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  1. This study revisits the material properties of solid “liquid crystalline” films made from synthetic helical polypeptides and explores their structure–property relationships. Poly(γ-benzyl-l-glutamate) (PBLG) with various molecular weights and architectures (linear, comb-, and brush-like) were transformed into films through mechanical hot pressing. The resulting materials are composed of helical PBLGs arranged in a near-hexagonal lattice, similar to those formed by casting from a concentrated solution in 1,2-dichloroethane (EDC). Despite exhibiting lower apparent crystallinity, these films showed superior mechanical strength, potentially due to the promotion of more interrupted helices and their entanglements under high temperature and pressure. A pronounced chain length effect on the tensile modulus and mechanical strength was observed, aligning with the “interrupted helices” model proposed by us and others. Macromolecules with a polynorbornene (PN) backbone and PBLG side chains mirrored the mechanical and viscoelastic properties of linear PBLGs. Our findings suggest that the folding structures of polypeptide chains and the discontinuity of the folding in longer chains are more influential in determining the macroscopic mechanical properties of the resultant materials than crystallinity, packing ordering, or macromolecular architecture, emphasizing the critical role of cohesive chain network formation in achieving enhanced mechanical strength. This research also presents a versatile approach to fabricating solid-state polypeptide materials, circumventing solubility challenges associated with traditional solution-based processing methods. 
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    Free, publicly-accessible full text available March 12, 2025
  2. Polypeptides, as the synthetic analogues of natural proteins, are an important class of biopolymers that are widely studied and used in various biomedical applications. However, the preparation of polypeptide materials from the polymerization of N-carboxyanhydride (NCA) is limited by various side reactions and stringent polymerization conditions. Recently, we report the cooperative covalent polymerization (CCP) of NCA in solvents with low polarity and weak hydrogen-bonding ability (e.g., dichloromethane or chloroform). The polymerization exhibits characteristic two-stage kinetics, which is significantly accelerated compared with conventional polymerization under identical conditions. In this Account, we review our recent studies on the CCP, with the focus on the acceleration mechanism, the kinetic modeling, and the use of fast kinetics for the efficient preparation of polypeptide materials. By studying CCP with several initiating systems, we found that the polymerization rate was dependent on the secondary structure as well as the macromolecular architecture of the propagating polypeptides. The molecular interactions between the α-helical, propagating polypeptide and the monomer played an important role in the acceleration, which catalyzed the ring-opening reaction of NCA in an enzyme-mimetic, Michaelis–Menten manner. Additionally, the proximity between initiating sites further accelerated the polymerization, presumably due to the cooperative interactions of macrodipoles between neighboring helices and/or enhanced binding of monomers. A two-stage kinetic model with a reversible monomer adsorption process in the second stage was developed to describe the CCP kinetics, which highlighted the importance of cooperativity, critical chain length, binding constant, [M]0, and [M]0/[I]0. The kinetic model successfully predicted the polymerization behavior of the CCP and the molecular-weight distribution of resulting polypeptides. The remarkable rate acceleration of the CCP offers a promising strategy for the efficient synthesis of polypeptide materials, since the fast kinetics outpaces various side reactions during the polymerization process. Chain termination and chain transfer were thus minimized, which facilitated the synthesis of high-molecular-weight polypeptide materials and multiblock copolypeptides. In addition, the accelerated polymerization enabled the synthesis of polypeptides in the presence of an aqueous phase, which was otherwise challenging due to the water-induced degradation of monomers. Taking advantage of the incorporation of the aqueous phase, we reported the preparation of well-defined polypeptides from nonpurified NCAs. We believe the studies of CCP not only improve our understanding of biological catalysis, but also benefit the downstream studies in the polypeptide field by providing versatile polypeptide materials. 
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    Free, publicly-accessible full text available July 28, 2024
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    With PEG-like properties, such as hydrophilicity and stealth effect against protein absorption, oligo(ethylene glycol) (OEG)-functionalized polypeptides have emerged as a new class of biomaterials alternative to PEG with polypeptide-like properties. Synthesis of this class of materials, however, has been demonstrated very challenging, as the synthesis and purification of OEG-functionalized N -carboxyanhydrides (OEG-NCAs) in high purity, which is critical for the success in polymerization, is tedious and often results in low yield. OEG-functionalized polypeptides are therefore only accessible to a few limited labs with expertise in this specialized NCA chemistry and materials. Here, we report the controlled synthesis of OEG-functionalized polypeptides in high yield directly from the OEG-functionalized amino acids via easy and reproducible polymerization of non-purified OEG-NCAs. The prepared amphiphilic block copolypeptides can self-assemble into narrowly dispersed nanoparticles in water, which show properties suitable for drug delivery applications. 
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  6. Abstract

    The recent advances in accelerated polymerization ofN-carboxyanhydrides (NCAs) enriched the toolbox to prepare well-defined polypeptide materials. Herein we report the use of crown ether (CE) to catalyze the polymerization of NCA initiated by conventional primary amine initiators in solvents with low polarity and low hydrogen-bonding ability. The cyclic structure of the CE played a crucial role in the catalysis, with 18-crown-6 enabling the fastest polymerization kinetics. The fast polymerization kinetics outpaced common side reactions, enabling the preparation of well-defined polypeptides using an α-helical macroinitiator. Experimental results as well as the simulation methods suggested that CE changed the binding geometry between NCA and propagating amino chain-end, which promoted the molecular interactions and lowered the activation energy for ring-opening reactions of NCAs. This work not only provides an efficient strategy to prepare well-defined polypeptides with functionalized C-termini, but also guides the design of catalysts for NCA polymerization.

     
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