Research peptides are short chains of amino acids. Easy enough. Then you put two vials with the same label into the same assay and get two different answers, and suddenly it isn’t easy. Usually the culprit is stuff you can’t see: leftover salts, chains missing a residue, junk from synthesis. Here we go through what peptides are, how they get made, how purity is checked, and why they fall apart on the shelf. We’ve linked the primary papers so you can check us.
Key findings
- Over 80 approved peptide drugs by 2021. Insulin kicked it off a century back.
- Solid-phase synthesis (Merrifield, 1963) still makes most of them.
- Deletion and insertion sequences are common synthesis impurities, and they can skew early functional studies.
- Leftover TFA from HPLC, at just 10 to 100 nanomolar, slowed osteoblast growth in culture. Yes, really.
Peptide, protein, whatever?
Amino acids, joined by amide bonds, in a chain. That’s a peptide. When it becomes a protein is honestly a bit fuzzy. People usually say peptides run up to about 50 residues, but you’ll see different cutoffs in different papers, and nobody’s going to arrest you over it.
Size is the whole appeal. The size is why we like them. Big enough to grab a receptor properly. Small enough that a chemist can just build one, no cell culture needed. Muttenthaler and colleagues tallied more than 80 marketed peptide drugs in a 2021 Perspective, covering everything from diabetes to HIV to chronic pain [1]. A separate review from Ferring scientists tracked over 150 peptides in active clinical development [2].
That’s the pharma side. For a lab, it means one thing. There’s a deep literature on how these molecules are built, tested and broken down, and you can lean on it.
How research peptides get made
Almost every research peptide on the market is made by solid-phase peptide synthesis, or SPPS. Merrifield came up with it in 1963. You stick amino acid one onto a resin bead. Add number two. Rinse. Add three. Rinse again. It’s almost boring, which is sort of the genius of it. At the end you cleave the finished chain off the bead.
The early version had a real flaw. Purity dropped with every coupling step, so longer peptides came out dirtier. Boc and then Fmoc protecting groups improved things a lot, and later methods like native chemical ligation let chemists join unprotected fragments to build longer chains [3].
Here’s what that means in practice. Every step is a chance to make a mistake, and a 30 residue peptide has a lot of steps. Some chains will miss an amino acid. Some will pick up an extra one. Those leftovers end up in your vial unless purification catches them.
Purity: what HPLC and mass spec actually tell you
A Ghent University review lays out the usual suspects. Deletion sequences come from incomplete Fmoc removal. Insertions come from excess amino acid reagent. And the authors make a point we think gets ignored far too often: these impurities can change the results of early functional studies and lead to wrong conclusions [4].
Two tests do most of the work. Reversed-phase HPLC pulls the main peptide apart from its near relatives; that purity number is just the main peak’s slice of total peak area. Then mass spec weighs the thing. Right mass, right molecule (probably). You want both. HPLC alone can’t tell you the main peak is the right peptide, and MS alone can’t tell you how much junk is sitting next to it.
Small aside. “98% pure” on HPLC doesn’t mean 98% of the powder by weight is peptide. Water and counterions count too, which is why net peptide content is a separate number. Our explainer on reading a certificate of analysis covers that in more detail.
The salt nobody talks about
HPLC runs on trifluoroacetic acid, so most purified peptides leave the column as TFA salts. Big deal? Turns out it can be. Cornish and colleagues found that TFA at 10 to 100 nanomolar reduced cell numbers and thymidine uptake in fetal rat osteoblast cultures within 24 hours. Chondrocytes and mouse calvariae showed the same thing. When they compared TFA and hydrochloride salts of amylin and calcitonin, the TFA versions consistently gave lower proliferation. That’s enough to bury a real effect, or invent a fake one [5].
Their fix: swap to a hydrochloride salt (or something equivalent) before any biology above 1 nanomolar. If your cell work looks off, check the counterion before you rewrite your hypothesis.
Storage and stability
Peptides degrade, and the chemistry is well mapped. Deamidation, oxidation, hydrolysis and aggregation are the big ones. A widely cited review of protein stability, which covers the same pathways, splits the problem into chemical and physical instability and treats the dried state separately from aqueous solution, because they behave very differently [6].
What do we actually do? Powder goes in the freezer, dry and dark. A cold vial sits on the bench until it’s at room temperature before anyone pops it open, otherwise you get condensation right on the powder (ask us how we learned that). Dissolved peptide is on a much shorter timer, so we aliquot instead of thawing one stock tube ten times. And we keep an eye on sequences carrying Met, Cys or Trp, which oxidize, or an Asn sitting next to Gly, the classic deamidation spot.
None of this is exotic. It’s just easy to skip on a busy day, and then you’re chasing a ghost in your data three weeks later.
What to ask a supplier
Ask for HPLC and MS on your lot. Not some sheet from 2019 that gets attached to every batch. Results from a U.S.-based third-party laboratory mean you aren’t just taking the seller’s word for it. What’s the counterion? If it’s a long or awkward sequence, how was it cleaned up? Crude purity on those is usually lower, and a good supplier won’t mind the question.
Want to see how we handle real compounds? Try our BPC-157 compound page and our ipamorelin, GHRP-2 and GHRP-6 comparison. Both lean hard on primary papers.
References
- Muttenthaler M, King GF, Adams DJ, Alewood PF. Trends in peptide drug discovery. Nat Rev Drug Discov. 2021. PMID 33536635
- Lau JL, Dunn MK. Therapeutic peptides: Historical perspectives, current development trends, and future directions. Bioorg Med Chem. 2018. PMID 28720325
- Chandrudu S, Simerska P, Toth I. Chemical methods for peptide and protein production. Molecules. 2013. PMID 23584057
- D’Hondt M, Bracke N, Taevernier L, et al. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014. PMID 25044089
- Cornish J, Callon KE, Lin CQ, et al. Trifluoroacetate, a contaminant in purified proteins, inhibits proliferation of osteoblasts and chondrocytes. Am J Physiol. 1999. PMID 10567002
- Manning MC, Chou DK, Murphy BM, Payne RW, Katayama DS. Stability of protein pharmaceuticals: an update. Pharm Res. 2010. PMID 20143256
Disclaimer: This content is intended for research purposes only and is not meant to constitute medical advice.
This educational material discusses laboratory research concepts only. Loti Labs products are intended strictly for research and development use and are not for human consumption.
