Solid-phase peptide synthesis, step by step

Almost every research peptide you will encounter was built by solid-phase peptide synthesis (SPPS). The method was developed by Bruce Merrifield in the early 1960s and earned him a Nobel Prize, and the basic idea has not changed since: anchor one end of the chain to something solid, then add residues to the free end one at a time.

Why anchoring solves the problem

Building a peptide in solution means purifying the growing chain away from reagents after every single step. For a twenty-residue peptide that is twenty purifications, with losses at each one. The yields become hopeless.

SPPS avoids this entirely. The chain is attached to an insoluble resin bead. Reagents are washed over the beads, react, and the excess is simply rinsed away while the product stays put. Purification between steps becomes a wash rather than a separation.

The cycle

The first amino acid is attached to the resin. Then the same cycle repeats for every residue in the sequence:

• Deprotection. The growing chain’s reactive end is capped with a protecting group to stop it reacting at the wrong moment. That cap is removed to expose the end.
• Washing. The deprotection reagents are rinsed away.
• Coupling. The next amino acid — itself protected, and activated to make it reactive — is introduced and forms a peptide bond with the exposed end.
• Washing. Excess reagent is rinsed away, and the cycle begins again.

Note that synthesis runs from the C-terminus to the N-terminus — backwards relative to how sequences are conventionally written, and backwards relative to how a ribosome does it.

Protecting groups

Amino acids have several reactive sites. Without protection they would join in every direction at once and produce a tangle. Protecting groups mask everything except the one position that should react, and are removed selectively at the right moment.

Two strategies dominate, named after the group protecting the chain end: Fmoc, removed under mild basic conditions, and Boc, removed under acid. Fmoc is the more common choice in modern research synthesis.

Why errors accumulate

Each coupling step is efficient but not perfect. If a step runs at 99% completion, a small fraction of chains fail to receive that residue — and those truncated chains carry on through the remaining cycles.

The arithmetic is unforgiving. Thirty steps at 99% each leaves roughly 74% of chains correct. At 98% it falls to about 55%. This is why longer peptides are harder and more expensive to produce at high purity, and why the crude product always needs purification before it is of any use.

Cleavage

When the sequence is complete, the chain is cut from the resin and the remaining protecting groups are stripped, usually with a strong acid cocktail. What results is crude peptide: the target compound mixed with truncated sequences and by-products. That mixture then goes to purification.