X-ray diffuse scattering and rigid-body motion in crystalline lysozyme probed by molecular dynamics simulation.

Rigid-body motions are determined from a 1 ns molecular dynamics simulation of the unit cell of orthorhombic hen egg-white lysozyme and their contribution to X-ray diffuse scattering intensities are examined. Using a dynamical cluster technique, groups of backbone atoms that move as approximately rigid bodies are derived from the intramolecular interatomic fluctuation matrix. These groups tend to be local in the sequence or connected by disulphide bonds, and contain on average five residues each, X-ray diffuse scattering patterns, which are sensitive to collective motions, are calculated from the full simulation trajectory (including all the protein degrees of freedom). The results reproduce the main features of the experimental scattering. Diffuse scattering is also calculated from fitted trajectories of the rigid bodies. The full simulation diffuse scattering and atomic displacements are found to be well reproduced by a model in which the backbone atoms form the rigid groups determined using the dynamical cluster technique and the individual side-chains behave as separate rigid bodies: the resulting R-factor with the full simulation scattering is 5%. Quantitatively poorer agreement is obtained from trajectories in which the secondary structural elements of the protein are considered rigid. Rigid whole-molecule and domain motions make only minor contributions to the protein atom displacements. Finally, correlations in the interatomic fluctuations are examined directly using a canonical method.