Crystallography

Newton
13 Colonies
Prov. 10:1
"The proverbs of Solomon. A wise son maketh a glad father: but a foolish son is the heaviness of his mother."

St.James Angels
The stained glass windows of the St. James Cathedral in Toronto tell the stories of the Christian Church through history.
Chess: "Newton" "13 Colonies" "Optiks" "Budweisser" "Moody Blues"
"Crystallography"

X-ray crystallography is an extremely precise, but also difficult and expensive means of imaging the exact structure of a given molecule or macromolecule in a crystal lattice. Because a diverse set of materials produce crystals, including salts, metals, minerals, semiconductors, and various inorganic, organic, and biological molecules, x-ray crystallography is essential to many scientific fields. A crystal is any regularly repeating arrangement of unit cells which range in size from less than 100 atoms — small-molecule crystallography — to tens of thousands — macromolecular crystallography).
X-ray crystallography is famous for being the tool first used to discover the structure of DNA, but it was also used to determine the structure of diamond, table salt, penicillin, numerous proteins, and entire viruses. In all, over 400,000 structures have been described using x-ray crystallography. These can be found in the Cambridge Structure Database.
To analyze a sample using x-ray crystallography, first one must obtain a high-purity crystal of the material to be studied with a very regular structure. This is often the hardest part as numerous crystals have nanometer-scale defects which make x-ray crystallography difficult.
Next, the sample is subjected to an intense beam of x-rays of a uniform wavelength. These x-rays produce a diffraction pattern as they reflect off the sample. This diffraction pattern is somewhat similar to what is observed when multiple stones are tossed into a pond – where the waves cross are peaks which make up the diffraction pattern.
By slowly rotating the crystal, pounding it with x-rays, and meticulously recording the diffraction patterns at each orientation, an electron density map may be derived. This electron density map is then used to formulate a hypothesis about the atomic structure it corresponds to. The diffraction patterns are then analyzed in light of the hypothesized structure, and if it looks plausible that the given structure would produce the observed diffraction pattern, a conclusion is drawn. The result is then uploaded to central databases of the type mentioned earlier.

-->
What is X-ray Crystallography ?
X-ray crystallography is an experimental technique that exploits the fact that X-rays are diffracted by crystals. It is not an imaging technique. X-rays have the proper wavelength (in the Ångström range, ~10^{-8 cm) to be scattered by the electron cloud of an atom of comparable size. Based on the diffraction pattern obtained from X-ray scattering off the periodic assembly of molecules or atoms in the crystal, the electron density can be reconstructed. Additional phase information must be extracted either from the diffraction data or from supplementing diffraction experiments to complete the reconstruction (the phase problem in crystallography). A model is then progressively built into the experimental electron density, refined against the data and the result is a quite accurate molecular structure.}
-->
Why Crystallography ?
The knowledge of accurate molecular structures is a prerequisite for rational drug design and for structure based functional studies to aid the development of effective therapeutic agents and drugs. Crystallography can reliably provide the answer to many structure related questions, from global folds to atomic details of bonding. In contrast to NMR, which is an indirect spectroscopic method, no size limitation exists for the molecule or complex to be studied. The price for the high accuracy of crystallographic structures is that a good crystal must be found, and that limited information about the molecule's dynamic behavior in solution is available from one single diffraction experiment. In the core regions of the molecules, X-ray and NMR structures agree very well, and enzymes maintain their activity even in crystals, which often requires the design of non-reactive substrates to study enzyme mechanisms.