumin may be accounted for assuming eiether weak monomer/trimer or weak monomer/dimer/tetramer association. The association do not approach saturation at the hightest concentrations studied, and the standard-state free energy change accompanying self-association amount to less than 4 kcal/mol off intermolecular contacts, suggesting that non-specific clustering of protein molecules at high concentration rather than the formation of specific complexes is being observed.