Peptide synthesis is a process that produces peptides. Peptides are organic molecules consisting of amino acids linked by a peptide (amide) bond. Many natural processes produce peptides. Chemists have borrowed these processes in bacteria to produce the peptide that they need. However, the scope of this approach is limited. More recently, peptide chemists have used solid-phase synthesis (SPPS) to produce difficult and unnatural sequences of peptides in a more controlled manner.
History of solid phase peptide synthesis (SPPS)
In 1963 a ground-breaking publication by Merrifield revolutionised peptide synthesis. Merrifield and his team began the synthesis of a peptide sequence by attaching the first amino acid to a solid polymer support.1 The rest of the sequence was built by deprotecting the terminal amine and adding the next protected amino acid in the sequence. Because the growing peptide is tethered to the solid support at one end, it is simple to wash away the excess of any unreacted monomer. The next deprotection and coupling stage begins with the newly cleaned peptide chain. Merrifield’s solid-phase approach set the stage for the production of complex and unnatural peptide sequences. Over the past 50 years the process has been significantly optimised.
Peptide synthesis today
Today, the focus of therapeutic pharmaceuticals is beginning to shift away from small molecular medicines and towards larger, peptide based treatments. Peptides have enormous potential as medicinal compounds. They are highly specific and often potent. Pharmaceutical groups have been attracted by the opportunity to modify peptide sequences to introduce greater physiochemical stability and solubility. As of 2016 there are over 50 peptide drugs in the market (non-insulin derived) with approximately 170 in various stages of clinical development.2 Total sales of peptide drugs in 2015 are estimated to be approximately $50 billion.3 Currently, most commercial peptide synthesis is performed in sequential or multi-parallel reactors with scale ranging from 1 µmol to 5 mmol. This peptide synthesis is often followed by a chromatographic purification. Bespoke batch vessels are often used for a clinically viable therapeutic peptide.
Benefits of continuous flow chemistry for peptide synthesis
Continuous flow technology (flow chemistry) offers a number of advantages to peptide synthesis versus traditional batch methodologies.4 The use of autosampler technology allows the user to prepare stock solutions of amino acids and couple them in any order. This enables the peptide synthesis of known sequences, and rapid screening of new sequences in a single instrument. Conventionally, peptide synthesis requires the use of protecting groups. Large excesses of reagents are also needed to ensure coupling is completed. This rather wasteful approach causes high costs and generates large quantities of waste.5
Using flow chemistry, it is straightforward to pre-heat and pre-activate the amino acid. This ensues fast, more efficient coupling cycles. This approach vastly increases the speed of each coupling cycle. Complete coupling and deprotection cycles of just 15 minutes are easily achieved. This more efficient coupling also greatly reduces the need for excesses of reagent. This reduces the cost and amount of waste generated by the synthesis. Integration of inline real-time analysis, such as UV/Vis spectroscopy and our Variable Bed Flow Reactor into the flow after the reactor permit immediate identification of any non-standard coupling events. This allows the user to recognise and optimise difficult coupling events during small scale syntheses. The peptide synthesis can then be scaled up using the optimised sequence.
At Vapourtec, during a collaboration with New Path Molecular, we have optimised and scaled-up the synthesis of ACP (65-74). We found that the optimised conditions translated to the large-scale synthesis without any effect to the purity of the crude peptide (94%). This approach makes it possible for the user to use a small amount of resin for an initial synthesis and optimisation. The optimised conditions can be translated to the now well understood large scale synthesis.
3. Peptide Therapeutics Market: Global Industry Analysis, Size, Share, Growth, Trends and Forecast 2012-2018, Transparency Market Research,
https://www.transparencymarketresearch.com/peptide-therapeutics-market.html,[Accessed Nov. 2017]