Interest of the normalized second virial coefficient and interaction potentials for crystallizing large macromolecules

It has been shown for several years that the second virial coefficient, A₂, can be helpfully used to describe the thermodynamic behavior of biological macromolecules in solution prior to crystallization. The coefficient, which reflects either repulsive or attractive interactions between particles, can allow a rapid determination of crystallization conditions. Different biological systems, from 14 kDa to 4600 kDa, were studied by small angle X-ray scattering. With large macromolecules, the A₂ values were found at the low end of the crystallization slot described by George & Wilson [(1994) Acta Cryst. D50, 361-365]. This led us to investigate the physical meaning of the second virial coefficient and to propose the use of the dimensionless second virial coefficient independent of the molecular weight and the size of the particle, which only takes into account the interaction potential between macromolecules, to predict successful crystallization conditions for large macromolecules. With this normalized coefficient (a₂), the effect of salt on small proteins becomes equivalent to the effect of PEG on large macromolecules in terms of interaction potentials.