A small change to a peptoid that crystallizes in one step (left) sends the modified peptoid down a more complicated path that progresses from disordered clump to crystal (right). Image: Jim De Yoreo/PNNL.
A small change to a peptoid that crystallizes in one step (left) sends the modified peptoid down a more complicated path that progresses from disordered clump to crystal (right). Image: Jim De Yoreo/PNNL.

Silky chocolate, a better medical drug and solar panels all require the same thing: just the right crystals making up the material. Scientists trying to understand the pathways crystals take as they form have now shown they can influence that pathway by modifying the starting ingredient. The insights gained from this work, reported in a paper in Nature Materials, could eventually give scientists better control over the design of a variety of products for energy and medical technologies.

"The findings address an ongoing debate about crystallization pathways," said materials scientist Jim De Yoreo at the US Department of Energy's Pacific Northwest National Laboratory (PNNL) 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."

Diamonds are one of the simplest crystals, composed of one atom – carbon. But in the living world, crystals, like the ones formed by cocoa butter in chocolate or the ill-formed ones that cause sickle cell anemia, are made from molecules like proteins that are long, floppy and contain a lengthy, well-defined sequence of many atoms. They can crystallize in a variety of ways, but only one way is best. In pharmaceuticals, these various crystallization pathways can mean the difference between a drug that works versus one that doesn't.

Chemists don't yet have enough control over the crystallization process to ensure the creation of the best crystal form, partly because chemists aren't sure how the earliest steps in crystallization 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. Chemists call this a one-step process, the mathematical rules for which they have long understood.

The other possibility is that crystals require two steps to form. Experiments suggest that, at the beginning of the crystallization process, molecules first form a disordered clump and then start rearranging into a crystal from within that group, as if the cards have to be mixed into a pile first before they can form a deck. De Yoreo and his colleagues wanted to determine if crystallization always requires the disordered step, and if not, why not.

To do so, the scientists formed crystals from a somewhat simplified version of a protein, known as a peptoid. This peptoid was not complicated – just a string of two repeating chemical subunits (think ‘ABABAB’) – yet was still 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 ABABAB... 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 immersed the peptoid molecules in solution to let them crystallize. Then they used a variety of analytical techniques to see what shapes the peptoids made and how fast, discovering that the two different peptoids formed crystals in very different fashions.

As the scientists mostly expected, the simpler peptoid formed initial crystals a few nanometers 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 crystallization process.

But thrusting a tail into the mix disrupted the calm, causing a complex series of events to take place before the crystals appeared. Overall, the team showed that this more complicated peptoid first clumped together into small clusters that were 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."

This story is adapted from material from Pacific Northwest National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.