The problem with amino acids

Every amino acid has at least two reactive ends: an amino group (the "N" end) and a carboxyl group (the "C" end). A peptide bond forms when the carboxyl of one amino acid joins the amino group of the next.

Here is the problem. If you throw two amino acids, say A and B, into a flask and activate them, you do not just get A-B. You get A-A, B-B, B-A, and chains of every length. It is chaos.

And many amino acids have a third reactive group on their side chain. Lysine has an extra amine. Aspartic and glutamic acid have extra carboxyls. Cysteine has a thiol. Serine, threonine and tyrosine have hydroxyls. Each one is another place for a reaction to go wrong.

The solution, which the Peptide Synthesis and Applications manual explains in its first chapter, is protection: temporarily capping every reactive group you do not want to react, then removing the caps at exactly the right moment.

Why peptides are built backwards

In your body, proteins are made starting from the N end and adding towards the C end. Solid-phase synthesis does the opposite.

The chapter explains that protected amino acids are coupled one by one so that the chain grows from the C terminus toward the N terminus. The first amino acid, the one that will end up at the C terminal end, is attached to the resin through its carboxyl group. Then each new amino acid is added to the free amino end.

The manual even has a small rule for chemists drawing it: always draw the nitrogen on the left and the carboxyl on the right, and so when showing solid-phase reactions, the resin must be on the right.

Two kinds of cap

Every amino acid used in synthesis carries two kinds of protection:

A temporary cap on its amino end. This is removed after each coupling so the next amino acid can attach. It comes off dozens of times during a synthesis.

Semi-permanent caps on its side chain. These stay on for the entire synthesis, and come off only at the very end.

The trick is that the conditions used to remove the temporary cap must not touch the semi-permanent ones. The chapter describes this as each strategy defining a "chemical window" of what else is possible.

Fmoc versus Boc

There are two main strategies, named after their temporary caps.

Fmoc (fluoren-9-ylmethyloxycarbonyl) is removed by a mild base, typically a 1 in 5 solution of piperidine in the solvent DMF. The side chains are protected with groups that come off in acid (trifluoroacetic acid, TFA). Base for one, acid for the other: the two never interfere. Chemists call this orthogonality.

Boc (tert-butyloxycarbonyl) is the original strategy Merrifield used. It is removed by TFA, a moderately strong acid. The side chains are protected by groups that need an even stronger acid to come off, traditionally hydrogen fluoride (HF). This relies on graduated acid strength rather than true orthogonality.

The manual states plainly that because of the need for corrosive and toxic HF and special equipment to handle it, the Fmoc strategy is often preferred for routine work. Part 3 explains why Boc is still used anyway.

The standard side-chain caps

For Fmoc chemistry, the chapter lists the modern standard caps, which are worth knowing because they turn up in almost any peptide synthesis discussion:

  • tert-butyl (tBu) esters for aspartic and glutamic acid
  • tert-butyl ethers for serine, threonine and tyrosine
  • Pbf for arginine
  • Trityl (Trt) for cysteine, asparagine, glutamine and histidine
  • Boc for lysine and, often, tryptophan

All of these come off in concentrated TFA at the end of synthesis.

Special caps for special jobs

Some caps are designed to stay on even through the final TFA step. The chapter highlights Acm (acetamidomethyl) for cysteine. Because Acm survives both acid and base, and can be removed separately with iodine, it lets chemists control exactly which cysteines pair up to form disulfide bridges. That is essential for making peptides like insulin with multiple bridges, which Part 2 of our follow-up series covers.

There are also selectively removable caps for lysine (Alloc, Mmt, Dde, ivDde), which let chemists expose one specific lysine while leaving everything else protected. That is how you attach something, such as a fatty acid chain or a fluorescent label, to one exact position on a peptide.

Joining the pieces: coupling reagents

To make a peptide bond, the carboxyl group of the incoming amino acid has to be "activated", turned into something reactive enough to join the amine on the chain.

The chapter walks through the reagents chemists use. Older carbodiimides (DCC, DIC). Additives like HOBt and HOAt that reduce a key side reaction called epimerisation (more on that in Part 7). And the faster "in situ" reagents with names like HBTU, HATU, PyBOP and COMU.

One small, interesting detail: the manual notes that Oxyma, a newer additive, was developed partly as a replacement for HOBt-type additives with a lower risk of explosion. Peptide chemistry involves some genuinely hazardous materials, which is one reason it belongs in properly equipped labs.

Why this matters to you

This is why a synthetic peptide is not a simple product. Each amino acid added involves removing a cap, activating the next building block, coupling it, washing, and checking. A 30 amino acid peptide involves 30 cycles of that, plus the final release. Each step is very efficient, but none is perfect.

Understanding the caps also explains something you will see in Part 6: when chemists check a finished peptide and find it weighs 56 or 242 more than expected, they can often tell which cap failed to come off, just from the number.

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