In the 1950s, Linus Pauling and Robert Corey proposed the idea of rippled beta-sheets — sections of proteins— called peptides— that are mirror-images of each other, held together by hydrogen bonds in a strictly alternating fashion. Scientists knew of more orderly pleated beta-sheets, but the related, yet distinct rippled sheets were not unambiguously confirmed by observation until 2022, nearly seventy years later.
In a recent paper published in the Journal of the American Chemical Society, scientists from the University of California, Santa Cruz, synthesized peptides from proteins associated with Alzheimer’s and Type II Diabetes and described five new rippled beta-sheet structures. Previous work showed the formation of rippled sheets in smaller peptides consisting of three amino acids.
This paper increases the scale to peptides of up to seven amino acids. The results suggest that the formation of rippled beta-sheets is a more common process than previously thought.
The scientists used peptides from Amyloid beta — a protein that builds up in the brains of patients with Alzheimer’s and causes damage to nerve cells — and amylin, which can build up as amyloid deposits in the pancreas of patients with type II diabetes.
The structures that these peptides fold into influence how toxic they are, so understanding how to manipulate their shape has important therapeutic implications. Amyloid beta, for example, may be less toxic when organized into fibrils rather than more unstable, disorderly structures called oligomers. With this in mind, the chemists found a way to push the peptides towards forming fibrils rather than oligomers.
To influence the crystallization, they used racemic mixtures, which contain equal amounts of a molecule and its non-superimposable mirror image. This is in contrast to enantiomerically pure substances, which contain only one molecule without its mirror image counterpart.
UC Santa Cruz chemist Jevgenij Raskatov had studied crystallization before and used his interdisciplinary background to design the concept.
“I knew that racemic mixtures often crystalize more easily than enantiomerically pure substances, and amyloid fibril formation is, in many ways, a one-dimensional crystallization,” he said. “So if we can enhance crystallization by using mirror images, that means to me that we could potentially enhance amyloid fibril formation using mirror-image-amyloid beta and get protections from toxicity.”
An emerging field
The five structures that the scientists describe form similar rippled patterns with a few key differences between them. Three of the peptides pack tightly together, while the other two contain solvent molecules within their structures.
“And if you have a system that can encapsulate small molecules, maybe some of those small molecules could be therapeutically interesting,” said Jevgenij, describing the potential use of rippled beta-sheets for drug delivery.
He expects many more uses for rippled sheets to become apparent soon.
“Amyloids are not only toxic substances. Amyloids are also all sorts of materials like nylons and spider silks and god knows what else,” he said. “I have a hunch, based on what I’ve seen so far, that there is a very big room behind that curtain.”
But in the near future, Jevgenij and the rest of the team will continue exploring the formations and structures of rippled beta-sheets. The Seaver Institute announced in January that it will continue supporting the work with a new $200,000 grant.
“Without their contributions and donations, this would not have been possible,” said Jevgenij.
The American Chemical Society also recently indicated support for the budding field. Chemists will gather at the first rippled sheet symposium on March 17. The conversation has come a long way in just a few years, said Jevgenij.
“In 2019 there were not only no crystal structures, people continued to actively doubt that there is such a thing as a rippled sheet,” he said.
The progress has not been easily made. The team of researchers spent several months collecting and refining data on the peptide structures for the recent paper.
“It’s been a lot of work,” said Jevgenij. “Asking the right questions, picking the right sequences, choosing the right conditions and not giving up because it’s failed however many times.”
The response to the hard work has been overwhelmingly positive. In an email to Raskatov, Samuel Gellman, a leading peptide chemist at the University of Wisconsin, Madison, called the work, “a landmark in experimental characterization of the rippled sheet.”
And for Jevgenij, the progress is personal.
“My mother had a stroke which almost killed her when she was 40,” he said. “So I always had great interest in neuro-degeneration, because I’ve seen what it does to people and to people’s children.”
As the field of rippled sheet chemistry emerges, the scientists expect it to inspire new treatments for Alzheimer’s as well as uses far beyond.
