Chapter 3, Problem 1Q

What structural features of protein molecules allow them to form the crystals required for imaging by X-ray crystallography?

Summary Introduction

To determine: The structural features of protein molecules that allow proteins to form crystals required for imaging by X-ray crystallography.

Introduction: X-ray crystallography is a technique that enables scientists to determine the exact position of thousands of atoms that are present in a typical molecule of protein. In X-ray crystallographic technique, a sharp beam of X-rays is directed over the crystals where it diffracts at specific angles to form diffraction pattern of spots.

Explanation of Solution

The formation of crystals of proteins is a very crucial step for imaging of proteins by X-ray crystallography techniques. By forming a molecule of protein into a crystal, in which atoms align themselves in a well-defined and repetitive manner, researchers can measure diffraction of X-rays by atoms. It helps in determining the position of atoms within the molecule of protein.

For the formation of protein crystals, the size of crystals should be sufficiently large (20-100 micrometer on each side) and must be free of any kind of imperfection such as a fracture that would lead to scattering of X-rays and their analysis.

Crystals of proteins are usually grown by decreasing solubility of proteins by adding precipitants. All these precipitants bind with molecules of water and reduce the amount of free water in the solution in which dissolution of proteins takes place. Formation of crystals takes place when the solution becomes supersaturated.