Diffraction analysis of motion in proteins

Analysis of the x-ray or neutron diffraction from cyrstals gives only an average picture of the contents of a crystallographic unit cell, the average being over a long time-(usually may hours) and the entire lattice. However, this picture can rather accurately define not only the mean atomic positions but also the distribution of displacements from rest positions. These displacements may include components from three main sources: (a) thermal vibration, both of individual atoms and of rigid groups; (b) dynamic disordering among thermally accessible conformational states; and (c) static variations among the structures within different unit cells, either due to lattice imperfections or because of conformational heterogeneity that is frozen in at the experimental temperature. Thus the accumulated spoor of atomic motions is accessible to the diffraction experiment even though the dynamic pathways are not.