The final step is not one reaction

After a peptide has been built on its resin bead, it has to be set free. At the same time, all the protective caps on its side chains (Part 2) have to come off.

Chapter 3 of Peptide Synthesis and Applications, by Søren Pedersen and Knud Jensen, opens with a sentence that sums up why this step is harder than it sounds: treating the resin with a cleavage cocktail does not trigger one simple reaction, but several simultaneous reactions that can compete and interfere with each other.

This is the moment where a well-built peptide can still be damaged.

Trifluoroacetic acid does the heavy lifting

For modern Fmoc chemistry, the final step uses trifluoroacetic acid (TFA), usually at around 95%. The chapter explains why TFA works so well: most resins swell well in it, peptides usually dissolve in it, and it is volatile, so it can be evaporated off afterwards.

In one treatment, TFA breaks the link between peptide and resin and strips the acid-sensitive caps off the side chains.

The problem: stray fragments

When those caps come off, they do not simply vanish. They break away as highly reactive, positively charged fragments called carbocations. The chapter describes them as good electrophiles, meaning they look for electron-rich places to attack.

And a peptide has plenty of those. The chapter lists the vulnerable amino acids:

  • Tryptophan is the most at risk. The chapter warns that the fragment released from the arginine cap (Pbf) can permanently attach to tryptophan's ring, and that this modification generally cannot be reversed.
  • Methionine is easily oxidised to methionine sulfoxide during cleavage and synthesis.
  • Cysteine can pair up with other cysteines when you did not want it to, forming unwanted bridges within or between chains.
  • Histidine and tyrosine can be alkylated in the same way as tryptophan if the fragments are not properly caught.
  • Glutamine at the start of a chain can cyclise into pyroglutamate under acidic conditions.

Scavengers: the sacrificial catchers

The solution is to add scavengers to the TFA, molecules that react with the stray fragments faster than the peptide does, sacrificing themselves to protect it.

The chapter lists the common ones:

  • Water, the simplest scavenger, usually enough for tert-butyl fragments.
  • Silanes like triethylsilane (TES) or triisopropylsilane (TIS), mainly to catch trityl fragments.
  • Thiols such as ethanedithiol (EDT), which protect against oxidation and catch other fragments.

A typical modern cocktail is TFA, silane and water in a 95 : 2.5 : 2.5 ratio. For peptides with several cysteines, methionines or arginines, a thiol is added.

The chapter includes a table of named cocktails that peptide chemists have used for decades, with letters for names: Reagent B, K, L, P, R, T. Reagent K, a mix of TFA, phenol, water, thioanisole and EDT, was the universal standard for years. The chapter notes newer mixes have replaced it because they are less toxic and less foul smelling.

A note about the smell

That last point is not trivial. The chapter's practical notes warn that thiols are noxious and must be handled in a well-ventilated fume hood, and recommends soaking all glassware and disposables in 50% bleach after use to deal with the smell. It also mentions a newer thiol, DODT, which was chosen partly because it is non-malodorous.

Peptide chemistry, in other words, is not a clean, quiet process. It involves strong acids, reactive fragments and substances you would not want in your kitchen.

Timing matters

The chapter stresses a principle that applies to all of this: expose the peptide to the cleavage mixture only for the minimum time needed. Typical times are 2 to 4 hours. Too long causes side reactions. Too short leaves caps on and peptide stuck to the resin.

It also recommends a test cleavage, a small trial on a little bit of resin, before cleaving the whole batch. That way, problems with a particular sequence show up before the entire synthesis is committed.

Getting the peptide out

After cleavage, the chapter describes the classic work-up:

  1. Filter off the resin.
  2. Concentrate the TFA solution under a stream of nitrogen.
  3. Add ice-cold diethyl ether, which makes the peptide precipitate while many of the cap fragments stay dissolved.
  4. Spin it down in a centrifuge, pour off the ether, and wash again.
  5. Dissolve the peptide in water and acetonitrile and freeze-dry it.

That freeze-dried fluffy powder is the "crude peptide". It is not finished. It still contains deletion sequences, modified chains and other impurities, which usually need to be removed by purification (HPLC).

A small, practical tip from the chapter: keep the ether you poured off until you have confirmed your peptide is in the solid, because very short or very hydrophobic peptides sometimes do not precipitate and stay in the ether.

Why this matters to you

This chapter explains where many impurities in a finished peptide come from. Some are not errors in building the chain at all. They are damage done in the final hours: an oxidised methionine, a tryptophan with a fragment stuck to it, a cap that did not quite come off.

That is exactly why the next part matters. After all this, how does a chemist actually confirm that the powder in the vial is the right molecule, in the right form?

This series explains laboratory chemistry for general readers. It is not a guide to making or using any compound. Nothing here is medical advice.