Abstract
Synthetic polypeptides prepared from N-carboxyanhydrides (NCAs) of α-amino acids are useful for elucidating the relationship between the primary structure of natural peptides and their immunogenicity. In this study, complex copolypeptide sequences were prepared using a recently developed technique; specifically, the random copolymerization of l-alanine NCA with NCAs of l-glutamic acid 5-benzylester (Bn-Glu NCA), S-benzyl-cysteine (Bn-Cys NCA), O-benzyl-l-serine (Bn-Ser NCA), and l-phenylalanine (Phe NCA) was performed using N-heterocyclic carbene (NHC) catalysts. The NHC-initiated Ala NCA/Bn-Glu NCA and Ala NCA/Bn-Cys NCA copolymerization reactions achieved 90% conversion within 30 min. The reactivity ratio values estimated using the Kelen and Tüdos method show that poly(Bn-Glu-co-Ala) and poly(Bn-Cys-co-Ala) have random repeating units with rich alternating sequences, whereas poly(Bn-Ser-co-Ala) and poly(Phe-co-Ala) contain a larger proportion of Ala-repeating units than Bn-Ser and Phe in random placement.
Highlights
Synthetic peptide-based polymers are a class of promising materials that exhibits interesting self-assembly behaviors and shows promise for applications in drug release, gene delivery, tissue engineering, and regenerative medicine [1,2,3,4,5] as well as metal-free polypeptide-based battery applications [6]
In recent decades, improved methods in the ring-opening polymerization (ROP) of α-amino acid-N-carboxyanhydrides (NCAs) have enabled the preparation of increasingly complex copolypeptide sequences having controlled molecular weights (MWs) that display properties far superior to those of illdefined homo- and copolypeptides, which are difficult to prepare using conventional methods owing to the slow polymerization rate and suffering from various side reactions, including chain termination, chain transfer, and water-induced NCA degradation [5,7,8,9,10,11,12,13]
A general and highly straightforward strategy for the synthesis of both linear and cyclic polypeptides that employs the N-heterocyclic carbene (NHC)-mediated living polymerization of NCA was recently reported by our group [12]
Summary
Synthetic peptide-based polymers are a class of promising materials that exhibits interesting self-assembly behaviors and shows promise for applications in drug release, gene delivery, tissue engineering, and regenerative medicine [1,2,3,4,5] as well as metal-free polypeptide-based battery applications [6]. The recent developments in the polymerization of NCAs have improved the efficiency and simplicity of the techniques for the preparation of polypeptides and enabled the preparation of polymers that were previously difficult to prepare using conventional methods, studies of the random copolymerization process in terms of the reactivity ratio of the various NCA monomers are surprisingly scarce [14,15,16,17,18,19,20,21,22,23,24], especially for recently developed ROP systems. Based on conventional copolymerization experiments using a range of NCAs and primary amines as initiators, the reactivity order decreases as follows: Gly NCA >Ala NCA > γ-BzGlu NCA > Leu
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