Most people picture research peptides being grown, like something brewed in a vat of cells. Short ones usually aren't. They're built chemically, one link at a time. And the way they're built explains most of the numbers on your certificate.
Short peptides are built, not grown
For a short chain, chemists use a method called solid-phase peptide synthesis. The first amino acid gets anchored to a tiny resin bead. Then the next one is added, then the next, with a rinse in between, until the chain is finished and cut free. Today's version, called Fmoc synthesis, is the method of choice for making peptides (Behrendt et al., J Pept Sci 2016).
The proof: the bead that won a Nobel Prize
Let's get into it.
It started with one chemist and a bead. In 1963, Bruce Merrifield at the Rockefeller Institute in New York showed how to build a short chain, four amino acids long, on a solid bead. The trick was simple. The growing chain stays stuck to the bead, so you can wash away the leftovers after every step instead of purifying the whole thing each time. In 1984 it won him the Nobel Prize in Chemistry (Nobel Prize).
Then he went big. In 1969, Merrifield and Bernd Gutte built ribonuclease A, an enzyme 124 amino acids long, from scratch, and the finished chain worked as an enzyme (Gutte and Merrifield, JACS 1969).
It even runs backwards. Cells build protein chains from the front end to the back. Solid-phase synthesis usually starts at the back end, anchored to the bead, and adds links toward the front. (Chemists like to keep things interesting.)
Every step is a small bet. Here's the part nobody tells you. Each link has to snap on, and not every chain gets every link. Say each step works 99% of the time. Over 20 links, that's 0.99 multiplied by itself 20 times, so about 82% of chains come out complete. The rest are near-twins missing a link or two, called deletion sequences.
So why should you care? That's what testing catches. Those near-twins look almost exactly like the real thing, so the raw product gets purified, then tested. HPLC purity shows what share of the peptide is the full, correct chain. LC-MS identity weighs it, and a chain missing a link weighs less. Net peptide content shows how much of the powder is peptide at all, since the finished powder also carries salts and water.
"Is synthetic the same as natural?" If the sequence is the same, it's the same molecule. A chain of amino acids doesn't remember where it came from. What can differ is everything else in the vial, and that's why the certificate matters more than the word "synthetic." Some proteins, like insulin, are usually made by engineered cells instead.
The big learning lesson
Keep reading: How to read a peptide certificate of analysis · What "third-party tested" really means · Made in USA vs imported peptides
Frequently asked questions
How are peptides made?
Most short peptides are made by solid-phase synthesis. Amino acids are added one at a time to a chain anchored on a tiny resin bead, then the finished chain is cut free and purified. Larger proteins are often made by engineered cells instead.
Who invented solid-phase peptide synthesis?
Bruce Merrifield at the Rockefeller Institute, who published it in 1963 and won the 1984 Nobel Prize in Chemistry for it.
Are synthetic peptides the same as natural ones?
If the amino acid sequence is identical, the molecule is identical. Differences come from impurities, such as chains missing a link, which purification and testing are meant to catch.