Tutorial 5: Seeding
There are different kinds of seedinng: macroseeding, microseeding, streakseeding, epitaxial seeding, cross-seeding. Here are some pictorial examples.
Remember: you can click on any picture to get a magnified view.
Take a cat whisker. Stroke the parent crystal to pick up invisible seeds. Now, draw a streak line across a new drop. Crystals will grow along the streakline. Sometimes you will also get new self nucleated crystals in parts of the drops farther away from the streakline.
Microseeding step 1. CBH II Example
These crystals of cellobiohydrolase II grew from a single nucleation site. (Notice that you also have a phase separation in the drop. To learn what that is go to tutorial 3.) We wanted to improve the quality as well as grow them as single crystals. Therefore these crystals were crushed and suspended in a slurry of mother liquor.
Microseeding step 2. CBH II Example
A new drop was innoculated with microseeds from the slurry. Centrifuge the slurry for 30 seconds at 1-2 g. Take an acupuncture needle and dip it into the slurry. Now touch the new drop with the needle. The experiment worked as you can see. The crystals now grow singly.
Microseeding step 3. CBH II Example
The cycle of microseeding was repeated again, using the new, improved crystals obtained in step 2. This crystal is the result. (It is over 2 mm long).
Microseeding step 1. Acyl coA Binding Protein (ABP) Example.
Here is another example of using seeding to improve crystal quality. These crystals grew in an enormous number of conditions, but as masses of needles. These were used as microseeds.
Microseeding step 2. ABP Example.
After microseeding into a new drop, the crystals grew singly. Alas, they are still only thin needles.
Microseeding step 3. ABP Example.
Microseeding doesn't solve everything. Seeding did not produce anything but needles. Cation choice was decisive in changing the crystal habit. Over 20 cations were tested as additives. All gave needles except this one, nickel sulfate.
Microseeding step 4. ABP Example.
When a ligand was added, these crystals grew. These were the ones that were with the best diffraction but took 4 months to grow. You can speed up the process considerably by using seeds of the apo-protein to seed the drops containing the holo-protein.
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