Diastereomers: the impurity no mass spectrum will find

A peptide made from L-amino acids in which one residue has become D is a diastereomer of the target. It has the same molecular formula, the same monoisotopic mass, the same UV spectrum and, frequently, dramatically reduced biological activity. It is the most consequential impurity class in peptide chemistry and the one least often reported.

Where it comes from

Epimerisation happens during activation. When a carboxyl group is activated for coupling, the α-proton becomes more acidic; base-mediated abstraction and reprotonation from the other face inverts the stereocentre. Residues most at risk are those whose activated species is most stabilised — cysteine and histidine notoriously, then serine and aspartate — and the risk rises with base strength, temperature, and time between activation and coupling.

It also happens by design in reverse: several compounds in this index deliberately contain a D-residue, and their principal impurity is the all-L analogue. Triptorelin (D-Trp6), leuprolide (D-Leu6), desmopressin (D-Arg8) and melanotan II (D-Phe7) are all in that position, and in each case the mass-identical impurity is a much weaker peptide.

Detection

What the index does about it

• Every compound fingerprint names the diastereomer species expected for that compound and states the resolution achievable on the reference method.
• Purity figures from gradients shorter than the class minimum are marked METHOD LIMITED, with the specific diastereomer named as unresolved.
• Where the diastereomer is partially resolved — a valley between 40 % and 80 % of the smaller peak height — the record reports it as "co-eluting diastereomer, estimated" rather than as a quantified impurity.
• The index does not claim to quantify diastereomeric purity anywhere. It claims to say when a purity figure does not include it, which is the honest available position.