X-Ray Crystallography is a method by which, under the proper conditions, the three-dimensional structure of a protein or other macromolecule may be determined. The X-rays themselves, with their characteristic high frequencies, provide an order of resolution that is unobtainable by light microscopy. In fact, the wavelength of X-rays allows for the resolution between objects separated by 1 Angstrom, or the radius of a hydrogen atom (from nucleus to 1s electron orbit). One may imagine the potential uses of such powerful technology.
The accurate 3-D structure of a protein or molecule is critical for pharmaceutical companies who may wish to synthesize drugs that interfere with the function of a protein implicated in a specific disease. For instance, success in the fields of cancer treatment and research has been accelerated through the use of X-ray crystallography. Basic research in Biology and Chemical fields have also benefited form this method.
How Does X-Ray Crystallography Work?
X-rays are scattered as they encounter matter in the same way that visible light does. This fact is illustrated in the following figure, in which two X-rays strike two electron clouds separated by a distance d.
X-rays scatted in specific directions will add an subtract through their phase relationship to yield regions of more intense light (in phase) and regions of less intense light (out of phase). Bragg’s Law, which is 2d sinq = l, gives the relationship between the phase angle (theta) and the distance d. By performing this diffraction analysis on a crystal at a multitude of angles, then determining the phase angles of individual reflections by repeating the analysis with heavy metals bound to the molecule, one obtains a diffraction pattern that can then be analyzed and processed using computers and a mathematical calculation called the Fourier Transform.
This yields a final 3-D picture of the Electron Density Map of the molecule in question. With this knowledge in place, it is possible to assign the special relationships between individual atoms.
Stick Structure and 3-D Arrangement of Atoms in a DNA-Protein Complex