Move one thing, change everything
In a normal peptide, each amino acid's side chain (the part that makes it alanine, lysine or phenylalanine) hangs off a carbon atom in the backbone, called the alpha carbon.
What if you moved that side chain one atom over, onto the neighbouring nitrogen instead?
That is a peptoid. Chapter 11 of Peptide Synthesis and Applications, by Paul Hansen and Jens Munk at the University of Copenhagen, describes peptoids as mimics of normal peptides in which the side chains are attached to the backbone nitrogen instead of the alpha carbon. Chemically they are chains of N-substituted glycines.
It sounds like a tiny rearrangement. Its consequences are large.
Three things that change
1. Enzymes cannot cut them. The body's protein-cutting enzymes, proteases, have evolved to recognise the normal peptide backbone. Moving the side chains onto the nitrogen changes the backbone enough that proteases largely fail to recognise it. The chapter states that peptoids are stable to proteases.
That fixes the biggest weakness of peptide drugs: their short life in the body.
2. They have no handedness. Normal amino acids (except glycine) are chiral: they come in left and right-handed mirror-image forms, and only one is natural. Because the peptoid's backbone carbon no longer carries a side chain, the chapter notes peptoids are achiral. That removes a whole category of synthesis problems, including the mirror-image impurities discussed in our earlier series.
3. They fold differently. Without the backbone N-H groups, peptoids cannot form the same hydrogen bonds that shape normal peptides into helices and sheets. The chapter notes they adopt different conformations from peptides. And because both cis and trans geometries of their backbone bonds are common, longer peptoids are complicated to study structurally.
But they often still work
Here is the remarkable part. Despite the rearrangement, the chapter says peptoids retain the overall properties of peptides, and often retain the biological activity of the parent peptide.
Why would that be? Many peptides act largely through the chemical character and spacing of their side chains: a positive charge here, a greasy group there. Peptoids keep the same side chains in roughly similar arrangement, just attached differently. For some activities, that is enough.
The antibiotic angle
The chapter focuses on one application in particular: antimicrobial peptoids.
Antimicrobial peptides are part of the innate immune system across nature. The chapter explains they act as a first line of defence against bacteria and fungi, and that unlike conventional antibiotics, which target specific processes like cell wall synthesis or protein production, most antimicrobial peptides work by disrupting the bacterial membrane itself, though the exact mechanism is not fully understood.
That membrane attack is harder for bacteria to evolve resistance to than a single enzyme target. So antimicrobial peptides are attractive as a response to antibiotic resistance. Their weakness is the usual one: proteases, including those made by bacteria, break them down.
Peptoids offer a way around this. The chapter notes that in recent years they have attracted attention as potential antibiotics against multiresistant bacteria, through new designs, cyclic peptoids, libraries, and converting natural antimicrobial peptides into peptoid versions.
The chapter's worked example is a seven residue antimicrobial peptoid built from a mix of positively charged (lysine-like) and bulky, greasy aromatic side chains, the classic combination for disrupting bacterial membranes.
Beyond antibiotics
The chapter lists other areas where peptoids have been used:
- Studying how molecules get into cells
- Binding to receptors (G protein-coupled receptors, the largest family of drug targets)
- Disrupting protein-protein interactions
- Protease inhibitors and calcium channel blockers
- A peptoid that binds expanded polyglutamine, proposed as a potential therapeutic approach for Huntington's disease
- Combinations with lipids ("lipitoids"), sugars ("glycopeptoids") and DNA-like structures
The submonomer trick
Peptoids have one more advantage: they are easy to make with enormous variety.
The chapter describes the submonomer method, by far the most popular. Instead of pre-making each building block, you build each unit on the resin in two quick steps:
- Attach bromoacetic acid.
- Add any primary amine you like, which displaces the bromine and becomes the side chain.
Repeat. Because thousands of primary amines are commercially available, chemists can install side chains that do not exist in any natural amino acid, cheaply and quickly. The chapter notes peptoids of up to 50 residues have been made this way in reasonable yield.
It also lists the side reactions to watch for, including certain amines forming ring-shaped by-products and some being unstable in the final acid step.
Why this matters to you
Peptoids show that the line between "peptide" and "not a peptide" is blurrier than people assume. Chemists can keep the useful features of a peptide (its side chain chemistry and binding) while swapping out the parts that make it fragile.
They also offer a hopeful angle on one of medicine's most serious problems. Antibiotic resistance is rising, and new classes of antibiotic are rare. Membrane-disrupting molecules that bacteria struggle to resist, built on a backbone their enzymes cannot digest, are one of the more interesting directions being explored.
This series explains laboratory chemistry for general readers. It is not a guide to making or using any compound. Nothing here is medical advice.
