In a standard coupling procedure the HOBt ester is generated by the reaction between the protected amino acid and HOBt. The reaction is mediated by DCC or DIC.
Racemization can occur following cyclization of the activated species to a 5(4H)-oxa-zolone which racemizes via enolization and the subsequent reopening by the amino component yields epimers.
One or two parameters may be changed in the second coupling however changes of coupling conditions are certainly due if positive tests are obtained after the second coupling!
- Change solvent: DMF/DCM 1:1 instead of DMF,
- Change coupling reagent: TATU instead of TBTU,
- Change additive: TBTU+HOAt instead of TBTU+HOBt.
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If the test is slightly positive the recoupling is usually made using a smaller amount of reagents, typically 50% of the quantities used in the first coupling.
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If the color tests are positive after the predetermined coupling time the resin is filtered off, washed with DMF and IPA and a second coupling is usually performed.
When the color tests keep revealing the presence of unreacted amino functions after recoupling, it is necessary to cap these to avoid the formation of deletion sequences. The capping will yield a truncated sequence (shortened pep- tide) but the truncated sequences differ generally considerably from the final peptide and can be readily separated.
This approach can even be further simplified and accelerated, as MALDI-MS turned out to be a simple and very effective method for monitoring an SPPS as only a few beads of peptide resin have to be cleaved to perform a mass spectrum [43,44,45].
After the cleavage the sample is analyzed by HPLC as Fmoc protected and free amino sequences are usually well separated in a standard HPLC gradient.
Capping is realized through a short treatment of the peptide resin with a large excess of a highly reactive unhindered acid derivative and abase, usually acetic anhydride or benzoyl chloride and pyridine. At the end of the capping step the reagents are filtered off and the resin is carefully washed before proceeding to the next deprotection step.
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Resin cures thermally (preferably under pressure) under anaerobic conditions and is capable of producing the highest quality hard, clear, colorless castings.
For that purpose a deficient amount of the C-terminal or penultimate Fmoc amino acid is coupled to the unloaded or preloaded resin. The resulting load is determined and when the desired level of substitution has been reached the remaining free amino groups are blocked by acetylation.
Cleavage of samples may seem a rather elaborate way of monitoring an SPPS, but it is the most comprehensive method. It is especially useful when the synthesis has to be optimized or documented or when negative color tests results are obtained because of scarcely accessible coupling sites.