PHASES_ A Program Package for the Processing and Analysis of

Document Sample
PHASES_ A Program Package for the Processing and Analysis of Powered By Docstoc
					Introduction to Isomorphous Replacement
    and Anomalous Scattering Methods

 Measure native intensities
 Prepare isomorphous heavy atom derivatives
 Measure derivative intensities
 Scale native and derivative data
 Determine heavy atom positions and occupancies
 Correlate origin and hand between derivatives
 Compute protein phases
   An isomorphous derivative crystal is one in which the only
   changes in electron density between it and the native crystal
   are peaks at the sites of heavy atom substitution.

The above definition implies no appreciable changes in packing contacts or unit cell
constants and no change in space group.

 Does it ever happen?
 No. Changes usually occur in protein conformation near the heavy atom binding sites, and
solvent molecules are displaced. Fortunately, these effects are often small, localized and
mostly alter the low resolution data. Nevertheless, for isomorphous crystals as we have
defined them, the structure factor for any reflection in the derivative crystal can be
expressed as the vector sum of the corresponding structure factors for the native and
“heavy atom” crystals, where the “heavy atom” crystal is a hypothetical crystal with the
same cell and symmetry as the native but containing only the heavy atoms.

 It is instructive to consider the consequences of this relationship from both an algebraic
and geometric point of view.
PHASE DETERMINATION BY SINGLE ISOMORPHOUS REPLACEMENT.

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:4
posted:5/6/2012
language:English
pages:29