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Is solid-phase synthesis the right strategy for your project?

When it comes to the production of peptides, solid-phase peptide synthesis (SPPS) is the method of choice. First described in the early 1960s, Merrifield’s method of attaching peptide chains to polymeric solid supports has become a mature industrial process valued for being flexible and cost-effective.

Today, SPPS is the preferred chemistry for generating peptides with diverse structural architectures containing anywhere between 5 to 150 amino acids. It is also the best method to assemble peptides with unnatural or modified residues. 

Over the past decades, peptides synthesized by SPPS have garnered popularity in multiple applications, including the production of peptide drugs, vaccines, cosmetics, and components for drug and gene delivery applications, among others.

What are the advantages of solid-phase peptide synthesis?

SPPS is a scalable, practical, economical, convenient, and efficient process of chemical synthesis of peptides. In comparison to conventional solution-phase strategies, SPPS offers significant advantages. Because new chains are anchored to solid supports, a large excess of reagents can be used to ensure the proper coupling of new amino acids or segments (fragments composed of several amino acids) to growing peptide chains.

Due to the stability of the solid support, the excess reagents can also be easily removed by washing and filtration, significantly simplifying the purification process. Plus, all steps of SPPS can be performed in a single reactor (one-pot synthesis) without the need to transfer materials, effectively reducing the production cost and hands-on time. Alternatively, solid supports may be packed in columns and used in a continuous-flow mode. In this way, excess reagents and washing chemicals can be pumped through the columns to accelerate the process of synthesis.

The flexibility of solid supports makes solid-phase synthesis amenable to automation, making it ideal for the industrial-scale production of peptides.

Major steps in solid-phase peptide synthesis

Whether peptides are synthesized in solution or on a solid support, peptide chain elongation can be achieved in a stepwise manner (one amino acid at a time), through segment condensation (coupling peptide fragments instead of single amino acids), or a combination of the two. Peptide synthesis is based on six general principles, regardless of reaction chemistry:

  1. The use of amino acids modified with protecting groups (on the termini and side chains)
  2. Application of effective termini deprotection strategies
  3. Correct activation of new amino acids or segments
  4. Formation of bonds between new amino acids and the growing chain, also known as coupling
  5. Rapid and efficient washing of reagents prior to the next cycle of synthesis
  6. Final cleavage of the solid support and side chain protecting groups

Although termini protecting groups allow a finer degree of control over the synthesis process, they must be selectively removed before another protected amino acid or segment can be coupled to the growing chain.

Due to the cyclic nature of SPPS, deprotection takes place several times during the synthesis and thus should be fast, free of side reactions, preferentially take place in mild conditions – to avoid removing side chain protecting groups – and render easily eliminated byproducts.

When it is successfully achieved, another step must take place before a new residue can be coupled to the growing chain – the activation of the carboxylic acid moiety of each incoming amino acid. This process makes use of diverse coupling reagents such as carbodiimides (e.g., DCC or DIC), or in situ coupling reagents, often based on phosphonium or aminium/uronium-imonium salts (e.g., PyBOP, TBTU, HATU, COMU).

Deprotection, activation, and coupling steps happen cyclically until the solid-phase peptide synthesis is completed. At this point, it is vital to detach the peptide from its solid support and remove all remaining side chain protecting groups without significantly altering the peptide’s composition.

In standard SPPS processes, this cleavage and global deprotection are carried out in the presence of 1-5% of trifluoroacetic acid (TFA). But despite its effectiveness, this reagent also tends to generate highly reactive cationic species which, unless trapped, can alter amino acids in the peptide backbone, particularly those containing nucleophilic groups (e.g., Trp, Met, Tyr, and Cys). 

To prevent this undesired effect, nucleophilic reagents, also known as scavengers, are added to the mix to quench the reactive cations. Examples of popular scavengers include triethylsilane, EDT, and phenol.

Most used protecting groups for SPPS

The terminal protection of amino acids is one of the most challenging aspects of peptide chemistry. Most peptide synthesis processes are carried out in the C to N direction where α-amino-protecting groups are removed once every peptide synthesis cycle.

These groups should not only confer solubility but also prevent or reduce epimerization, particularly during the coupling stage. The most common α-amino-protecting groups for solid-phase peptide synthesis are 9-fluorenylmethoxycarbonyl and tert-butyloxycarbonyl, also known as Fmoc and Boc groups, respectively.

Presently, Fmoc groups are more commonly used than Boc. These are stable under acidic conditions but susceptible to removal by weak bases, mainly secondary amines such as piperidine or piperazine. In conventional SPPS reactions, Fmoc group removal can be achieved when reasonable amounts of piperidine (about 20%) are used in relatively polar media like N, N-dimethyl-formamide (DMF), or N-methylpyrrolidone (NMP).

Conversely, Boc chemistry is more stable in basic conditions but susceptible to removal by acids such as TFA. Terminal deprotection is usually carried out in the presence of 20-50% TFA using a relatively non-polar solvent such as dichloromethane (DCM).

Interestingly, Boc chemistry was the most mature and efficient system for the first 20 to 25 years of SPPS research. However, because it requires strong acids for side chain deprotection (i.e., hydrofluoric acid), it was quickly replaced by the milder Fmoc methods when those first became available.

Choosing the right solid support for solid-phase peptide synthesis

Solid supports for SPPS consist of a stable resin and a reversible linker, also known as handles. Merrifield’s original polystyrene (PS) resin is still widely used today as the inert component of SPPS’ solid support. However, numerous resins with enhanced properties have since become commercially available, including polyethylene glycol polyacrylamide (PEGA), cross-linked acrylate ethoxylate resin (CLEAR), and augmented surface polyethylene prepared by chemical transformation (ASPECT).

However, the success of SPPS relies in the choice of a suitable linker. Ideally, linkers should:

  • Facilitate the attachment of the starting amino acid to the support
  • Be sufficiently stable under a wide range of reaction conditions
  • Allow for selective cleavage at the end of the synthesis without degrading the final product

A list of the most used linkers can be found in the table below:

Linker C-terminal function Use cases
Rink amide linker Amide Easy-to-handle linker useful for the synthesis of peptide amides
Wang linker Carboxyl Useful for the synthesis of peptide acids – usually attached to the first amino acid
2-chlorotrityl linker Carboxyl Recommended for the synthesis of peptides with C-terminal Proline or Cysteine – cleaves easily in low-TFA solutions
HMBA linker Diverse Universal linker
Sieber amide linker Amide Suitable to produce protected peptide fragments – cleaves easily in low-TFA solutions

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