Structural Biology
Structural biology is a branch of molecular biology concerned with the study of the architecture and shape of large biological molecules (macromolecules)----including proteins and nucleic acids (DNA and RNA) in particular. Structural biology examines the physical and chemical basis for the complex structures adopted these molecules. This subject is of great interest to biologists, because macromolecules carry out most of the functions of living cells. Typically, these functions depend on proteins, DNA and RNA adopting a specific three-dimensional shape. These shapes depend, in ways that can be predicted and measured, on the basic chemical structure of the molecules----specifically, on the amino acid sequence of proteins and on the nucleotide sequence of DNA or RNA.

Protein Crystallography
X-ray crystallography is the primary method for determining the molecular conformations of proteins. Since proteins do not occur naturally as crystals, such molecules are typically placed in solution and allowed to crystallize over days, weeks, or months through a very slow process of evaporation (vapor diffusion). The proteins are sealed in a container with a reservoir containing a hygroscopic solution----that is, a solution that tend to bind up the water. Water in the drop diffuses to the reservoir, slowly increasing the concentration and allowing a crystal to form. When X-rays are shined on the crystals, they will scatter in a characteristic pattern. Experts aided by powerful computers are then able to deduce the three-dimensional structure of the protein from the pattern of scattered X rays.