Measurement of the second osmotic virial coefficient for protein solutions exhibiting monomer-dimer equilibrium

The second osmotic virial coefficient (B) is a measure of solution nonideality that is useful for predicting conditions favorable for protein crystallization and for inhibition of aggregation. Static light scattering is the technique most commonly used to determine B values, typically using protein concentrations less than 5 mg/mL. During static light scattering experiments at low protein concentrations, frequently the protein is assumed to exist either as a single nonassociating species or as a combination of assembly states independent of protein concentration. In the work described here, we examined the limit for ignoring weak reversible dimerization (K-d >= 1 mM) by comparing B values calculated with and without accounting for self-association. Light scattering effects for equilibrium dimer systems with K-d < 20 mM and K-d < 1 mM will significantly affect apparent B values measured for 20 and 150-kDa proteins, respectively. To interpret correctly light scattering data for monomer-dimer equilibrium systems, we use an expanded coefficient model to account for separate monomer-monomer (B-22), monomer-climer (B-23), and dimer-dimer (B-33) interactions. (c) 2008 Elsevier Inc. All rights reserved.