Pseudoproline dipeptides contain oxazoles formed from serine or threonine. When treated with trifluoroacetic acid (TFA), the oxazoles are converted to serine or threonine. The oxazoles have structural similarities to proline, thus they are called pseudoprolines. Like proline, the pseudoprolines create a “kink” in the polyamide backbone of peptides. The kink disrupts the interchain hydrogen bonding responsible for aggregation. Thus, incorporating pseudoprolines into the peptide sequence enhances the coupling efficiency, and in turn, improves the purity and yield of the peptide. Coupling to pseudoprolines is difficult and coupling yields are usually low. Thus pseudoprolines are utilized in peptide synthesis as preformed pseudoproline dipeptides.
Pseudoprolines are stable in weak acid conditions and pseudoproline-containing peptides can be prepared and on 2-chlorotrityl resin or Sieber resin and cleaved with the pseudoprolines in place. The pseudoproline-containing peptides have improved solubility compared to the corresponding standard peptide. This makes the pseudoproline-containing peptides easier to purify and use in fragment condensation reactions. Additionally, peptides containing a pseudoproline at the C-terminal can be coupled without racemization of the C-terminal residue.
The kink that pseudoprolines produce in linear peptides assists in forming cyclized peptides. Incorporating a pseudoproline is reported to increase the yield of cyclized peptide and to accelerate the cyclization reaction.
Pseudoprolines block the formation of aspartimide side products and the byproducts arising from aspartimide formation. Fmoc-Asp(OtBu)-Ser(ψMe,Mepro)-OH was used in place of Fmoc-Asp(OtBu)-OH/Fmoc-Ser(tBu)-OH in a semisynthetic preparation of the R2 subunit of E. coli ribonucleotide reductase. This methodology has also been used recently in a synthesis of the glycopeptide domain of erythtopoietin.
Guidelines for Using Pseudoproline Dipeptides
1. If possible, insert pseudoprolines before hydrophobic regions. 2. There should be at least 2 amino acids between pseudoprolines or between pseudoprolines and prolines. 3. The optimum spacing is 5-6 amino acids between pseudoprolines or between pseudoprolines and prolines.