The Acid That Needs Its Own Room

An old method that refuses to retire

Part 2 introduced the two main strategies for building peptides: Fmoc and Boc. Fmoc is the modern favourite. Boc is Merrifield's original method from the 1960s.

You might expect the older method to have faded away. It has not. Chapter 4 of Peptide Synthesis and Applications, written by researchers at the University of Copenhagen and the Torrey Pines Institute, explains why Boc chemistry still has a place, and why it comes with one of the more alarming requirements in all of chemistry.

Hydrogen fluoride

The final step in Boc chemistry, releasing the finished peptide from the resin and stripping off the side chain protection, traditionally uses anhydrous hydrogen fluoride (HF).

The chapter describes HF as hazardous and notes that its use requires special equipment. That is an understatement worth unpacking.

HF is unusual among acids. It is not the strongest acid in the chemical sense, but it is uniquely dangerous to people. It passes through skin easily and binds calcium in the body, which can damage deep tissues and disrupt heart rhythm even from exposures that look minor at first. It also dissolves glass, so it cannot be handled in normal lab glassware. Labs that use it have dedicated HF apparatus made of resistant plastics, specific safety procedures, and antidote gels on hand.

That is why the chapter's advantages and disadvantages table lists "Requires special equipment: Yes" and "Safety: Potentially dangerous" for Boc chemistry, versus "No" and "Relatively safe" for Fmoc.

So why use it at all?

The same table shows what Boc chemistry offers in exchange:

Those are real advantages, and the chapter explains the chemistry behind them.

The charged chain trick

The most interesting explanation concerns aggregation, the tendency of growing peptide chains to stick to each other on the resin and become inaccessible (Part 4 goes into this in detail).

When the Boc cap is removed with acid, the newly exposed amino group on the chain carries a positive charge. When the Fmoc cap is removed with base, the amino group is neutral.

Positive charges repel each other. So in Boc chemistry, the growing chains are less likely to fold up and clump together. The chapter describes the charged amino group as limiting the formation of secondary structure on the resin.

It is a lovely example of how a small chemical detail, whether an atom carries a charge, changes the outcome of a whole synthesis.

In situ neutralisation

That positive charge has to be dealt with before the next amino acid can attach. The chapter describes a technique called in situ neutralisation: adding extra base (DIEA) during the coupling step, so the charge is neutralised at the same moment the next amino acid is joined. Using excess activated amino acid, two to four times the needed amount, the chapter says coupling yields above 99% per step are routinely achieved.

When Boc is the right choice

The chapter lists situations where Boc chemistry is preferred:

• Hydrophobic peptides, such as sequences from membrane-anchored proteins, which aggregate badly under Fmoc conditions.
• Peptides with ester bonds in their backbone (depsipeptides), which would be damaged by the base used in Fmoc chemistry.
• Peptide thioesters, needed for the native chemical ligation technique used to stitch peptides together into proteins. Thioesters do not survive the repeated base treatments of Fmoc synthesis.

The chapter on cyclic, disulfide-rich plant peptides (by David Craik's group in Queensland) says the same thing from experience: despite needing HF, Boc is their preferred method because of higher deprotection and coupling yields.

When it is the wrong choice

Boc chemistry is not suitable for everything. The chapter notes that peptides with acid-sensitive modifications, particularly those carrying sugars (glycopeptides) or phosphate groups (phosphopeptides), cannot be made this way, because the strongly acidic final step would strip those modifications off.

A detail about salts

One more point from the chapter that connects to later parts of this series. Peptides released with HF end up as fluoride salts, which the chapter says tend to be more soluble than the TFA salts produced by Fmoc chemistry.

That is a reminder that a synthetic peptide is almost never just the peptide. It comes with a counterion, a small charged partner left over from the final step. Part 7 explains why that matters when you read a purity figure.

Why this matters to you

The Boc versus Fmoc story shows that peptide synthesis involves genuine trade-offs, and sometimes the best chemistry is also the most dangerous. It is one more reason peptide manufacturing belongs in properly equipped facilities with trained people, and one more reason the question "who made this, and how?" is never trivial.

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