Isothermal compressibility of macromolecular crystals and macromolecules derived from high-pressure X-ray crystallography

The compressibility of several nucleic acid and globular protein crystals has been investigated by high-pressure macromolecular crystallography. Further, crystal structures at four different pressures allowed the determination of the intrinsic compressibility versus pressure of d(GGTATACC)₂ and hen egg-white lysozyme. For lysozyme, the values for the intrinsic molecular compressibility at atmospheric pressure and the nonlinearity index were 0.070 GPa^-1 and 8.15, respectively. On the basis of two crystal structures at atmospheric and high pressure, similar, albeit less complete, information was derived for d(CGCGAATTCGCG)₂ and bovine erythrocyte Cu,Zn superoxide dismutase. Using these data and accurate calculations of the solvent-excluded volume, the apparent solvent compressibility in the crystalline state was determined as a function of pressure and compared with results from a simple model that assumes invariant unit-cell content, with the conclusion that solvent compressibility was abnormal for three out of the five crystals investigated. Experimental results suggest that macromolecular crystals submitted to high pressure may have a variable unit-cell mass due to solvent exchange with the surrounding pool, as already observed in other hydrated crystals such as zeolites.