This week's Chemistry Nobel puts mirror-image molecules in focus. Here is what that means for peptide testing

On 7 October 2026 the Royal Swedish Academy of Sciences awarded the Nobel Prize in Chemistry to Henri B. Kagan of Université Paris-Sud and Kenso Soai of Tokyo University of Science "for the discovery of non-linear effects and autocatalysis in asymmetric organic synthesis." [1]

The prize is about chirality, sometimes called handedness. Many molecules exist in two forms that are mirror images of each other, like a left and a right hand. The Academy's background material notes that amino acids exist as two mirrored variants, yet only one of them is found in the proteins of living cells. [2] Kagan showed in 1986 that the purity of a catalyst's handedness does not have to carry over to the product in a straight line, and Soai later designed a self-copying reaction in which one mirror-image form takes over almost the entire product. [2]

Why a chemistry prize matters to peptide labs

Peptides are chains of amino acids, and every amino acid in a typical peptide sequence except glycine has a stereocentre that can, in principle, sit in the "wrong" mirror-image orientation. The European Medicines Agency's guideline on the development and manufacture of synthetic peptides, which came into effect on 1 June 2026, makes the same point: with the exception of glycine, all natural amino acids in synthetic peptides are optically active, and isomeric impurities in starting materials can lead to stereoisomers of the final peptide. [3]

The guideline names two routes for these impurities. The first is the starting materials: protected amino acid building blocks can carry enantiomeric and diastereomeric impurities. The second is the synthesis itself, where a process called epimerisation can flip a single residue's orientation during coupling. [3] A 2023 review in Molecules covers the factors that drive epimerisation during peptide synthesis and the methods chemists use to suppress it. [4]

How labs measure it

Measuring handedness in a finished peptide is harder than it sounds. The usual approach breaks the peptide back down into its amino acids with acid, then separates the left- and right-handed forms on a chiral column. The catch is that the acid step itself causes some racemisation, which would overstate the impurity. [3][5]

The workaround, described in a 1995 Journal of Chromatography A paper, is to run the hydrolysis in deuterated acid. Any amino acid that flips during the test picks up a deuterium atom at its alpha carbon, adding one mass unit, so a mass spectrometer can tell test-induced flips apart from impurities that were already in the sample. [5] The EMA guideline describes the same principle, with chiral gas chromatography coupled to mass spectrometry (GC-MS), and also lists liquid chromatography and other chiral techniques. [3] A 2020 open-access study in Foods shows the scale of the problem: a pure L-histidine standard containing 0.07% of the D-form measured 10.4% D-form after standard acid hydrolysis. [6]

What it means when reading a COA

Two points from the guideline are useful for anyone reviewing analytical paperwork. First, it says separating diastereomers "may require the development of specific methods," so a single standard purity method may not catch every mirror-image variant. [3] Second, it allows enantiomeric purity to be handled as a characterisation test rather than a routine release test, when that is justified by risk analysis or prior knowledge of the process. [3] In practice, that means a routine certificate of analysis will not always report enantiomeric purity.

For more on what a typical COA does and does not cover, see our guide to reading a COA and our explainer on HPLC area percent versus net peptide content.

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