Crystal paths uncovered

Materials World magazine
18 Apr 2017

Scientists trying to understand the paths crystals take as they form have been able to influence that path by modifying the starting ingredient. Their research could eventually help to better control the design of products for energy or medical technologies.

‘The findings address an ongoing debate about crystallization pathways,’ said materials scientist Jim De Yoreo at the Department of Energy's Pacific Northwest National Laboratory and the University of Washington. ‘They imply you can control the various stages of materials assembly by carefully choosing the structure of your starting molecules.’

Crystals, like those formed by cocoa butter in chocolate or ill-formed ones that cause sickle cell anaemia, are made from molecules that are long and floppy and contain a well-defined sequence of many atoms. They can crystallize in a variety of ways, but only one way is the best. In pharmaceuticals, that can mean the difference between a drug that works and one that doesn't.

Chemists don't yet have enough control over crystallisation to ensure the best form, partly because chemists aren't sure how the earliest steps in crystallisation occur. A particular debate has focused on whether complex molecules can assemble directly, with one molecule attaching to another, like adding one playing card at a time to a deck. They call this a one-step process, the mathematical rules for which scientists have long understood.

The other side of the debate argues that crystals require two steps to form. Experiments suggest that the beginning molecules first form a disordered clump and then, from within that group, start rearranging into a crystal. De Yoreo and his colleagues wanted to determine if crystallisation always required the disordered step, and if not, why not.

To do so, they formed crystals from a simplified version of the sequence-defined molecules found in nature, a version they call a peptoid. It was a simple string of two repeating chemical subunits, yet complex because it was a dozen subunits long. Based on its symmetrical chemical nature, the team expected multiple molecules to come together into a larger structure, as if they were Lego blocks snapping together.

In a second series of experiments, they wanted to test how a slightly more complicated molecule assembled. So, the team added a molecule onto the initial A-B-A-B-A-B sequence that stuck out like a tail. The tails attracted each other, and the team expected their association would cause the new molecules to clump. But they weren't sure what would happen afterwards.

The researchers put the peptoid molecules into solutions to let them crystallise and then used variety of analytical techniques to see what shapes the peptoids made and how fast. It turns out the two peptoids formed crystals in very different fashions. As the scientists expected, the simpler peptoid formed initial crystals a few nanometres in size that grew longer and taller as more of the peptoid molecules snapped into place. The simple peptoid followed all the rules of a one-step crystallisation process. But thrusting the tail into the mix caused a complex set of events to take place before the crystals appeared. Overall, the team showed that this more complicated peptoid first clumped together into small clusters unseen with the simpler molecules.

Some of these clusters settled onto the available surface, where they sat unchanging before suddenly converting into crystals and eventually growing into the same crystals seen with the simple peptoid. This behavior was something new and required a different mathematical model to describe it, according to the researchers. Understanding the new rules will allow researchers to determine the best way to crystallize molecules.

‘We were not expecting that such a minor change makes the peptoids behave this way,’ said De Yoreo. ‘The results are making us think about the system in a new way, which we believe will lead to more predictive control over the design and assembly of biomimetic materials.’