Second-generation Vapor Diffusion Apparatus

The Vapor Diffusion Apparatus (VDA-2) uses the "hanging drop" method often used on Earth to crystallize proteins. The method is quite simple. Proteins are mixed into a solution that makes them grow - the precipitant solution - and squeezed onto the tip of a syringe. While exposed to the air part of the liquid evaporates, the concentration rises, and crystals form.

The effects of Earth's gravity limit the success of this method on Earth. Growing crystals are more dense than their solution and will sink to the bottom while the less dense depleted solution floats to the top. The result can be crystals that actually are a jumble of miniature crystals piled together. These can be useless for analysis.

The design of VDA-2 is based on the experience from several missions using the original VDA. That hardware comprised double-barrel syringes. One barrel contained the protein solution, the other the precipitant solution. The two plungers together injected their fluids into a single droplet.

Several VDA experiments grew small specimens, or none at all, apparently because the droplets did not mix completely (when Skylab astronauts joined droplets of grape and orange drinks, they saw the two flavors stayed separate even though the drop behaved as a unit).

VDA-2 adds a third barrel that, several times, will suck up the protein/precipitant droplet and inject it back onto the tip of the syringe. Shown at left at about 50% larger than actual size (click for a larger image), these syringes will ensure that the samples are fully mixed.

As with the original VDA hardware, VDA-2 comprises 24 syringes mounted on a plate with miniature viewports so astronauts can photograph the droplets. Four trays will be carried in a temperature-controlled locker. The astronauts will start and stop the experiments by cranking the syringes (they are connected).

Crystals grown by the VDA-2 on a previous flight (STS-72 ) are shown below: Fab'734 (left; 0.7 mm long), an antigen binding fragment from an antibody developed for medical diagnosis and turkey egg white lysozyme (right; 1.0 to 1.2 mm long) grown for comparison with crystals grown on Earth. Click on the images for larger views.

Principal investigator for the VDA-2 experiment is Dr. Larry DeLucas of the University of Alabama at Birmingham. At Marshall, the project scientist is Laurel Karr.

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Author: Dave Dooling
Curator: Bryan Walls
NASA Official: John M. Horack