When wethink about a protein crystal, we tend to imagine all the buildingblocks of the crystal lined up perfectly in the space. However, theblocks (and therefore the unit cells of the crystals) show defectsbetween them in form of orientational divergence. So, we can say thatthere is a certain degree of spatial disorder inside the proteincrystals that is measured as the mosaicity or mosaic spread of acrystal. Mosaicity is an intrinsic feature of each crystal that isaffected by the hydrodynamic and mechanical stability of the growthenvironment. Also, mosaicity can change after crystals are formedbecause of chemical or physical damage such as changes in pH, sal,cryo-cooling, handling… In other words, mosaicity is a measure ofhow disordered (in terms of spatial orientation) are the buildingblocks of a crystal due to chemical and physical processes that canundergo during or after crystal formation.
The main problemassociated with mosaicity is that it broadens the diffraction spotprofiles in the diffraction pattern, so it can cause problems in thefollowing steps of structure determination. Because of the innerdisorder of high mosaic crystals, the atoms of the unit cells are notperfectly lined up and the diffracted beams do not generate aperfectly shaped spot in the detector. This is because the littledisplacements between the same atoms in different unit cells generateoverlapping spots. However, the problem with mosaicity is not justassociated with problems in determining the correct position of eachspot, but also with its intensity. Hence, the intensity of each spotis distributed in a wider angular region, so the peak width for theintensity of each spot appears as a gaussian curve instead of a thinpeak.
Because of this, mosaicity can be defined as the full width athalf maximum (FWHM) of diffraction peaks. In conclusion, mosaicity isan inner property of crystals that causes problems with the actualmeasurement of the intensity and position of each spot in adiffraction pattern. As a result, the indexation, space group andcrystal structure of the protein are way more complicated todetermine when we work with high-mosaicity crystals.