Modeling the Correlation betweenZandBin an X-ray Crystal Structure Refinement

Abstract

AbstractWe have examined how the refinedB-factor changes as a function ofZ(the atomic number of a scatterer) at the sulfur site of the [4Fe:4S] cluster of the nitrogenase iron protein by refinement. A simple model is developed that quantitatively captures the observed relationship betweenZandB, based on a Gaussian electron density distribution with a constant electron density at the position of the scatterer. From this analysis, the fractional changes inBandZare found to be similar. The utility ofB-factor refinement to potentially distinguish atom types reflects theZdependence of X-ray atomic scattering factors; the weaker dependence of electron atomic scattering factors onZimplies that distinctions between refined values ofBin an electron scattering structure will be less sensitive to the atomic identity of a scatterer than for the case with X-ray-diffraction. This behavior provides an example of the complementary information that can be extracted from different types of scattering studies.