Soaking:: the effect of osmotic shock on tetragonal lysozyme crystals

Protein crystals crack when they are soaked in a solution with ionic strength sufficiently different from the environment in which they grew. It is demonstrated for the case of tetragonal lysozyme that the forces involved and the mechanisms that lead to the formation of cracks are different for hypertonic and hypotonic soaking. Tetragonal lysozyme crystals are very sensitive to hypotonic shocks and, after a certain waiting time, cracks always appear with a characteristic pattern perpendicular to the crystallographic c axis. Conversely, a hypertonic shock is better withstood: cracks do not display any deterministic pattern, are only visible at higher differences in ionic strength and after a certain time a phenomenon of crystal reconstruction occurs and the cracks vanish. At the lattice level, the unit-cell volume expands in hypotonic shock and shrinks under hypertonic conditions. However, the compression of the unit cell is anisotropic: the c axis is compressed to a minimum, beyond which it expands despite the unit-cell volume continuing to shrink. This behaviour is a direct consequence of the positive charge that the crystals bear and the existence of channels along the crystallographic c axis. Both features are responsible for the Gibbs-Donnan effect which limits the free exchange of ions and affects the movement of water inside the channels and bound to the protein.