Basic X-ray diffraction equations

There are three fundamental equations in X-ray diffraction these are described in this section before subsequent discussion of the methods of protein structure determination. 

Bragg s Law In 1913, Bragg provided the explanation for the formation of diffraction spots seen in Von Laue s first X-ray crystal photographs taken in 1912. Bragg visualised the scattering of X-rays by a crystal in terms of reflections from planes of atoms. The condition for diffraction is given by (Fig. 3a) 

Bragg [26] modestly remarked I have always felt the association of my name with it to be an easily earned honour because it is merely the familiar optical relation giving colours reflected by thin films, in another guise . 

3 A resolution course of polypeptide chain can be traced and topology of folding established. Amino acid side chains can be identified only with the help of amino acid sequence. 

2 A resolution details such as identification of carbonyl oxygens allow the conformation of polypeptide to be established with precision. Details of side chains, e.g., forking of isoleucine residues, becomes apparent and in certain cases assignment of amino acid type can be made without sequence data. 

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