Introduction to Solid Phase Peptide Synthesis

Solid phase peptide synthesis, commonly abbreviated SPPS, is the dominant method for producing research peptides. First described in the 1960s, it allows a defined peptide sequence to be assembled one amino acid at a time in a controlled, stepwise manner. The approach is well suited to producing short-to-medium length peptides at research scale and has been widely adopted because it is amenable to automation and delivers reproducible results. The description below is a factual educational overview; it is not a synthesis protocol and contains no usage guidance for any material.

The defining feature of solid phase synthesis is that the growing peptide chain is attached to an insoluble solid support, usually referred to as a resin. The resin consists of small polymer beads that carry chemical groups capable of forming a covalent bond with the first amino acid in the target sequence. Anchoring the chain to the resin allows reagents and by-products to be removed simply by washing, since the peptide stays on the support while everything else flows away. This is what makes SPPS practical: purification between each step is reduced to a washing procedure rather than a separation.

Different resins are chosen according to the chemistry being used and the desired form of the C-terminal end of the final peptide. Some resins release the peptide as a free acid, while others yield an amide. The choice of resin is part of the design of the synthesis rather than a detail of the individual coupling steps.

Why protecting groups are needed

Amino acids carry multiple reactive groups. Without protection, adding one amino acid to a growing chain would result in unwanted reactions at sites other than the intended bond-forming position. Protecting groups are chemical modifications that block these reactive sites temporarily, ensuring that each new amino acid is added only at the correct position.

Fmoc chemistry

The most widely used approach in contemporary SPPS is Fmoc chemistry, named after the fluorenylmethyloxycarbonyl group used to protect the alpha-amino group during synthesis. The Fmoc group is removed at the start of each cycle under mild basic conditions, revealing the amino group for coupling. Fmoc chemistry is compatible with a wide range of side-chain protecting groups and with the labile resins typically used in modern instruments, which is why it has become the standard approach.

Boc chemistry

The earlier approach uses a tert-butyloxycarbonyl (Boc) group to protect the alpha amino group. Boc chemistry requires acid conditions for deprotection and a stronger acid for final cleavage from the resin. While less commonly used for routine synthesis today, Boc chemistry remains relevant for certain sequences and modifications where Fmoc approaches face limitations.

Each amino acid is added to the growing chain through a repeating sequence of steps. The cycle begins with deprotection: the protecting group on the terminal amino group is removed, freeing it to react. The next amino acid to be added is then activated, meaning its carboxyl group is chemically primed to form a new peptide bond efficiently. Coupling joins the activated amino acid to the free amino group of the chain. A capping step is often included to block any unreacted chains so they do not accumulate as deletion sequences. Finally, the resin is washed to remove all reagents before the next cycle begins.

This cycle is repeated for each residue in the target sequence, starting from the C-terminal amino acid and extending toward the N-terminus. Because the sequence is built up one residue at a time, the design of the synthesis directly dictates the identity of the product. Automation of the cycle, including reagent addition and washing, is routine in modern synthesisers, which supports high throughput and reproducibility.

Once the sequence is complete, the peptide must be detached from the resin and the remaining side-chain protecting groups must be removed. This is achieved in a cleavage step using an appropriate reagent, typically an acid mixture. The reaction releases the peptide into solution and removes the protecting groups simultaneously. The resulting crude material is a mixture of the target peptide and the removed protecting groups, resin fragments, and other components. This crude mixture is then processed further before purification.

After cleavage, the crude peptide is purified to isolate the target sequence from by-products. Reversed-phase high-performance liquid chromatography is the standard purification method and takes advantage of differences in hydrophobicity between the target peptide and related impurities such as deletion sequences or incompletely deprotected material. For an explanation of how reversed-phase chromatography works, see Reverse Phase Chromatography Fundamentals.

After purification, analytical methods are used to characterise the material. Chromatography indicates the proportion of the sample that corresponds to the target peptide, and mass spectrometry helps confirm identity by comparing the measured molecular weight against the expected value. For these methods, see Understanding HPLC Analysis and Understanding Mass Spectrometry. A broader account of the overall manufacturing process is given in How Peptides Are Manufactured. Produced materials can be viewed in the catalogue.