Immobilized acyl-transfer molecular reactors enable the solid-phase synthesis of sterically hindered peptides.

Incorporating unnatural amino acids, such as hindered N-methylated or α,α-disubstituted amino acid(s), into peptides can improve their properties for application in the pharmaceutical and biomedical fields. However, the current solid-phase peptide synthesis (SPPS) faces sluggish reaction rates and low yields when incorporating sterically hindered amino acids, owing to the poor kinetics of the two-phase acyl-transfer process from solution to solid. Here we introduce an immobilized ribosome-mimicking molecular reactor to facilitate on-resin proximity-induced intra(inter)-reactor acyl transfers. This strategy bypasses the two-phase acyl-transfer mechanism in conventional SPPS and boosts coupling efficiency in the solid-phase synthesis of N-methylated and/or α,α-disubstituted amino acid(s)-containing sterically hindered peptides, including cyclosporin A and alamethicin F analogues. The ribosome-mimicking molecular reactor SPPS can be integrated into existing SPPS platforms using commercially available resins and reagents, and displays high compatibility with standard synthesizers, enabling the automated synthesis of pharmaceutically important, sterically hindered difficult peptides.