D-amino acid substitution in peptides
Last reviewed 2026-10-10 · Sable Peptides · Research use only
Sable research method explainer. Updated 2026-10-10. Research use only.
Short answer
Every amino acid except glycine is chiral and exists as an L and a D form, which are mirror images. Proteins made by ribosomes use L-amino acids almost exclusively. Synthetic peptides can include D-amino acids at chosen positions. A single D residue changes the local shape of the chain and usually makes that site much harder for proteases to cut. Sequences mark these residues with a D- prefix.
Why D residues are used in research peptides
Protease resistance. Proteolytic enzymes evolved to recognise L-peptide bonds. Replacing a residue at or next to a cleavage site with its D form often slows hydrolysis there. A 2026 study of a D-amino acid-modified antimicrobial peptide is a recent example of this being measured directly (Khlaychinda et al., 2026).
Shape. A D residue can stabilise a turn, or present a side chain in a different direction, which is why structure-activity studies of short peptides often scan D substitutions position by position.
Synthesis is straightforward. Protected D-amino acids are standard building blocks in solid-phase synthesis, so a D residue costs little extra to install.
Examples in the catalogue
- Triptorelin is native GnRH with glycine 6 replaced by D-tryptophan: pGlu-His-Trp-Ser-Tyr-D-Trp-Leu-Arg-Pro-Gly-NH2. Compare it with gonadorelin, the unmodified native sequence, to see a single-position D substitution.
- GHRP-6 (His-D-Trp-Ala-Trp-D-Phe-Lys-NH2) contains two D residues. Hexarelin is GHRP-6 with 2-methyl-D-tryptophan at position 2.
- Ipamorelin (Aib-His-D-2Nal-D-Phe-Lys-NH2) combines D residues with a non-natural amino acid.
- SS-31 begins with D-arginine: D-Arg-Dmt-Lys-Phe-NH2.
- CJC-1295 carries D-alanine at position 2 of its GHRH(1-29) backbone.
- Melanotan II and PT-141 each contain D-phenylalanine inside their lactam ring.
Going further: retro-inverso peptides
If every residue is replaced by its D form and the sequence is also reversed, the result is a retro-inverso peptide. Its side chains point in roughly the same directions as the parent L-peptide's, but the backbone is reversed. Chorev and Goodman reviewed this design strategy (Chorev and Goodman, 1993). FOXO4-DRI, listed on its own /ref page, is described as a D-retro-inverso peptide.
What this means analytically
A D residue has exactly the same mass as its L form, so mass spectrometry alone cannot confirm stereochemistry. Diastereomers (peptides that differ at one stereocentre) can often be separated by reversed-phase HPLC, and chiral amino-acid analysis can confirm the configuration of each residue. Racemisation during synthesis, where an intended L residue partly converts to D or the reverse, is one source of closely eluting impurities. This is part of why HPLC purity and mass identity are reported together.
At Sable
The /ref pages give each sequence with D residues marked. Batches are released at ≥99% purity by HPLC with identity checked against the expected mass, and the certificate for your batch is available for every lot at /coa. Sable Peptides has been operating since 2019 and supplies every compound mentioned here as laboratory research material only. Nothing on this page is advice on use in people or animals, and no administration or medical-use information is provided.
Key terms
- Chirality
- The property of a molecule that is not superimposable on its mirror image.
- L and D forms
- The two mirror-image forms of a chiral amino acid. Proteins use L forms.
- Diastereomer
- A stereoisomer that is not a mirror image, such as two peptides that differ at one stereocentre.
- Racemisation
- Partial conversion of one stereoisomer into the other during synthesis or handling.
- Retro-inverso
- A peptide of D residues in reversed order.
Common questions
Does a D-amino acid change molecular weight?
No. D and L forms have identical mass. The difference is stereochemistry.
How are D residues shown in a sequence?
With a D- prefix, for example D-Trp in triptorelin.
What is a retro-inverso peptide?
A peptide built from D-amino acids in reversed order. FOXO4-DRI is described this way.
Related explainers
- Non-natural amino acids in peptide analogues: Aib, Nle, Dmt and others
- Cyclic vs linear peptides: what ring closure changes
Related reference pages
Sources
- Khlaychinda T, et al. Enhanced proteolytic stability and distinct mechanisms of a D-amino acid-modified antimicrobial peptide against Pseudomonas aeruginosa. Sci Rep. 2026;16. https://doi.org/10.1038/s41598-026-49275-8
- Chorev M, Goodman M. A dozen years of retro-inverso peptidomimetics. Acc Chem Res. 1993;26(5):266-273. https://doi.org/10.1021/ar00029a007
- Merrifield RB. Solid phase peptide synthesis. I. The synthesis of a tetrapeptide. J Am Chem Soc. 1963;85:2149-2154. https://doi.org/10.1021/ja00897a025
Questions
Does a D-amino acid change molecular weight?
No. D and L forms have identical mass. The difference is stereochemistry.
How are D residues shown in a sequence?
With a D- prefix, for example D-Trp in triptorelin.
What is a retro-inverso peptide?
A peptide built from D-amino acids in reversed order. FOXO4-DRI is described this way.
Sources
CoA for every lot — request yours and we email it within 2 hours.
Research use only. Not for human or veterinary use. No health claims.