Characterizing Your Crystal 
4 Things You Should Do Once You Have A Native Dataset 
subtitle: Getting to Know You
Original Twinning Server
Improved Twinning Server

Is your crystal twinned? Merohedral twinning is an anomaly that is
impossible to detect by viewing the diffraction pattern. It occurs
when the lattices of two or more distinct crystal domains coincide
exactly in three dimensions. The twinning server can detect twinning
by comparing diffraction data to that expected by Wilson statistics.
If left undetected, you could spend months agonizing over poor phasing
results. You will save yourself untold mental anguish if you find out
now! The following crystallographic point groups support twinning: 3,
4, 6, 321, 312, 23.
Requirements: Unit cell parameters, space group, and
observed structure factors in XPLOR or
CIF format.
References:
T. O.
Yeates, Methods Enzymol. 276, 344358, 1997.

Matthews Coefficient

How many macromolecules are in your asymmetric unit? Are there
multiple molecules? A single molecule? Half a molecule? All these
are possible. The Matthews coefficient allows you to estimate this
number. The Matthews coefficient (Vm) is
easily calculated as
___________volume of your unit cell__________
the molecular weight of your macromolecule * Z * X
Where Z is the number of
asymmetric units in the unit cell (i.e. the number of symmetry
operators in your space group). The unknown variable, X, is the number
of molecules in the asymmetric unit. You may perform the calculation
using the
Vm server
. Calculate a series of Matthews coefficients with
X= 0.5, 1.0, 2.0, etc. The most probable values of X are those which
give Matthews coefficients within the empirically observed range. This
range is shown in the bar graph at left, or click on the reference
below to see the original paper in PDF format.
Requirements: Unit cell parameters, space group,
molecular weight of your macromolecules.
References:
B. W.
Matthews, J. Mol. Biol. 33, 491497, 1968.

Self Rotation Function

Did the Matthews Coefficient suggest 2 or more molecules in the
asymmetric unit? The self rotation function will tell you if the
molcules in your asymmetric unit are related by rotational symmetry
axis and if so, how the axis is oriented in your unit cell. This
information is crucial if you want to take advantage of
noncrystallographic symmetry (NCS) averaging.
Requirements: Unit cell parameters, space group,
and observed structure factors in any format.
References:
M. G.
Rossmann and D. M. Blow, Acta Cryst. 15, 2431, 1962.

Native Patterson Map

Did the Matthews Coefficient suggest 2 or more molecules in the
asymmetric unit? The native Patterson map will tell you if the
molcules in your asymmetric unit are related by a pure translation
and if so, the length and direction of the vector that relates the
two molecules. This
information is crucial if you want to take advantage of
noncrystallographic symmetry (NCS) averaging.
Requirements: Unit cell parameters, space group,
and observed structure factors in XtalView's fin format.
References:
Reference for the Native Patterson Map:
J. Drenth, Principles of Protein XRay Crystallography (Springer Advanced Texts in Chemistry)
Hardcover (February 1999) . See pages 130139.

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