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Mosaicity of a Macromolecular Crystal
What is mosaicity?
The quality of a crystal described by the parameter, mosaicity (aka mosaic, mosaic spread, rocking angle), involves the degree of perfection of the lattice translations throughout the crystal. A macromolecular crystal may contain a variety of imperfections in packing and heterogeneity of contents. However, the traditional way of describing mosaicity simplifies the situation by considering the crystal to be composed of mosaic blocks, each block being a perfect crystal. If all blocks are perfectly aligned, the diffraction from each block will be perfectly contiguous with that from all other blocks and the mosaicity will be 0. As the mosaic blocks become disordered (as may happen during flash-cooling), the resulting diffraction is spread out. In the rotation technique of diffraction data collection, this is seen as broadening the rotation angle required to collect the intensity of a given reflection. Thus, mosaicity is given in degrees.
Mosaicity is sometimes referred to as a "rocking angle". This measurement requires specialized equipment and refers to the angle through which a diffractometer must be swung to encompass a reflection (actually, fwhm of the intensity peak). In normal usage, that is, as reported by data reduction software, mosaicity incorporates contributions from the instrument as well as from crystal quality. Instrumental contributions include beam divergence and spectral quality of the X-rays.
What is an acceptable mosaicity?
A very good crystal on a standard X-ray generator and area detector will have
a mosaicity in the range of 0.2 to 0.4 degrees. However, structures have been
solved using crystals with mosaic spread well above 1.0 degree.
Acceptable mosaicity depends on the instrumentation and the unit cell. Individual reflections must not overlap. Area detectors with high resolution can be used to extract useful data from mosaic crystals. Small unit cells with correspondingly higher separation between reflections can exhibit higher mosaicity and still yield useful data.
Obviously, the lower the mosaicity value the better the data so one should always strive to keep mosaicity low. This can be done by optimizing flash-cooling conditions or through annealing.
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Mosaicity of a Macromolecular Crystal
What is mosaicity?
The quality of a crystal described by the parameter, mosaicity (aka mosaic, mosaic spread, rocking angle), involves the degree of perfection of the lattice translations throughout the crystal. A macromolecular crystal may contain a variety of imperfections in packing and heterogeneity of contents. However, the traditional way of describing mosaicity simplifies the situation by considering the crystal to be composed of mosaic blocks, each block being a perfect crystal. If all blocks are perfectly aligned, the diffraction from each block will be perfectly contiguous with that from all other blocks and the mosaicity will be 0. As the mosaic blocks become disordered (as may happen during flash-cooling), the resulting diffraction is spread out. In the rotation technique of diffraction data collection, this is seen as broadening the rotation angle required to collect the intensity of a given reflection. Thus, mosaicity is given in degrees.
Mosaicity is sometimes referred to as a "rocking angle". This measurement requires specialized equipment and refers to the angle through which a diffractometer must be swung to encompass a reflection (actually, fwhm of the intensity peak). In normal usage, that is, as reported by data reduction software, mosaicity incorporates contributions from the instrument as well as from crystal quality. Instrumental contributions include beam divergence and spectral quality of the X-rays.
What is an acceptable mosaicity?
A very good crystal on a standard X-ray generator and area detector will have a mosaicity in the range of 0.2 to 0.4 degrees. However, structures have been solved using crystals with mosaic spread well above 1.0 degree. Acceptable mosaicity depends on the instrumentation and the unit cell. Individual reflections must not overlap. Area detectors with high resolution can be used to extract useful data from mosaic crystals. Small unit cells with correspondingly higher separation between reflections can exhibit higher mosaicity and still yield useful data. Obviously, the lower the mosaicity value the better the data so one should always strive to keep mosaicity low. This can be done by optimizing flash-cooling conditions or through annealing.| Return to Roadmap | Previous |
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Joel Harp- modified on January 30, 2009 |