GHRP-6, a hexapeptide containing two D-amino acids (D-Trp and D-Phe), is a growth hormone secretagogue that acts as a ghrelin receptor agonist. Previous studies from our group have shown that this sequence can be synthesized in high purity using just 2 x 1 minute double couplings at room temperature.
Elevated temperatures are known to raise reaction rates, including deprotections and couplings during solid phase peptide synthesis. While often applied in the synthesis of challenging sequences to improve purity and reduce deletions by accelerating difficult deprotections and couplings, heating can also provide greater efficiency for the synthesis of simple peptides. For example, reaction times might be reduced, or single couplings used in place of double couplings, without a loss in purity. To examine the effect of temperature on the purity of GHRP-6 peptides synthesized using single couplings, the simultaneous, independent induction heat capability of PurePep Chorus was used to perform a temperature scan for deprotection and coupling reactions.
GHRP-6 Sequence: His-D-Trp-Ala-Trp-D-Phe-Lys-NH2
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 - 1 min, 6-fold excess HCTU® with DIPEA in DMF at 25°C, 75°C, 90°C. D-Amino acids were coupled using prime-free Single-Shot™ additions
Synthesis using single couplings at room temperature provided a lower crude purity (71.1%) than had previously been observed with double couplings (91.4%). However, by raising the temperature to 75°C and 90°C, the crude purities increased to 88.9% and 93.8%, respectively. Thus, heating enabled an equivalent or better result using half the coupling time and reagent of the previously reported synthesis.
Table: Reaction temperatures and corresponding measured purities of GHRP-6
Figure: HPLC traces of crude GHRP-6 synthesized with single couplings at three reaction temperatures