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 X-PLOR or CIF format.

References: T. O. Yeates, Methods Enzymol. 276, 344-358, 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, 491-497, 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 non-crystallographic 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, 24-31, 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 non-crystallographic 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 X-Ray Crystallography (Springer Advanced Texts in Chemistry) Hardcover (February 1999) . See pages 130-139.



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