UC Santa Barbara Team Develops Efficient Method for Synthesizing Non-Natural Amino Acids

A UC Santa Barbara research team has developed a technique for efficiently synthesizing non-natural amino acids and applying them to peptide construction. The methodology, published in the Journal of the American Chemical Society, will significantly advance peptide research, giving scientists greater access to amino acids beyond the 22 found in nature.

"The key advantage is that these amino acids come out of the process already in a form that can be used directly to make peptides, without extra modification steps," said first author Phil Kohnke, a doctoral student in senior author Liming Zhang's lab in the Department of Chemistry & Biochemistry. "Compared to existing approaches, this is one of the most straightforward and broadly useful methods reported so far."

The technique allows scientists to access amino acids beyond the 22 found in nature with far greater efficiency. By producing amino acids in a form ready for peptide synthesis, the method removes several difficult steps required in traditional approaches.

Amino acids are the building blocks of proteins, making them among the most fundamental biological molecules. Linking together 10 to 50 amino acids produces a peptide. While proteins are longer, more complex and may consist of multiple peptides. The amino group of one always links to the carboxylic acid group of another. The order of amino acids is a defining characteristic of peptides and proteins.

Although there are hundreds of types of amino acids, only 22 are naturally used by lifeforms to build proteins. These include 20 canonical flavors that are coded for in DNA, and two that are produced by other mechanisms. Scientists can already produce natural amino acids cheaply. "But we have developed an efficient chemical synthesis for making non-natural or noncanonical amino acids in a way that they can be used directly for peptide synthesis," Zhang said.

The recently published paper details a new technique for synthesizing amino acids and then binding them together into peptides using a resin scaffold. The team uses gold catalysis to create amino acids from cheap, readily available chemical ingredients. The technique is highly stereoselective, meaning that it can produce amino acids with a specific handedness instead of an undesired mixture of right-handed and left-handed ones.

Current synthetic techniques require removing the constituent that shields the amino group as well as activating the acid group during peptide synthesis. However, their method produces amino acids where the acid group is already primed to react; only the amino group requires unmasking.

The team used a resin scaffold to assemble peptides from the amino acids. The framework attaches to one side of the growing peptide, enabling them to add amino acids one by one to the molecule in a rinse-and-repeat process. "We basically attach things to resin and then just grow the chain," he said.

This technique is popular in industry because it greatly simplifies the purification process. Rather than go through the tedious effort of purifying the peptides from a solution, the molecules can be cleaved from the scaffold and washed off. "Our method can be ported into this process with very little friction or accommodation," Kohnke added.

Having access to more amino acids opens up entirely new possibilities for biochemists, medical researchers and materials scientists. But making non-natural amino acids is often difficult, expensive or impractical. "Many existing methods either involve many time-consuming steps, only work for a narrow set of molecules, or require further manipulations before ready for peptide synthesis," Kohnke said. The new technique mostly solves these problems, easily and cheaply producing amino acids that are immediately useful for peptide synthesis.

Zhang is particularly interested in developing new peptide therapeutics. Peptides have found use in over 80 drugs worldwide since insulin was first synthesized in the 1920s, which changed type 1 diabetes from a death sentence to an entirely manageable condition. Peptides containing non-natural amino acids can be made more resistant to enzymes or shaped to fit target receptors more effectively.

While the current work is primarily a chemistry breakthrough, it could inform preclinical drug studies in the future by providing medicinal chemists with a richer toolkit for designing and testing peptide therapeutics. The team is now seeking collaborations to make the methodology accessible to researchers working on drug development and materials science.