‣ developed by R . B . Merrifield in the early 1960s . Each peptide bond is formed in a cyclic sequence , based on Fmoc- or Boc-protected N-terminal amino acids . 1 For solid support , a variety of resins can be used . They consist of a polymer matrix with linkers attached to it , where one peptide will be built for each linker . The polymer matrix swells with solvent , which enables reagents to flow through their micropores . Excessive compression of the resin beads will reduce the yield in coupling reactions . Firstly , the amino acid linked to the resin needs to be deprotected . The next one in the sequence needs to be converted into a more reactive species , an amino ester , for the reaction to complete . Once this is coupled , the solid supported peptide needs to be washed to remove any excess reagent . This minimises crosscontamination between couplings . Figure 2 shows the sequence . This is repeated for as many amino acids as the peptide sequence contains . Once the peptide has been built , the cleavage and removal of the side protection groups is the last step to obtaining the desired product . Flow chemistry is an ideal platform for automated synthesis and repeated SPPS cycles have already been implemented successfully in continuous flow . 2-7
The benefits of continuous flow are vital for the development of SPPS in industrial laboratories . In continuous flow , the solid resin beads ( supported media ) are usually packed in a cylindrical reactor . Because the beads are spheres , they will occupy the minimum possible volume when packed . This reduces the amount of required solvent on the wash steps , as well as the excesses of reagents needed for the reaction to complete . 4 , 8 The explanation for this phenomenon is simple : as the plug of amino ester flows through the packed resin , each active site on the resin will be exposed to a more concentrated solution over time than in a typical batch reactor . In addition to a more efficient synthesis , another benefit of flow chemistry is the ease of connecting inline detectors ( i . e . FTIR or UV- Vis ), which provide more accurate insight into the chemical reactions . Experimental inline data allows scientists to evaluate , for example , whether a deprotection step has gone to completion or if the pumped reagents have reached steady state conditions . One of the drawbacks of traditional flow chemistry is the limited ability to accommodate volume changes in solid reagents . This becomes apparent in SPPS ; as the sequence advances , more and more amino acids are coupled to the peptide , adding mass and volume . Thus , in the synthesis of a 30-mer peptide , the final volume of the resin is usually double the initial volume . 9 When a peptide is being built , each coupling reaction adds mass and therefore volume to the reactor packing materials , which compresses the resin matrix against the reactor ’ s walls , creating high back pressure . Peptide chemists have taken two different approaches to this problem : starting with a packed reactor and running at increasingly high pressures every cycle , or starting with a headspace in the reactor , so the resin can grow . Although they are working alternatives , these are not actual solutions to the problem that needs to be resolved if we want flow chemistry to be the leading technology for SPPS .
VBFR
Over the last three years , Vapourtec has worked towards a mechanical solution for a chemical problem in the flow arena . The reactor we have developed has the ability to measure and control the packing density of a given solid media . In this case , resin for SPPS can be packed with high precision throughout the individual coupling reactions , allowing packing to the minimum possible volume . This is achieved by means of a movable plunger that can
50 SPECIALITY CHEMICALS MAGAZINE ESTABLISHED 1981