Why a peptide’s molecular weight can differ: salts, caps and oxidation
The molecular weight printed on a product page is usually for the free peptide. The mass on a certificate of analysis can differ for several ordinary reasons. Knowing them makes a COA much easier to read.
Terminal modifications
- N-terminal acetylation adds about 42 g/mol. TB-500 (Ac-LKKTETQ) and thymosin alpha-1 are acetylated.
- C-terminal amidation (–NH2 instead of –OH) lowers the mass by about 1 g/mol. Kisspeptin-10, VIP and ipamorelin are amidated.
- Pyroglutamate at the N-terminus forms when glutamine or glutamate cyclizes. ARA-290 begins with pyroglutamate.
Salt forms
Peptides are often supplied as acetate or trifluoroacetate (TFA) salts. The counter-ions add weight to the powder but are not part of the peptide, so mass spectrometry shows the peptide itself. AOD-9604, for example, is about 1815 g/mol as the free base and heavier as the acetate salt.
Oxidation and other shifts
- Methionine oxidation adds 16 g/mol per methionine. SNAP-8, MOTS-c and Semax all contain methionine.
- Disulfide bonds remove 2 g/mol (two hydrogens). AOD-9604 has one.
- Charge states: electrospray mass spectra show ions such as [M+2H]2+, which appear at about half the mass. The report usually converts these back to the neutral mass.
Free peptide vs a related form
Some names cover more than one molecule. TB-500 may mean the 889 g/mol fragment or the 4963 g/mol thymosin beta-4, and KPV may be free or capped (Ac-KPV-NH2). The identity panel settles which one is in the vial.
Related guides
How mass spectrometry confirms identity · HPLC purity: what the percentage means
More guides
- BPC-157 vs TB-500: two research peptides compared
- Reading a mass spectrum on a peptide COA: m/z, charge states and adducts
- GHK-Cu blends explained: GLOW and KLOW compositions
- CJC-1295 with and without DAC: what the names mean
- Short peptide bioregulators: an overview for researchers
- Thymosin alpha-1: structure, history and research
- Epitalon (AEDG): what the research covers
- KPV: the α-MSH tripeptide explained
- MOTS-c: the mitochondrial-derived peptide explained
- Adamax and Semax: what is known about this analog
- Selank: the tuftsin-derived heptapeptide explained
- SNAP-8 vs Argireline: two SNAP-25 mimic peptides compared
- AHK-Cu vs GHK-Cu: comparing two copper tripeptides
- GHK-Cu: structure, history and published research
- HPLC purity: what the percentage on a COA means
- Mass spectrometry: how a peptide's identity is confirmed
- How to verify a lot with the QR code on the vial
- Lyophilized peptides: what they are and how labs store them
- How to choose a research compound supplier: 7 points
- How to read a certificate of analysis (COA)
Open the COA Portal · Browse the catalog
All LA Peptides products are sold for laboratory research only. Not for human or veterinary use.
For laboratory research use only. Not for human or veterinary use. This guide summarizes published scientific literature for context. It is not medical advice and does not describe any product as suitable for human use.