Solid-Phase Peptide Synthesis, Explained

The core idea

Solid-phase synthesis anchors the growing chain to an insoluble resin bead. Because the product stays attached to the solid support, excess reagents and by-products can be washed away after every step instead of requiring purification between residues.

Chains are assembled from the C-terminus toward the N-terminus, which is the opposite direction to how sequences are written and how ribosomes work.

The repeating cycle

Each residue is added by the same cycle: remove the temporary protecting group from the chain's N-terminus, couple the next activated and protected amino acid, then wash. In Fmoc chemistry the temporary group is removed with a mild base while side-chain protection stays intact.

• Deprotection of the terminal amine
• Coupling of the next protected residue with an activator
• Washing away excess reagent
• Repeat until the sequence is complete
• Cleave from resin and remove side-chain protection

Where impurities come from

No coupling step is perfectly efficient. A failure at one position produces a deletion sequence — the full peptide minus one residue — which is the classic impurity seen just beside the main HPLC peak.

Incomplete deprotection, oxidation of sensitive residues during cleavage, and side reactions with scavengers account for most of the rest. Longer sequences accumulate more of all of them, which is why a fifty-residue peptide is harder to purify to 99% than a five-residue one.

Purification and isolation

Crude peptide is purified by preparative reverse-phase HPLC, the pure fractions are pooled, and the solution is lyophilized to a stable powder. Counter-ions from the mobile phase are carried into the final product, which is why net peptide content is below 100%.