Peptide bond formation using unprotected N-carboxyanhydrides under green chemistry conditions

Peptide bond forming reactions are fundamental to the synthesis of peptides and peptidomimetics. Unfortunately, the chemistry of the peptide bond rouses much concern in terms of economic cost, environmental impact, and production of wastes, especially toxic organic solvents. In a Green Chemistry perspective, we reconsidered the use of unprotected N-carboxy anhydrides (NCAs) of standard alpha-amino acids under minimal liquid-assisted grinding (LAG) conditions. NCAs are very attractive in peptide synthesis for their simplicity and atom economy. Indeed, the molecular bulk of these N-protected/C-activated bidentate reactants remains with the newly formed peptide. However, the NCAs are prone to polymerization, a formidable obstacle to the development of mechanochemical conditions. Nevertheless, polymerization can be prevented by operating under controlled basic conditions. In this work, we describe peptide bond forming reactions from NCAs and amino partners by LAG in amalgam with nanocrystalline hydroxyapatite (HAp) powder as a bio-compatible, reusable inorganic base. The experiments supported the combination of HAp and the green solvent gamma-valerolactone, and highlighted the importance of NCAs' purity and crystallinity, as studied by NMR, SEM, and XRD. The efficacy of the procedure was assessed by the synthesis of the 13-mer cytotoxic peptide PTP7.