Good coupling performance of PyBOP in the solid-phase synthesis of tetrapeptide, OH-Pro-Leu-Ala-Ileu-NH2

PyBOP (benzotriazol-1-yl-oxytripyrrolidinophosphonium hexafluorophosphate) was found as a good coupling reagent in the solid-phase synthesis of a tetrapeptide, OH-Pro-Leu-Ala-Ile-NH21. The peptide was chosen as the target of synthesis due to its bioactivity as an antibacterial agent. The synthesis was carried out on solid support, 2-chlorotrityl chloride resin with Fmoc strategy. Proline was selected as the first amino acid to be attached on the resin that was followed by the attachment of leucine, alanine and isoleucine. In the synthesis, Fmoc deprotection step was undertaken by taking advantage of 20% piperidine in DMF and coupling reaction was done by the addition of amino acid and PyBOP in a mixture of dichloromethane and DMF (1:1) and in the presence of basic DIPEA. The analytical RP-HPLC of the final product showed a single peak at 22.0 minutes (20-90% of acetonitrile in water with 0.1% of TFA during 30 minutes), indicating that each coupling could give a good coupling performance that resulted in a...