X-ray crystallography
From ArticleWorld
X-ray crystallography is a method used in the study of the structure of crystals by which a pattern is generated through the diffraction of X-rays through the atoms of a crystal. The pattern generated provides information that can aid in the determination of the molecular structure of the material. Interaction occurs not with the nucleus of the atoms present in the structure by rather with their surrounding electrons. This technique has applications in chemistry and biochemistry.
Single crystal and powder diffraction
Single crystal methods have been used in the analysis of inorganic as well as organometallic systems. The method also has applications to the field of pharmaceuticals as it can be applied to the problem of polymorphs. One limitation of this technique is the crystal quality used in the determination.
Powder diffraction is useful to the field of material science. Sample reparations are simple and the analysis is non-destructive and fast. The majority of the materials under study in this field are crystals and for those that are not information can still be obtained about them using this technique. The size, purity and texture of the material can be quickly determined.
Crystallization
Crystallization of the compounds under study must first be done for this method to be of use. This is required as single molecules do not have enough diffraction power. This is why crystals are used in this method as they are essentially infinite repeating units of the molecules being studied. Crystallization methods include:
- Diffusion gradient
- Concentration
- Sublimation
Materials such as DNA which do not occur as crystals require other techniques to bring them into the crystal form in order to be studied by crystallography. These molecules are placed in a solution are allowed to slowly crystallize over a period of days, weeks or months by vapor diffusion. This method uses a drop of solution which is composed of the molecule, buffer and precipitants sealed in a container with a reservoir of a hygroscopic solution. It is seen that the water in the drop diffuses and this concentrates the solution and precipitates the molecule over time. When the crystal is obtained the analysis of the structure can then take place.