Re: [ccp4bb] [QUAR] Re: [ccp4bb] embarrassingly simple MAD phasing question (another)

[Bart Hazes]

On 10-10-14 01:34 PM, Ethan Merritt wrote: ... The contribution from normal scattering, f0, is strong at low resolution but becomes weaker as the scattering angle increases. The contribution from anomalous scattering, f' + f", is constant at all scattering angles. ... My simple/simplistic mental picture for this is that electrons form a cloud surrounding the atom's nucleus. The larger the diameter of the cloud the more strongly the atomic scattering factor decreases with resolution (just like increased B-factors spread out the electrons and reduce scattering). Anomalous scattering is based on the inner electron orbitals that are much closer to the nucleus and thus their scattering declines more slowly with resolution. By this reasoning f' and f" would still decline with resolution but perhaps the difference is so substantial that within the resolution ranges we work with they can be considered constant. By the same reasoning you'd expect neutron diffraction to have scattering factors that are for all practical purposes independent of resolution, assuming b-factors of zero. In addition, the different fall-off in the scattering factors for f0 and f' or f" will be much less noticeable for anomalous scatters with high B-values where the latter dominates the 3D distribution of the electrons. Bart ============================================================================ Bart Hazes (Associate Professor) Dept. of Medical Microbiology & Immunology University of Alberta 1-15 Medical Sciences Building Edmonton, Alberta Canada, T6G 2H7 phone: 1-780-492-0042 fax: 1-780-492-7521 ============================================================================