A Brighter View Of A Hidden World

While there are several ways in which researchers can determine the structure of a protein, the most widely used method is protein crystallography. Proteins can be made to crystallize in much the same way sugar crystals can be formed from sugar water to make rock candy.

If you were to mix sugar into warm water, then place a string or wooden stick into the mixture and let it sit undisturbed, you would see crystals begin to grow. These crystals form because the concentration of the sugar increases as the water evaporates, and when the concentration of sugar is more than the remaining water can hold, the sugar crystallizes onto the string or stick. The resulting crystals will continue to grow in size as the process continues, making a sweet treat known as rock candy. 

 Protein crystallographers, however, don't use sugar, jars, and string. They do make use of the same principles to form protein crystals through a process known as vapor diffusion. The proteins to be crystallized are in a solution that contains water. When this solution is exposed in a sealed experiment chamber, differences in vapor pressure between the solution and the chamber causes the water to move from the solution into the chamber, just as the water evaporates from the rock candy. As the amount of water decreases, the protein solution becomes concentrated and begins to form crystals. As the experiment continues, these crystals will grow larger.

On Earth, crystals are likely to have defects in their structure. Gravity causes heavier objects, and chemicals, to sink while lighter ones rise -- a process known as sedimentation. This also happens within fluids, creating currents that can cause uneven mixing or damage delicate structures. Gravity also causes other phenomena that can interfere with crystal growth. As a result, it can be difficult on Earth to grow near-perfect crystals of some proteins to a size that can be analyzed. 

In microgravity, however, these gravity-induced effects are reduced or eliminated. This quiet environment provides the opportunity for researchers to grow crystals that are more nearly perfect.

These crystals can then be studied using a process called X-ray diffraction. Scientists can send a beam of X-rays through the crystal, and measure how the beam is split up by the atoms of the crystal. By studying the pattern made by the X-rays, they can then map the locations of the different atoms, allowing them to create a diagram of the protein's structure. With this as a guide, researchers can then determine how the protein does its job.