Harvard’s salt trick could turn billions of tons of hair into eco-friendly materials

• SEAS researchers have discovered the chemical mechanism by which certain salt compounds break down protein waste, like wool and feathers.
• The discovery enables a gentler and more sustainable protein recycling process.

The textile and meat-processing industries produce billions of tons of waste annually in the form of feathers, wool and hair, all of which are rich in keratin – the strong, fibrous protein found in hair, skin and nails.

Turning all that animal waste into useful products – from wound dressings to eco-friendly textiles to health extracts – would be a boon for the environment and for new, sustainable industries. But upcycling proteins is challenging: Breaking down, or de-naturing, proteins into their component parts typically requires corrosive chemicals in large, polluting facilities, keeping any cost-effective protocol out of reach.

Researchers in the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have uncovered key fundamental chemistry of how proteins like keratin de-nature in the presence of certain salt compounds – an insight that could take protein recycling to the next level.

A team led by Kit Parker, the Tarr Family Professor of Bioengineering and Applied Physics at SEAS, combined experiments and molecular simulations to better illuminate the chemical mechanisms by which salts cause proteins to unfold. They’ve shown that a solution of concentrated lithium bromide, a salt compound known to break apart keratin, interacts with the protein molecules in a completely unexpected way – not by binding to the proteins directly, as was conventional wisdom, but by changing the structure of the surrounding water molecules to create a setting more favorable for spontaneous protein unfolding.

This insight allowed the researchers to design a gentler, more sustainable keratin extraction process, separating the protein out of solution easily and without the need for harsh chemicals. The process can also be reversed with the same salt mixture, enabling recovery and reuse of lithium bromide denaturants.

The research is published in Nature Communications and is also featured in a Behind the Paper blog post.

Inspired by keratin biomaterials

First author Yichong Wang, a graduate student in chemistry who works in Parker’s group, said the research builds on the lab’s longstanding interest in developing keratin biomaterials with shape memory for biomedical applications. They had previously observed that keratin extracted from lithium bromide solvents can form thick, shapeable gels that readily separate from the surrounding solution and solidify almost immediately when placed back in water. While useful, they found the behavior odd, and they wanted to understand it better.

“We thought there might be a gap between current mechanistic understanding of how de-naturation works, and what we were seeing,” Wang said. “That’s when we got very interested in the mechanism itself to see if we could optimize our extraction procedures by explaining this phenomenon better.”

Molecular dynamics reveals shifts in surrounding water

To dig deeper, the team turned to the lab of Professor Eugene Shakhnovich in the Department of Chemistry and Chemical Biology, whose expertise is in protein biophysics. Molecular dynamics simulations led by co-author Junlang Liu allowed them to see that the lithium bromides were not working on the proteins at all, but rather, on the water around them.

It turns out lithium bromide ions cause water molecules to shift into two different populations – normal water, and water molecules that become trapped by the salt ions. As the normal water volume decreases, the proteins start to unfold due to the thermodynamic shift in the environment, rather than being directly ripped apart like in other de-naturation methods. “Making the water less like water, allows the protein to unfold itself,” Wang said. They had similar results by testing simpler proteins like fibronectin, pointing to a universal mechanism.

Better understanding and designing protein extraction methods that are less energy-intensive and less polluting than conventional ones opens potential avenues for protein-upcycling industries. In the Parker lab, using keratin as a substrate for tissue engineering is a major research thrust; having a reliable, sustainable method to extract and re-use such products would bolster their efforts.

What’s more, the process could lay a path for a whole new biomaterials industry, turning a massive waste stream like hair or chicken feathers into low-cost recycled materials, possibly as an alternative for traditional plastics, for example.

The research had many sources of federal support, including the National Institutes of Health (R35GM139571 and R01EY030444) and the National Science Foundation through the Harvard University Materials Research Science and Engineering Center (DMR-2011764). Other funding came from the Health@InnoHK program of the Innovation and Technology Commission, part of the Hong Kong SAR Government; and the Medical and Health Informatics Laboratories at NTT Research, Inc.