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SPPS & solvent consumption
Solvent use is an intrinsic feature of solid-phase peptide synthesis ( SPPS ). SPPS allows peptide chains to be built on a solid support ( the solid phase resin ) through the repeated attachment of protected amino acids , followed by a deprotection step . SPPS is mostly performed in batch reactors , usually stirred tank reactors ( STRs ), which are equipped with a bottom filter that is able to retain the solid phase resin while draining the solution . STRs make it possible to have a homogeneous medium for the control of the reaction . They are the established technology for largescale manufacturing . The successive chemical steps ( coupling / deprotection ) used to assemble a peptide sequence are each following by a draining / washing sequence . This ensures that the reaction solutions are fully removed from the resin and undesired side reactions are avoided . These washing steps account for more than 75 % of the total solvent used for the assembly of the peptide . Therefore , an alternative washing
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procedure can help to reduce the environmental impact , while not jeopardising the quality of the product .
Batch washing
The standard way used to wash the peptide resin in an SPPS reactor is called ‘ batch washing ’. It functions as a repetition of successive dilutions of the reagents contained in the resin . After draining the reaction solution , the washing solvent is introduced into the reactor ( Figure 1 ). The peptide resin is stirred to dilute the remaining reagents and then drained through the bottom frit of the reactor . This operation needs to be repeated several times to reach the acceptable limit of reagent residues . Every washing step reduces the residual reagents toward the acceptable limit , as illustrated in Figure 2 , for the washing of the piperidine solution typically used in the deprotection step . The results show an exponential decrease in concentration , due to a constant dilution rate correlated to the ratio of the added solvent volume and the remaining solution contained in the wet resin .
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This dilution phenomenon can be modelled and the applied numerical model can be used to optimise the washing conditions , as illustrated in Figure 3 . Changing the volume of solvent for each batch washing has a drastic impact on the process performance . Increasing the solvent volume per washing to 9L instead of 6L reduces the number of washes required ( eight v . ten washes to achieve 100 ppm ) compared to a volume of 6L , but the solvent consumption is increased by ~ 20 %. On the other hand , a reduction in washing volume to 2L reduces solvent consumption by ~ 20 % but significantly increases the number of washing steps to 23 , with a detrimental impact on process productivity .
New washing protocol
Consequently , the standard batch washing protocol has a relatively poor solvent efficiency . Inspired by other processes , where solid or liquid phase systems are applied , we investigated a washing protocol , by applying a percolation strategy . This
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