Types of precipitates

Tutorial 2

One of the most difficult things for beginners is to recognize promising precipitates and distinguish them from preciptiates that are not worth pursuing. Except for drops where the protein is denatured, precipitate can be regarded as a positive drop result. This is because precipitate occurs in a state of supersaturation. Crystals can only grow from supersaturated states, and thus they can grow from precipitates.

How do I tell a "good" precipitate from a "bad" precipitate?

"Good" Precipitate

"Bad" Precipitate

Non-amorphous precipitateCharacteristic brownish tinge to the precipitate
Precipitate shows birefringenceSkins on the drop
Precipitate redissolves if given the right conditionsPrecipitate do not redissolve upon dilution

Here are some things to look for (visual examples of these are given below):

A light, amorphous precipitate

A heavy, amorphous precipitate

Most of the protein in the drop has precipitated.

Non-amorphous precipitate 1

This is NOT an amorphous precipitate. Look for things like this in your drop--precipitates which have some kind of anomalous pattern. Do you notice the darker, localized regions? These are often pre-cursors to crystals or at least spherulites.

Non-amorphous precipitate 2

Another example of an atypical (i.e. non-amorphous) precipitate in this microbatch drop. Notice that the precipitate has a pattern or different regions.

Non-amorphous precipitate 2

A few days later, crystals appeared in the drop.

Crystalline precipitate

The student who showed me this drop completely missed the crystals because: 1. he had only focused at one depth in the drop, and 2. only checked the drop at 40x magnification. Notice the blurry region in the upper one-third of this picture. If this region is IN focus, then the tiny microcrystals underneath it are OUT of focus and cannot be seen. By changing to 100X magnification and focusing through the entire depth of the drop, these microcrystals were discovered.

Bad precipitate

Brown amorphous precipitate. BAD! This brown tinge is characteristic of denatured protein. Denature proteins never crystallize. Okay, you might yet get crystals in this drop, but only if enough of the un-denatured protein is still in a state of supersaturation. This is unlikely. Note the skin on the drop; see below.

Skin

Skin on the drop. BAD! These skins are believed to be a layer of denatured protein or caused by the polyethylene glycol. Avoid them, they will certainly slow down the rate of vapor diffusion. Pick away the skin before mounting your crystal. If the skin wraps around the crystal it affects the limit of diffraction.

Precipitate with crystal

Crystal growing from a heavy, amorphous precipitate.

It often happens that precipitate occurs first and later crystals grow. For more examples of this see tutorial 6 on Ostwald ripening.