Gry Christensen was a 15-year-old year 11 student when she took part in a “citizen science” project to understand how the different crystals in mussel shells form. But unlike most school experiments, the samples she and her 1,000 fellow school students prepared were then blasted by scientists in a particle accelerator with X-rays 10 billion times brighter than the sun.
“It was a bit of an eye-opener,” Christensen says of the study, called Project M, which involved students from 110 schools. They prepared different samples of calcium carbonate (the main component of clam shells) which scientists then sent to the UK’s national synchrotron (a type of circular particle accelerator), the Diamond Light Source in Oxfordshire. The goal was to help scientists better understand how to form different types of crystal structures from the same chemical. “Afterwards I became more interested in chemistry,” says Christensen, who went on to study agricultural science at Gråsten Landbrugskole in Denmark. “The chemistry really helped me to have an insight into the natural world.”
But while such an approach may be new, understanding how crystals form is an old problem with serious consequences. Crystal structure can affect the strength of steel, and even the therapeutic activity of drugs developed to treat AIDS and Parkinson’s disease.
Calcium carbonate is the main compound in rocks such as chalk, limestone and marble, which are derived from organic materials including shells. It accounts for the annoying limescale stains around faucets, as well as useful applications from antacid tablets to concrete blocks. “Calcium carbonate is all around us,” says Dr Claire Murray, a chemist who led Project M in 2017 with colleague and fellow chemist at the Diamond light source, Dr Julia Parker. But one outstanding challenge is controlling its crystal forms.
A crystal is a solid in which components are arranged in a highly ordered and repeating pattern, and the shape of this pattern – the crystal structure – determines the material’s properties. A common example of the effect of crystal structure is carbon – useful for taking notes when the atoms lie in sheets of honeycomb lattices in pencil lead (graphite), but much more difficult, and much more expensive, when the atoms are arranged in the cubic crystal lattice that diamond shape.
In other materials, control over a substance’s possible crystal structures – or “polymorphs” – was a matter of life and death. In the early 1980s, life expectancy after an AIDS-related diagnosis was less than two years. Patient outcomes began to improve significantly by the mid-1990s, thanks to the development of antiretroviral treatments, including a drug called ritonavir. However, two years after its initial release in 1996, the drug was withdrawn from the market due to problems with the stability of its crystal structure.
The ritonavir capsules were originally delivered with the active ingredient in a highly concentrated solution. Unfortunately, these conditions prompted the active drug to change structure, becoming less soluble than the original and therefore much less effective as a drug. Further drug development has since solved the problem. However, the Parkinson’s drug rotigotine faced similar problems with a less soluble crystal structure that emerged in 2008, prompting a group recall in Europe, while the drug was listed out of stock in the US until 2012, when drug developers found a reformulation.
“There are many recent examples, but they are not all public,” says Dr. Marcus Neumann, CEO and scientific and technical director of Avant-garde Materials Simulation (AMS), a German company that develops software for crystal structure prediction. “Examples come to light when they affect a drug that is already on the market. And luckily, that doesn’t happen very often.”
For more than 20 years, AMS has been refining computer code that can predict which crystal structures can form for a given chemical compound, helping drug companies catch problematic polymorphs before bringing a drug to market. In 2019AMS has shown that its code can predict the occurrence of a problematic form of rotigotine. Recent updates after the algorithm incorporates the effects of temperature and humidity, and also uses comparisons with crystal structure data from drug companies AMS has worked with, including AstraZeneca, Novartis, AbbVie (which now produces reformulated ritonavir), and UCB Pharma (which produces the reformulated rotigotine patches).
Nevertheless, identifying the experimental conditions necessary to produce a particular crystal remains challenging, as different structures can occur with little change in conditions, and one structure can change into another. You can think of it like oranges stacked in a box. You can lay out a square grid of oranges and balance each orange in the layer above directly on top of the orange below, and they will balance delicately for a while. However, just a tap will cause oranges on top of the dip to nestle between oranges in the layer below – the more stable structure.
“There is still a lot of need for experiments because many factors are not 100% understood as far as how to achieve certain crystal structures,” says Dr. Adam Raw, head of materials science R&D in the life sciences division at Merck. He emphasizes the “huge number of factors that can come into play” when additives are introduced to push the system toward a certain crystal structure, exactly the approach that Project M explored.
Calcium carbonate has three possible crystal structures: aragonite, vaterite and calcite. A mussel selectively grows which one it needs – for example, the durable calcite for the outer shell – and “doesn’t use harsh chemical conditions,” says Dr Julia Parker. “Just additives, organic molecules.” Parker and Murray wondered if the right additive at the right concentration would help them control the growth of vaterite versus calcite.
At Diamond Light Source, the pair could quickly distinguish small changes in the crystal structure of hundreds of samples by examining the paths of the X-rays from the synchrotron as they spread from each crystal’s lattice. (The synchrotron accelerates electrons, which emit X-rays as they change direction to move around it.) The bottleneck was preparing all the samples – testing factors including the additive used, concentration and mixing time – until the idea of working with VK came up. schools, by exploiting the similarities in the laboratories and environmental conditions.
Christensen and fellow students at Didcot Girls’ School, near Diamond, were the first to test the sample preparation kits and helped guide Parker and Murray in the equipment and instructions needed in each kit. The data needed to characterize each sample was collected at the synchrotron in just one day.
The results, published in January of this year, helps shed light on the conditions that significantly favor or discourage vaterite formation, and provides insight into the ways in which these crystals form. “I think they’ve made progress in showing which factors are most likely to be consistent with biomineralization [living creatures making minerals] and formation of these calcium carbonate crystals in biological applications,” says Raw. “But of course there is much more work to be done.” Results of the project were not only scientific, however: school participants who were enthusiastic about chemistry later showed up at Diamond for internship interviews.
“With the project, it was like you were doing something for the real world, not just an experiment at school,” says Christensen.
