Chiral purity on a peptide COA: D-amino acids, racemization and why standard HPLC can miss them

What chiral purity means for a peptide

Amino acids other than glycine exist in two mirror-image forms, L and D. The EMA guideline notes that, with the exception of glycine, all natural amino acids in synthetic peptides are optically active with at least one stereogenic centre. [1] IUPAC notation assumes L unless a residue is marked D-, so an unmarked sequence claims every chiral residue is L. [2] [3]

One wrong residue in an otherwise all-L peptide does not make a mirror image of the peptide. It makes a diastereomer: same formula, same mass, different shape. The EMA says that in theory a multitude of different diastereoisomers of a target peptide can form. [1]

Where D-residues come from

The EMA names two sources. Starting materials: protected amino acid building blocks can carry enantiomeric and diastereomeric impurities, and isomeric impurities in them can lead to stereoisomers of the final peptide. Process: epimerisation during synthesis, for example via oxazolone formation. The guideline notes that urethane-type protecting groups such as Fmoc and Z are rather resistant to oxazolone formation, and that additives and bases in the process also suppress epimerisation. [1]

Bachem lists incorrect enantiomers among the impurities in synthetic peptides and describes optical purity as an important parameter for amino acid derivatives and loaded resins, the materials upstream of the peptide. [5] That is a reasonable control point: a manufacturer that tests every building block may not test every finished lot.

Why a standard certificate can miss it

Intact-mass MS cannot see a diastereomer, because it has the same formula as the target. Standard reverse-phase HPLC may or may not resolve it. The EMA says control of diastereomers may require the development of specific methods, and that when impurities co-elute as one peak the 1.0% qualification threshold applies to the combined peak unless justified. [1]

Illustrative arithmetic shows why this matters. Take a fictional 10-residue peptide with nine chiral residues. If each position independently carried 0.2% of the wrong isomer, about 1.8% of molecules would contain at least one D-residue (1 - 0.998 to the ninth power). At 0.1% per position the figure is about 0.9%. If those diastereomers co-elute with the main peak, an HPLC purity of 99% could be counting them as product. These are invented numbers to show scale, not typical values.

Methods that do measure it

The EMA characterisation table lists chiral GC-MS, LC, other chiral chromatography, and enantiomeric purity after partial or complete enzymatic digestion. It explains the usual approach: hydrolyse the peptide in deuterated hydrochloric acid, then separate amino acid enantiomers by chiral GC with a mass spectrometric detector. Hydrolysis itself causes some epimerisation; amino acids that epimerise during hydrolysis pick up deuterium and can be excluded by mass. [1]

C.A.T. GmbH, a contract laboratory that offers this test, describes the same principle. For its deuterated GC-MS method it publishes a quantification limit of 0.10% over a 0.10 to 5% range and asks for 4 mg (single) or 8 mg (duplicate) of peptide. Its HPLC method (quantification limit 0.1%) and a short-hydrolysis GC-MS variant (0.05%) are described for free amino acids and derivatives; the page says peptides are not suited to the short-hydrolysis variant. It notes that Asn is reported as Asp and Gln as Glu because hydrolysis converts the side chains, and that cysteine and tryptophan are challenging. [4] Those limits mean a result of "not detected" is a statement about the method, not proof of zero.

Is it supposed to be on the certificate?

Not necessarily, even for medicines. The EMA guideline allows enantiomeric purity to be a characterisation test with no routine release control, if a risk analysis or prior knowledge of epimerisation mechanisms justifies it. [1] A research certificate without a chiral line is normal. The question is whether anyone measured it, at the building-block stage or on the peptide, and whether the answer applies to your lot.

Supplier statements vary. Peptides EU states that purity is determined by reverse-phase HPLC and identity by mass spectrometry, and that it publishes selected independent certificates; its FAQ does not mention chiral or enantiomeric testing. [6] OMNI Peptides restricts its catalog to qualified researchers and its public homepage showed no analytical detail on 30 October 2026. [7] Neither statement tells you whether chiral purity was measured.

How to ask when the certificate is silent

Send the questions in writing and file the answer with the lot record. A supplier that cannot answer is giving you information too.

• Was enantiomeric purity measured on this lot of peptide? If so, by which method, with which quantification limit, and can you share the report?
• If not, is optical purity specified for the protected amino acid building blocks and loaded resin? What limits apply?
• Does the HPLC method used for the purity figure separate any known diastereomers of this sequence?
• Does the sequence contain residues known to be harder to analyse (cysteine, tryptophan) or that hydrolysis converts (asparagine, glutamine)?
• If the sequence includes an intended D-residue, how was its configuration confirmed?