How Peptide Synthesis Works: SPPS Explained

Every research peptide sitting in a freezer — from a 15-mer like BPC-157 to a much larger sequence — was almost certainly built using the same core method: solid-phase peptide synthesis, or SPPS. Developed by Bruce Merrifield in the 1960s (work that later earned him a Nobel Prize), SPPS remains the dominant technique for producing synthetic peptides, precisely because it converts a difficult solution-phase chemistry problem into a much more controllable, repeatable process.

The core idea

Before SPPS, peptides were built in solution, one bond at a time, purifying the product after every single step — a slow, low-yielding process that became essentially impossible for longer sequences. Merrifield's insight was to anchor the growing peptide chain to an insoluble solid support (a resin bead), so that after each reaction step, everything that isn't attached to the resin can simply be washed away. The peptide chain itself never leaves the resin until synthesis is complete.

The basic cycle

SPPS builds a peptide chain from the C-terminus to the N-terminus, one amino acid at a time, through a repeating cycle:

1. Deprotection. Each amino acid added to the chain has a temporary protecting group on its amine end, preventing it from reacting prematurely. This protecting group is removed chemically, exposing a reactive amine ready for the next coupling step.
2. Coupling. The next amino acid in the sequence — also protected, and typically "activated" to make its carboxyl group more reactive — is introduced and forms a new peptide bond with the exposed amine on the resin-bound chain.
3. Washing. Excess reagents and byproducts are washed away, since only the resin-bound, growing peptide chain is retained.
4. Repeat. The cycle repeats, one residue at a time, until the full sequence has been assembled.

Two major protecting-group chemistries dominate modern SPPS: Fmoc (fluorenylmethyloxycarbonyl) and Boc (tert-butyloxycarbonyl). Fmoc chemistry, removed under mild basic conditions, has become the more widely used approach in commercial and research peptide synthesis, largely because it avoids the highly corrosive hydrofluoric acid required to cleave Boc-protected peptides from resin at the end of synthesis.

Cleavage and purification

Once the full sequence has been assembled, the peptide is cleaved from the resin and any remaining protecting groups on amino acid side chains are removed — typically using a strong acid cocktail for Fmoc chemistry. What comes off the resin at this point is described as "crude" peptide: it contains the target sequence, but also a range of byproducts from incomplete couplings, side reactions, and deletion sequences (where a coupling step failed and synthesis continued anyway, producing a peptide missing one internal residue).

This crude material is then purified, almost always by preparative HPLC, separating the target peptide from these byproducts based on differences in hydrophobicity. The purified fractions are pooled, and the product is typically lyophilized (freeze-dried) into a stable powder — which is the point at which purity testing, discussed in our article on purity, comes into play.

Why chain length is a limiting factor

Every coupling step in SPPS has a yield less than 100% — even a highly efficient 99% coupling yield, applied over 50 sequential steps, compounds to roughly 60% overall yield of correctly formed peptide by the end (0.99 raised to the 50th power). This is the core reason longer peptides are harder and more expensive to synthesize by SPPS, and why very long sequences are often instead produced using recombinant (biological, rather than chemical) methods.

SPPS turned peptide synthesis from a bespoke, low-throughput solution chemistry problem into an automatable, repeatable process — which is a large part of why synthetic peptides are as widely available for research today as they are.