December 6, 2021

I, Science

The science magazine of Imperial College

Rainbow Crystals

X-ray crystallography is one way of finding out what a molecule consists of. It can show which atoms are present in a substance and how they are connected to one another. This involves crystallising the compound, firing X-rays at the crystal and analysing the diffraction pattern that results.

However, not all compounds naturally form crystals. Some must be crystallised in the lab, which can be a lengthy process. The above picture shows crystals of a drug molecule formed by dissolving a powder sample in a mixture of solvents and then leaving it to evaporate for several weeks.

A crystal is made up of atoms which form repetitive patterns known as unit cells. Generally, when looking at the crystals through a microscope, crystallographers will select those which are uniform in colour. This would suggest that the unit cells are all lined up and facing the same direction, so the crystal is suitable for X-ray diffraction.

The colour variation seen in the above crystals suggests that they are very thin and plate-like. These have been viewed under a polarising microscope, so light waves only travel in one direction through them. Since all the unit cells are lined up, there will be a point at which the light cannot shine through the crystal when it is rotated. It seems to disappear, or ‘extinguish’. If you look closely at the top left of the photo, you can see the ghost of a crystal which is almost fully extinguished.

It is impossible to predict what kind of crystal will grow based on the structure of the compound being studied. Depending on the crystallisation conditions, one compound may form several different types of crystals, whose physical properties vary widely. This is particularly important in the pharmaceutical industry, where different crystal structures can affect the body differently.

Image: Siobhan Chan