Temperature scanning in method development for a difficult peptide synthesis

Sterically hindered peptides can provide increased proteolytic stability while influencing peptide conformations by adding rigidity to the backbone and favoring specific secondary structures that can improve ligand binding to target receptors. These peptides are difficult to synthesize due to the reduced space available for the reactive groups to come into proximity during coupling and lower reactivity of many sterically hindered amino acids, with a high tendency to produce deletion sequences. An example of this type of sequence is so-called Aib ACP, where the two Ala residues in ACP^65-74 have been substituted by aminoisobutyric acid, Aib.

Elevated temperatures can raise reaction rates, accelerating difficult deprotections and couplings that cause deletion sequences. To examine the effect of temperature on the purity of Aib ACP peptides synthesized, the simultaneous, independent induction heat capability of PurePep Chorus was used to perform a temperature scan for deprotection and coupling reactions.

Figure: Structure of Aib ACP

Aib ACP Sequence: VQ-Aib-Aib-IDYING-NH₂

Methods:

Resin – Rink Amide MBHA resin at 25 μmol scale

Deprotections – 2 x 1 min with 20% Piperidine in DMF at 25°C, 75°C, 90°C

Coupling - 3 min for standard AAs with 10 min for the coupling of the 2nd Aib, 6-fold excess COMU® with DIPEA in DMF at 25°C, 75°C, 90°C

Results:

The highest crude purity, 84.1%, was observed for the synthesis performed at 75°C. Both elevated temperatures produced the peptide in higher purity than the ambient temperature synthesis, though the highest temperature did not produce the best result.

Conclusions:

• Simultaneous, independent induction heating was used to perform a temperature scan of deprotection and coupling reactions during the synthesis of Aib ACP
• Elevated temperature during synthesis can improve the crude purity of difficult peptide sequences
• The optimal reaction temperature may not be the highest available, illustrating the power of parallel screening to optimize this parameter

Table: Reaction temperatures and corresponding measured purities of Aib ACP

Figure: HPLC traces of crude Aib ACP at three reaction temperatures