Lecture notes on X-ray Crystallography

X-ray crystallography is the method used in the study of the molecular structure of complex organic molecules such as proteins and nucleic acids. The technique was originally developed by Lawrence Bragg and William Bragg to study the crystal structure. X-rays are short wavelength radiations (1-105.). X-rays can be reflected, refracted, diffracted, polarized and scattered like ordinary light. In this technique, the X-rays are allowed to impinge on a high molecular weight complex molecule such as DNA, RNA or proteins. As these X-rays penetrate the macromolecule, they get diffracted and are then allowed to fall on a photographic plate where it develops a pattern called diffraction pattern. So this technique is otherwise known as X-ray diffraction technique.
When a beam of X-rays strikes an object with considerable internal order such as a crystal, many of the waves pass through without significant disturbance. However, some of the waves are diffracted, which emerge in well-defined directions to form a symmetrical pattern of spots . lf a photographic plate is placed in the path of the reflected beam, a pattern of spots is produced with different spacing and intensity. From the position and intensity of these spots and their angles with respect to the irradiated crystal, the various distances between the layers of atoms in the crystal and thus the arrangement of the atoms in the crystal can be deduced.

X-ray diffraction method is a valuable tool in the study of 3- dimensional structure of biological macromolecules such as proteinsand nucleic acids. Watson and Crick (1953) elucidated the structure of DNA on the basis of a diffraction pattern prepared by RosalindFranklin and Willkins.