Classic techniques for observing molecules, such as the use of X-ray crystallography or NMR, often aren't effective enough to observe protein dynamics and how proteins combine to other molecules. This can slow down research into proteins and microscopic molecules that interact with them. Fortunately, hydrogen/deuterium exchange mass spectrometry (HDX) can make it possible to observe these chemical interactions. 

The HDX Process

The HDX exchanges hydrogen for deuterium. This is accomplished by soaking proteins in D2O. Deuterium has a similar weight to hydrogen and has a similar function as hydrogen. Hydrogen–deuterium exchange is a chemical reaction that involves a covalently bonded hydrogen atom that is replaced by a deuterium atom. This is mot easily accomplished through exchangeable deuterons and protons because the reaction will occur without a catalyst. 

Exchange With Non-Exchangeable Deuterons And Protons

If non-exchangeable deuterons or protons are used, a acid, base, or metal catalyst must be used. Also, increasing the pressure and temperature of the chemicals is often necessary. However, the substrate must be robust enough to where it can resist the conditions that it is placed under. If the conditions are right perdeuteration will occur. This is the exchange of all non-exchangable molecules in the solution. 

Neutron Scattering

One application for hydrogen-deuterium exchange is neutron scattering experiments. When perdeuteration of one component of a multi-component system is carried out, this can create a necessary contrast. However, creating contrast with deuterated is often sufficient. 

Limitations Of This Process

While this device is very helpful, it must be used in combination with an X-ray rather than being used by itself. Only the X-ray or the NMR will provide information on the structure of the protein. The limitations of mass spectrometry result from how protein structures slow exchange. The rate of exchange is determined by hydrogen bonding and the accessibility of the solvent. When an amide is part of a hydrogen bond, it will either exchange very slowly or will not exchange at all. Meanwhile, the amide that is on the surface of the hydrogen bonded to water will exchange very rapidly. Therefore, it will be difficult to determine which element is responsible for a particular exchange rate. 

Those who are using the HDX must be informed on how to properly operate it. This requires custom certified reference materials. However, with the proper information and the right equipment, it is possible to understand how proteins and microscopic molecules interact with each other.