DIFFUSE-SCATTERING IN ELECTRON-DIFFRACTION DATA FROM PROTEIN CRYSTALS

Diffuse background present in electron diffraction patterns from prote in crystals can produce significant systematic error in intensity meas urements. In certain regions of the diffraction pattern, this can even result in systematically negative intensity measurements. Thus, it is important to remove the diffuse background to produce more accurate m easurement of structure factor amplitudes. The variable part of the co ntinuous background arises from disorder in the crystal with an intens ity which depends on the type of disorder. We show how the diffuse sca ttering can be calculated from a model for the disorder leading to a c orrection of intensities measured from diffraction patterns. For cryst alline bacteriorhodopsin (bR) in native p3 (a = 62.45 Angstrom) purple membrane the best correction for several models tested is obtained by assuming that the disorder comes from random displacement of the trim er of bR molecules as a rigid unit. The correction was applied to inte nsity measurements using improved error estimates based on the experim ental errors. The number of intensities previously measured wrongly to be negative is about 10%. This is reduced by 4-fold to be in agreemen t with that expected from counting statistics. The final corrected amp litudes gave a lower R-factor when used for refinement of an atomic mo del.