Sea urchins

These often begin as spherulites (see previous tutorial) or they grow out from a spherulite. They are extremely thin needles clustered around a single nucleation site. Often they appear “fuzzy”. The white arrows in this picture show three-dimensional crystals growing from independent nucleation sites in the same drop.

Needles 1

Extremely thin needles growing from a single nucleation center. Since these needles are much longer, I would not call this a “sea urchin” any longer. Notice the large 3-D crystal growing in the same drop.

Needles 2

Still too many but at least they are single needles. The nucleation rate is too high which is why they are too many and too thin. Try reducing the protein or precipitant concentration or both. Another method is to put a layer of oil over the reservoir in the vapor drop setup (see N. Chayen, J. Appl. Cryst 1997, vol. 30, p. 198-202). See also Tutorial 5 on seeding.

Needles 3

In a vapor diffusion experiment, the level of supersaturation increases fastest at the edges (i.e., on the inside perimeter) of the drop. That is because the rate of evaporation is fastest there. Faster evaporation=faster supersaturation. The needles shown here nucleated all along the inside perimeter of the drop and are growing inwards, towards the bulk volume of the drop.  Take home lesson: Always check the edges of your drops carefully because if anything is going to happen, it usually happens here first.


Two-dimensional plates. It could be argued, but plates are usually considered an improvement over thin needles. The plates here are clearly growing from a single nucleation site and overlaying each other, which is far from optimal. The problem with plates is that they always diffract poorly in the thinnest direction. Thus you might get 2.5Å in one direction, and 8Å in the other. Optimize to grow them separately and thicker. To grow them separately, seeding often helps. To grow them thicker, try an additive screen.


A three-dimensional crystal. But check the diffraction before you get out the champagne.