Enhancing Peptide Cyclization: Unveiling the Key Roles of Coupling Reagents and Heat for Optimal Results.

Abstract

Cyclic peptides offer several advantages over their linear counterparts, including enhanced structural stability due to their rigid conformation and increased resistance to enzymatic proteolysis. Additionally, their ring structure and constrained conformation reduce the entropic cost upon binding to receptors and other biological targets, leading to higher binding affinity and specificity. However, peptide macrocyclization is often synthetically challenging due to reduced entropy, oligomer formation, and C-terminal epimerization. The conventional approach for synthesizing cyclic peptides involves the direct coupling of amine and carboxyl termini in solution phase, using protected side-chain peptides and coupling reagents. Despite this, improving the efficiency of head-to-tail cyclization remains a key challenge. In this study, we optimized the cyclization of a de novo octapeptide composed of alternating l- and d-amino acids. The impact of various factors on the cyclization was examined, including coupling reagents, temperature, heating, chaotropic agents, solvent and concentration. This investigation has not only led to the identification of efficient cyclization conditions, but it also provides a valuable framework for the cyclization of challenging peptide sequences. The insights gained in this study contribute to the field of peptide chemistry, expanding the understanding of peptide cyclization reactions, which could accelerate the development of cyclic peptide-based therapeutics.