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Biologists have long known that amino acids can help stabilize proteins, for example as additives to pharmaceutical formulations. In trying to understand why this works, EPFL and MIT researchers have discovered a fundamental stabilizing effect of all small molecules, creating exciting possibilities for controlling particles in solution.

For decades, amino acids have been added to medical formulations like insulin as stabilizers: these small molecules keep proteins (i.e. larger particles) from interacting in undesirable ways. And for decades, scientists have known that this works—but not why.

Now, an international team of scientists, led by the Supramolecular Nano-Materials and Interfaces Laboratory in EPFL's School of Engineering, has finally explained the 'why'—and in the process, unearthed a fundamental stabilizing effect of all small molecules in solution.

The discovery has been published in in collaboration with Alfredo Alexander-Katz at MIT and researchers at the Southern University of Science and Technology in China, including EPFL alumnus Zhi Luo.

"When suspended in solution, proteins are constantly changing shape around a central form, and so the prevailing theory has been that amino acids help keep proteins from misfolding," explains recent EPFL Ph.D. graduate and first author Ting Mao.

"Now, we show that this is not the case. In fact, the stabilizing effect of amino acids has little to do with biology but is rather a general property of all small molecules in relation to larger particles, known as colloids, in solution."

Balancing attraction and repulsion

To understand this colloidal effect of small molecules, Supramolecular Nano-Materials and Interfaces Laboratory head Francesco Stellacci suggests imagining two colleagues walking toward each other on opposite sides of a hallway.

"Imagine these two colleagues get along really well and always want to stop and chat. If the hallway is empty, they will immediately spot each other and come together. But if the hallway suddenly becomes crowded, they may not see each other until they have already walked past, or even miss each other entirely," Stellacci explains.

"This phenomenon, called screening attraction, is how amino acids affect larger particles: they play the role of the crowd in the hallway, discouraging passing interactions."

Interestingly, scientists have known for over a century that salts do the opposite: they screen repulsion. In the hallway example, salt also plays the role of the crowd; only in this case, it prevents two unfriendly colleagues from avoiding an awkward interaction.

"What we have discovered is that amino acids are essentially the anti-salt, because they have an opposite 'screening' effect. You can even see this in nature: it has been shown that when a plant is watered with salty water, its cells will produce more amino acids to help stabilize them as they become stressed by the increased salt concentration," says EPFL scientist and co-author Quy Ong.

Better control of molecular interactions

The researchers say that their work provides a strong argument for reporting amino acid concentrations in scientific studies going forward. "In biology, one would never do an experiment without reporting the ionic (salt) concentration of a solution. Our work shows that amino acid concentrations have just as much impact, and should therefore be reported just as rigorously," Stellacci says.

Indeed, Stellacci is already pursuing the untapped potential of these molecular effects. "We want to understand how small molecules like amino acids are central to healthy biological function ... our goal is ultimately to predict which molecules can stabilize which proteins, and how much—something that is currently done by trial and error in biomedical research."