Diffuse X-ray scattering from protein crystals provides information about molecular flexibility and packing irregularities. Here we analyse diffraction patterns from insulin crystals that show two types of scattering related to disorder: very diffuse, liquid-like diffraction, and haloes around the Bragg reflections. The haloes are due to coupled displacements of neighbouring molecules in the lattice, and the very diffuse scattering results from variations in atomic positions that are only locally correlated within each molecule. The measured intensity was digitally separated into three components: the Bragg reflections and associated haloes; the water and Compton scattering; and the scattering attributed to internal protein movements. We extend methods used to analyse disorder in membrane structures to simulate the diffuse scattering from crystalline insulin in terms of (1) the Patterson (autocorrelation) function of the ideal, ordered crystal structure, (2) the root-mean-square (r.m.s.) amplitude of the atomic movements, and (3) the mean distance over which these displacements are coupled. Movements of the atoms within the molecules, with r.m.s. amplitudes of 0.4-0.45 A, appear to be coupled over a range of approximately 6 A, as in a liquid. These locally coupled movements account for most of the disorder in the crystal. Also, the protein molecules, as a whole, jiggle in the lattice with r.m.s. amplitudes of approximately 0.25 A that appear to be significantly correlated only between nearest neighbours.

Different crystal forms of bovine pancreatic ribonuclease A and hen egg white lysozyme, 2Zn insulin, 4Zn insulin and crystals of concanavalin A were examined under controlled environmental humidity in the relative humidity (r.h.) range of 100 to 75%. Many of them, but not all, undergo reversible structural transformations as evidenced by discontinuous changes in the diffraction pattern, the unit-cell dimensions and the solvent content. Tetragonal, orthorhombic and monoclinic lysozyme and a new crystal form of ribonuclease A show transformations at r.h.'s above 90%. Monoclinic lysozyme transforms at low r.h. to another monoclinic form with nearly half the original cell volume. The well known monoclinic form of ribonuclease A grown from aqueous ethanol solution undergoes two transformations while the same form grown from 2-methyl-2,4-pentanediol (MPD) solution in phosphate buffer does not transform at all. Soaking experiments involving alcohol solutions demonstrate that MPD has the effect of decreasing the r.h. at which the transformation occurs. Triclinic lysozyme, 2Zn insulin, 4Zn insulin and the crystals of cancanavalin A do not transform in the 100 to 75% r.h. range before losing crystallinity. The results obtained so far indicate that the crystal structure has a definite influence on water-mediated transformations. The transformations do not appear to depend critically on the amount of solvent in the crystals but the r.h. at which they occur is influenced by the composition of the solvent. The transformations appear to involve changes in crystal packing as well as conformational transitions in protein molecules. The present investigations and other related studies suggest that water-mediated transformations in protein crystals could be very useful in 0108-7681/85/060431-06501.50 exploring conformational transitions in and the hydration of proteins.

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