DISTRIBUTION ANALYSIS OF THE VARIATION OF B-FACTORS OF X-RAY CRYSTAL-STRUCTURES - TEMPERATURE AND STRUCTURAL VARIATIONS IN LYSOZYME

The B-factor (isotropic temperature factor) data for X-ray structures of hen egg-white lysozyme from the study of Young et al. (Young, Dewan , Nave, and Tilton J. Appl. Cryst. 1993, 26, 309-319) potentially cont ain information about the relative contributions of static and dynamic variation to these factors. The six structures of the protein were ob tained at two widely different temperatures (100 and 298 K), with two crystal forms (monoclinic and tetragonal) and other experimental diffe rences. In addition, the monoclinic lysozyme crystals with two molecul es per asymmetric unit allow direct examination of variation between s tructures determined under identical conditions at both temperatures. The B-factors from these structures all have complex distribution func tions as might be expected considering all of the influences that thes e values must reflect. The empirical cumulative distribution functions (eCDF's) of these data show that they are representative of complex, multicomponent distributions. Distribution analysis using the DANFIP p rocedure (Wampler, Anal. Biochemistry 1990, 186, 209-218) of the data sets reveals that they can be modeled as four to six Gaussian subpopul ations, that these subpopulations do not correlate with specific atom types, specific amino acid residues or fixed locations in the structur e. While they do seem to correlate with localized groupings of atoms, these grouping vary from structure to structure even within the same c rystal under the same conditions. Temperature seems to have a global e ffect in this case, but it is clear that other factors including exper imental error influence the distribution of B-factors within a given s tructure. This analysis also helps explain the oft observed lack of at omic level correlation between experimental B-factors and calculated m ean square displacements from molecular dynamics simulations.