There are several strategies employed when a peptide synthesis requires optimization. Typically, the first thing considered is whether or not to double couple specific amino acids within the sequence. This is somewhat of a change in mentality from traditional room temperature synthesis strategies where double coupling is frequently used for the entire peptide sequence.
As the complexity of peptides continues to grow, so does the use of amino acids with side chain protecting groups that can be selectively removed, leaving the peptide on resin and the remaining side chain protecting groups intact. While there are protocols to be found in the literature, they may not work to the highest level of efficiency every single time. This can lead to disasterous results for any subsequent chemistry.
While resins loaded with the natural 20 amino acids are commercially available these days, there may be times when loading the first amino acid onto the resin in house may be necessary. And unlike loading the first amino acid onto amide-leaving resins, the first coupling reaction for C-terminal acids can be chemically more challenging.
While many of the standard amino acids can be purchased pre-loaded onto Wang type resins, there are still cases where coupling the first amino acid onto Wang resin manually is necessary. In my case, an unnatural amino acid was required on the C-terminus so there was not a commercially available source.
In today’s post I’ll answer the above question by comparing the crude purity of peptides synthesized using amino acid stock solutions or freshly dissolved amino acids.