Enhancing Synthetic Efficiency in Fmoc SPPS

Pseudoproline dipeptides are undoubtedly the most powerful tools described to date for enhancing synthetic efficiency in Fmoc SPPS. The routine use of pseudoproline dipeptides has been found to:

• help avoid costly and unnecessary repeat syntheses of failed sequences;
• increase purity of crude products;
• simplify HPLC purification;
• increase yield of crude and purified products;
• enable synthesis to be carried out on a lower scale.

Insertion of Pseudroproline Dipeptide

Pseudoproline dipeptides consist of a dipeptide in which the Ser or Thr residue has been reversibly protected as proline-like TFA-labile oxazolidine. The insertion of a pseudoproline dipeptide into a sequence disrupts the formation of the secondary structures thought responsible for problems during peptide assembly, leading to better and more predictable acylation and deprotection kinetics. The most dramatic results are seen in the preparation of highly aggregated sequences, where 10-fold increases in product yield have been achieved from insertion of a single pseudoproline. However, the enhanced and more uniform reaction rates also benefit routine synthesis, providing improved yields, purities and solubilities of crude products, and easier HPLC purification with higher product return. For longer peptides, the incorporation of several pseudoprolines at regular intervals throughout the sequence has been found to be particularly effective.

Pseudoproline dipeptides are extremely simple to use. They are introduced into the peptide sequence using standard coupling methods, substituting any Aaa-Ser or Aaa-Thr dipeptide. Experience has led to the development of the following empirical guidelines for their use:

• Optimal results are obtained if the PPs are spaced 5-6 residues apart throughout the sequence.
• The optimum separation between a PP and a Pro residue is 5-6 amino acid residues.
• The minimum separation between a PP and another PP or Pro residue is 2 residues.
• Aim to insert a PP before regions of hydrophobic residues.