A Faster Route to Non-Natural Amino Acids Could Expand the Next Generation of Peptide Drugs

Nature builds proteins from 22 amino acids. Twenty are encoded in DNA, and two are produced through specialized cellular machinery. That set of 22 covers everything from the structural fibers of tendons to the enzymes that drive metabolism - a remarkable range of function from a tightly constrained molecular vocabulary.

Chemists and drug developers have long known that this vocabulary could, in principle, be expanded. Hundreds of non-natural amino acids exist that biology never evolved to use. They can be incorporated into peptide chains using laboratory synthesis, and when they are, they can confer properties that natural amino acids cannot: resistance to enzymatic degradation, specific conformational shapes that lock onto target receptors, unusual electronic properties. The problem is making them. Non-natural amino acid synthesis is often difficult, expensive, and produces molecules in forms that require further chemical modification before they can be strung together into peptides.

A research team at UC Santa Barbara, led by Liming Zhang and doctoral student Phil Kohnke, has developed a synthesis method that addresses those obstacles. Their approach, published in the Journal of the American Chemical Society, uses gold catalysis to produce non-natural amino acids that emerge from the reaction already formatted for solid-phase peptide synthesis - the industrial standard method for assembling peptide chains.

Why existing synthesis routes fall short

Amino acids have a specific architecture: an amino group (NH2) on one end and a carboxylic acid group (COOH) on the other, with a variable side chain in the middle. When you build a peptide, the acid group of one amino acid reacts with the amino group of the next to form a peptide bond. But both groups need to be chemically protected or activated at the right time - otherwise the amino acids link to themselves or to each other in the wrong order.

Standard synthesis requires removing a protecting group from the amino end and activating the acid end at each step in the chain assembly. Many existing methods for making non-natural amino acids produce molecules where both groups need additional chemical manipulation before they are ready for this process. That adds steps, increases costs, and narrows the range of chemists who can practically use the compounds.

"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," said Kohnke. "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."

How the method works

The Zhang lab's approach uses gold catalysis - a class of reactions where gold atoms serve as catalysts to enable chemical transformations not easily achieved by other means - to build non-natural amino acids from cheap, commercially available starting materials. The reaction is highly stereoselective, meaning it produces the amino acid in a specific spatial orientation rather than a mixture of left-handed and right-handed forms. Chirality matters enormously in drug chemistry: the two mirror-image forms of a molecule often have different biological activities, and one may be therapeutic while the other is inert or harmful.

Crucially, the acid group of the resulting amino acid is already in an activated form ready to form peptide bonds. Only the amino group requires deprotection - one step instead of two - before the amino acid can be added to a growing peptide chain on the solid-phase synthesis resin.

That resin-based assembly is the standard industrial and research workflow. Peptides are grown one amino acid at a time while attached to a solid bead, then cleaved off and washed away at the end. It greatly simplifies purification and is used by both academic labs and pharmaceutical manufacturers. The new synthesis route ports directly into this workflow with minimal adaptation. "Our method can be ported into this process with very little friction or accommodation," Kohnke said.

Why access to more amino acids matters for drug development

Peptide drugs represent a growing sector of the pharmaceutical industry. More than 80 peptide drugs have been approved worldwide since insulin was first synthesized in the 1920s. The class of drugs known as GLP-1 receptor agonists - which includes semaglutide (Ozempic) and related molecules - are among the most commercially important drugs developed in recent years, and they depend on peptide engineering. Ozempic itself contains one non-natural amino acid plus a fatty acid modification that extends its half-life in the body.

Natural peptides are degraded quickly by enzymes circulating in the body. Incorporating non-natural amino acids at strategic positions can protect the peptide from those enzymes, extend its activity, or force it into a shape that binds its target more tightly. "By incorporating non-natural amino acids, drug designers can 'armor-plate' the peptide against enzymes or force it into a specific shape to lock onto a receptor better," Zhang said.

Having access to a larger and cheaper library of ready-to-use non-natural amino acids expands the chemical space that medicinal chemists can explore when designing new peptide drugs. Zhang's lab is now working to automate the synthesis method and make it accessible to non-chemist collaborators in drug development and materials research.

Scale and validation still ahead

The published work establishes the chemistry and demonstrates the method's selectivity and utility in a research context. Whether the gold catalysis approach can be scaled to the quantities required for pharmaceutical manufacturing - and whether it retains its stereoselective performance at scale - has not yet been demonstrated. Gold catalysts also carry their own cost and material supply considerations that would factor into large-scale adoption. The team is working to address those practical questions, with automation of the process as a near-term priority.