Over the past three decades, the field of peptide-based materials has been rapidly expanding and evolving, becoming a multidisciplinary area, with new developments and applications being consistently discovered. The purpose of this Peptide Materials Special Issue is to highlight research presented at the first Gordon Research Conference on Peptide Materials in January, 2023. Consequently, we invited eminent scientists with primary research interests in Peptide Materials to contribute original research articles or short reviews in this area. This thematic issue is focused on the materials aspects of peptides and their derivatives and mimics, including both fundamental research in peptide design, synthesis, assembly, micellization, gelation, and coacervation, as well as disparate technological applications, including functional materials for energy storage, catalysis, drug delivery, regenerative medicine, adhesion, protein purification, and nanotechnology. As peptides are composed of amino acids─the fundamental building blocks of proteins─they serve as a natural bridge between small-molecule supramolecular assemblies and large biomacromolecular constructs. Their ability to adopt well-defined secondary and tertiary structures, undergo hierarchical self-assembly, and exhibit tunable biochemical properties and distinct structural features highlights their importance and relevance within the broader landscape of biomacromolecular research. The peptide materials field has become a well-established, interdisciplinary area that attracts chemists, chemical engineers, material scientists, physicists, and biomedical engineers. The papers collected in this special issue demonstrate the growing recognition of peptides, polypeptides, proteins, and their derivatives and mimics as a versatile and critical class of biomacromolecules, poised to drive continued growth and innovation across diverse scientific and technological disciplines.

The controlled transformation of polymeric vesicles into stable nonspherical morphologies is of interest as a means to mimic cells, create nanoreactors, and improve their potential for therapeutic delivery applications. We have found that the poly(dehydroalanine) segments in poly(l-methionine sulfoxide)x-b-poly(dehydroalanine)y, MOxADHy, copolypeptides form membranes that provide plasticity and selective permeability in DMSO/water mixtures, which allow the predictable control of vesicle shape by the variation of dialysis conditions. The findings of this study expand vesicle shape transformation methods to these biodegradable block copolypeptide vesicles, which are amenable to development for applications in therapeutic delivery.

We report the synthesis of amphiphilic poly(l-methionine sulfoxide)x-b-poly(dehydroalanine)y, diblock copolypeptides, MOxADHy, and their self-assembly into submicrometer-diameter unilamellar vesicles in aqueous media. The formation of vesicles was observed over an unprecedented range of copolypeptide compositions due to the unique properties and chain conformations of ADH hydrophobic segments. These copolypeptides incorporate two distinct thiol reactive components where each segment can respond differently to a single thiol stimulus. Incubation of MO35ADH30 vesicles with glutathione under intracellular mimetic conditions resulted in vesicle disruption and release of cargo. Further, incubation of MO35ADH30 vesicles with thiolglycolic acid resulted in a reversal of amphipilicity and successful in situ inversion of the vesicle assemblies. This conversion of biomimetic polymer vesicles into stable inverted vesicles using a biologically relevant stimulus at physiological pH and temperature is unprecedented. These results provide insights toward the development of advanced functional synthetic assemblies with potential uses in biology and medicine.

There is considerable recent interest in the synthesis and development of peptide-based materials as mimics of natural biological assemblies that utilize proteins and peptides to form organized structures and develop beneficial properties. Due to their potential compatibility with living organisms, synthetic peptide materials are also being developed for applications such as cell grafting, therapeutic delivery, and implantable diagnostic devices. One desirable feature for such applications is the ability to design materials that can respond to stimuli by changes in their structure or properties under biologically relevant conditions. Peptide and protein assemblies can respond to stimuli, such as changes in temperature, solution pH, ions present in media, or interactions with other biomacromolecules. An exciting area of emerging research is focused on how biology uses the chemistry of sulfur-containing amino acids as a means to regulate biological processes. These concepts have been utilized and expanded in recent years to enable the development of peptide materials with readily switchable properties.The incorporation of sulfur atoms in polypeptides, peptides, and proteins provides unique sites that can be used to alter the physical and biological properties of these materials. Sulfur-containing amino acid residues, most often cysteine and methionine, are able to undergo a variety of selective chemical and enzyme-mediated reactions, which can be broadly characterized as redox or alkylation processes. These reactions often proceed under physiologically relevant conditions, can be reversible, and are significant in that they can alter residue polarity as well as conformations of peptide chains. These sulfur-based reactions are able to switch molecular and macromolecular properties of peptides and proteins in living systems and recently have been applied to synthetic peptide materials. Naturally occurring "sulfur switches" can be reversible or irreversible and are often triggered by enzymatic activity. Sulfur switches in peptide materials can also be triggered in vitro using oxidation/reduction and alkylation as well as photochemical reactions. The application of sulfur switches to peptide materials has greatly expanded the scope of these switches due to the ability to readily incorporate a wide variety of noncanonical sulfur-containing synthetic amino acids.Sulfur switches have been shown to provide considerable potential to reversibly alter peptide material properties under mild physiologically relevant conditions. An important molecular feature of sulfur-containing amino acid residues was found to be the location of sulfur atoms in the side chains. The variation of sulfur atom positions from the backbone by single bond lengths was found to significantly affect polypeptide chain conformations upon oxidation-reduction or alkylation/dealkylation reactions. With the successful adaptation of sulfur switches to peptide materials, future studies can explore how these switches affect how these materials interact with biological systems. This Account provides an overview of the different types of sulfur switch reactions found in biology and their properties and the elaboration of these switches in synthetic systems with a focus on recent developments and applications of reversible sulfur switches in peptide materials.

Three different series of amino acid side-chain functionalized homopolypeptides were prepared as variants of previously reported α-helical, coacervate-forming cationic polypeptides. Studies of the physical behavior of these polypeptides in aqueous media in the presence of multivalent counterions enabled a better understanding of the molecular requirements for coacervate formation of side-chain functionalized homopolypeptides. Variation in lengths of side-chain amino acid or linker segments in cationic α-helical polypeptides was found either to prohibit coacervate formation or to allow adjustment of the phase transition temperature. A series of charge-reversed, anionic amino acid side-chain functionalized homopolypeptides were also prepared and found to be α-helical and able to form coacervates similar to analogous cationic homopolypeptides. These results illustrate the ability to predictably tune coacervation properties via molecular adjustment of side-chains in homopolypeptides and show that amino acid side-chain functionalized homopolypeptides can be used as a general platform for development of biomimetic, coacervate-forming polymers.

Our group recently developed a family of side-chain amino acid-functionalized poly(S-alkyl-l-homocysteines), Xaa-CH (Xaa = generic amino acid), which possess the ability to form environmentally responsive coacervates in water. In an effort to further study how the molecular structure affects polypeptide coacervate formation, we prepared side-chain amino acid-functionalized poly(S-alkyl-rac-cysteines), Xaa-rac-C, via post-polymerization modification of poly(dehydroalanine), ADH. The use of the ADH platform allowed straightforward synthesis of a diverse range of side-chain amino acid-functionalized polypeptides via direct reaction of unprotected l-amino acid 2-mercaptoethylamides with ADH. Despite their differences in the main-chain structure, we found that Xaa-rac-C can form coacervates with properties similar to those seen with Xaa-CH. These results suggest that the incorporation of side-chain amino acids onto polypeptides may be a way to generally favor coacervation. The incorporation of l-methionine in Met-rac-C allowed the preparation of coacervates with improved stability against high ionic strength media. Further, the presence of additional thioether groups in Met-rac-C resulted in an increased solubility change upon oxidation allowing facile reversible redox switching of coacervate formation in aqueous media.

Background: Silicone gel removal after breast implant rupture is a difficult task. Silicone is hydrophobic and thus cannot be irrigated effectively with saline. Attempts at mechanical removal with sponges are often partially successful. Incomplete removal results in persistent silicone contamination with possible local inflammation, infection, and silicone granulomata. In this partially quantitative investigation, we assess the de-adhesion ability of different clindamycin formulations against known surfactant controls when combined with silicone gel. Methods: To demonstrate surfactant properties in vitro, clindamycin phosphate, clindamycin hydrochloride, and a known surfactant, sodium dodecyl sulfate (SDS), were compared. An amount of 170 g of silicone gel placed in a dry glass container exhibited strong adherence to the container walls. In separate trials, clindamycin phosphate (300 mg in 100 mL), clindamycin HCl (300 mg in 100 mL), and SDS (1 g in 100 mL) solutions with normal saline were added to the silicone aggregate, and de-adhesion properties were compared. Results: All solutions aided in the de-adhesion of the sticky silicone from glass substrate. The SDS had the strongest effect, followed by clindamycin phosphate and then clindamycin HCl. The observed interactions suggested that all of the solutions behaved as ionic surfactant coating the silicone with negative charges via adsorption. However, the phosphate anionic formulation was associated with a greater surfactant effect than HCl. Conclusions: Clindamycin acts as a surfactant to aid in the clinical removal of ruptured silicone gel. Clindamycin phosphate seems to have a stronger effect than clindamycin HCl, likely related to the negative charges on the phosphate groups.

We report development and preparation of synthetic polypeptide based, coacervate core polyelectrolyte complex micelles, PCMs, in aqueous media, which were characterized and evaluated for the encapsulation and in vitro release of a model single-stranded RNA, polyadenylic acid, poly(A). Cationic, α-helical polypeptides pegylated at their N-termini, PEG113-b-5bn and PEG113-b-5cn, were designed to form coacervate core PCMs upon mixing with multivalent anions in aqueous media. Sodium tripolyphosphate (TPP) and poly(A) were used as model multivalent anions that allowed optimization of polypeptide composition and chain length for formation of stable, nanoscale PCMs. PEG113-b-5c27 was selected for preparation of PCMs that were characterized under different environmental conditions using dynamic light scattering, atomic force microscopy and cryoelectron microscopy. The PCMs were found to efficiently encapsulate poly(A), were stable at physiologically relevant pH and solution ionic strength, and were able to release poly(A) in the presence of excess polyvalent anions. These PCMs were found to be a promising model system for further development of polypeptide based therapeutic delivery vehicles.

Axon regeneration can be induced across anatomically complete spinal cord injury (SCI), but robust functional restoration has been elusive. Whether restoring neurological functions requires directed regeneration of axons from specific neuronal subpopulations to their natural target regions remains unclear. To address this question, we applied projection-specific and comparative single-nucleus RNA sequencing to identify neuronal subpopulations that restore walking after incomplete SCI. We show that chemoattracting and guiding the transected axons of these neurons to their natural target region led to substantial recovery of walking after complete SCI in mice, whereas regeneration of axons simply across the lesion had no effect. Thus, reestablishing the natural projections of characterized neurons forms an essential part of axon regeneration strategies aimed at restoring lost neurological functions.

We report the preparation of a new water-soluble, nonionic homopolypeptide poly(L-homoserine) as well as poly(L- homoserine) block copolymers with controllable segment lengths. The conformational preferences of poly(L-homoserine) were also determined in both the solid state and in solution. Poly(L- homoserine) is soluble in water and adopts a disordered conformation that makes it a promising addition to the small class of nonionic, water-soluble homopolypeptides with potential for development for applications in biology. Toward this goal, a poly(L-homoserine) containing a block copolypeptide was prepared and found to assemble into micro-and nanoscale vesicles in water.

Neural progenitor cells (NPC) represent potential cell transplantation therapies for CNS injuries. To understand how lesion environments influence transplanted NPC fate in vivo, we derived NPC expressing a ribosomal protein-hemagglutinin tag (RiboTag) for transcriptional profiling of transplanted NPC. Here, we show that NPC grafted into uninjured mouse CNS generate cells that are transcriptionally similar to healthy astrocytes and oligodendrocyte lineages. In striking contrast, NPC transplanted into subacute CNS lesions after stroke or spinal cord injury in mice generate cells that share transcriptional, morphological and functional features with newly proliferated host astroglia that restrict inflammation and fibrosis and isolate lesions from adjacent viable neural tissue. Our findings reveal overlapping differentiation potentials of grafted NPC and proliferating host astrocytes; and show that in the absence of other interventions, non-cell autonomous cues in subacute CNS lesions direct the differentiation of grafted NPC towards a naturally occurring wound repair astroglial phenotype.

Via the design of a new, soluble poly(S-alkyl-l-cysteine) precursor, a route was developed for the successful preparation of long-chain poly(dehydroalanine), ADH, as well as the incorporation of dehydroalanine residues and ADH segments into copolypeptides. Based on experimental and computational data, ADH was found to adopt a previously unobserved "hybrid coil" structure, which combines the elements of 25-helical and 310-helical conformations. Analysis of the spectroscopic properties of ADH revealed that it possesses a strong inherent blue fluorescence, which may be amenable for use in imaging applications. ADH also contains reactive electrophilic groups that allowed its efficient modification to functionalized polypeptides after reactions under mild conditions with thiol and amine nucleophiles. The combined structural, spectroscopic, and reactivity properties of ADH make it a unique reactive and fluorescent polypeptide component for utilization in self-assembled biomaterials.

Selective lectin binding and sorting was achieved using thermosensitive glycoconjugates derived from recombinant elastin-like polypeptides (ELPs) in simple centrifugation-precipitation assays. A recombinant ELP, (VPGXG)40, containing periodically spaced methionine residues was used to enable chemoselective postsynthetic modification via thioether alkylation using alkyne functional epoxide derivatives. The resulting sulfonium groups were selectively demethylated to give alkyne functionalized homocysteine residues, which were then reacted with azido-functionalized monosaccharides to obtain ELP glycoconjugates with periodic saccharide functionality. These modifications were also found to allow modulation of ELP temperature dependent water solubility. The multivalent ELP glycoconjugates were evaluated for specific recognition, binding and separation of the lectin Ricinus communis agglutinin (RCA120) from a complex protein mixture. RCA120 and ELP glycoconjugate interactions were evaluated using laser scanning confocal microscopy and dynamic light scattering. Due to the thermoresponsive nature of the ELP glycoconjugates, it was found that heating a mixture of galactose-functionalized ELP and RCA120 in complex media selectively yielded a phase separated pellet of ELP-RCA120 complexes. Based on these results, ELP glycoconjugates show promise as designer biopolymers for selective protein binding and sorting.

We report the development of new side-chain amino acid-functionalized α-helical homopolypeptides that reversibly form coacervate phases in aqueous media. The designed multifunctional nature of the side-chains was found to provide a means to actively control coacervation via mild, biomimetic redox chemistry as well as allow response to physiologically relevant environmental changes in pH, temperature, and counterions. These homopolypeptides were found to possess properties that mimic many of those observed in natural coacervate forming intrinsically disordered proteins. Despite ordered α-helical conformations that are thought to disfavor coacervation, molecular dynamics simulations of a polypeptide model revealed a high degree of side-chain conformational disorder and hydration around the ordered backbone, which may explain the ability of these polypeptides to form coacervates. Overall, the modular design, uniform nature, and ordered chain conformations of these polypeptides were found to provide a well-defined platform for deconvolution of molecular elements that influence biopolymer coacervation and tuning of coacervate properties for downstream applications.

This chapter summarizes advances in the synthesis of well-defined block and statistical copolypeptides that can be assembled into stable membrane structures. Polymerization chemistries are briefly discussed that allow precision copolypeptide synthesis with good control over chain length, chain length distribution, and chain-end functionality. Using these methods, well-defined copolypeptides of controlled dimensions (including molecular weight, sequence, composition, and molecular weight distribution) can now be prepared, which can be assembled into vesicles composed of polypeptide membranes. Many of these membrane assemblies have been found to possess unique properties that are derived from the amino acid building blocks and ordered conformations of the polypeptide segments.

Background: Host defense peptides are a family of endogenous short peptides that are found in all living beings and play a critical role in innate immunity against infection. Methods: A nonsystematic review of host defense peptides was conducted with specific interest in properties and applications relevant to plastic and reconstructive surgery. Results: In addition to their direct antimicrobial actions against pathogens, including multidrug-resistant bacteria, they also demonstrate important functions in immunomodulation, tumor cell lysis, and tissue regeneration. These properties have made them a topic of clinical interest for plastic surgeons because of their potential applications as novel antibiotics, wound healing medications, and cancer therapies. The rising clinical interest has led to a robust body of literature describing host defense peptides in great depth and breadth. Numerous mechanisms have been observed to explain their diverse functions, which rely on specific structural characteristics. However, these peptides remain mostly experimental, with limited translation to clinical practice because of numerous failures to achieve acceptable results in human trials. Conclusions: Despite the broad ranging potential of these peptides for use in the field of plastic and reconstructive surgery, they are rarely discussed in the literature or at scientific meetings. In this review, the authors provide a summary of the background, structure, function, bacterial resistance, and clinical applications of host defense peptides with the goal of stimulating host defense peptide-based innovation within the field of plastic and reconstructive surgery.

A simple approach for producing supramolecular nanohybrids was developed allowing the preparation of soluble hybrid two-dimensional nanosheets in water through spontaneous organization of two-dimensional Cu(II) rubeanate hydrate and cationic diblock copolypeptide amphiphiles. These components self-assembled into nanosheets with a thicknesses of about several nm and widths on the order of several micrometers, which displayed electrooxidation reactions. The nanostructures were dependent on polypeptides, suggesting that both cationic and hydrophobic polypeptide components are involved in generating the two-dimensional (2D) nanosheet structures. More information can be found in the Full Paper by Keita Kuroiwa et al.

Biomaterials hold promise for therapeutic applications in the central nervous system (CNS). Little is known about molecular factors that determine CNS foreign body responses (FBRs) in vivo, or about how such responses influence biomaterial function. Here, we probed these factors in mice using a platform of injectable hydrogels readily modified to present interfaces with different physiochemical properties to host cells. We found that biomaterial FBRs mimic specialized multicellular CNS wound responses not present in peripheral tissues, which serve to isolate damaged neural tissue and restore barrier functions. We show that the nature and intensity of CNS FBRs are determined by definable properties that significantly influence hydrogel functions, including resorption and molecular delivery when injected into healthy brain or stroke injuries. Cationic interfaces elicit stromal cell infiltration, peripherally derived inflammation, neural damage and amyloid production. Nonionic and anionic formulations show minimal levels of these responses, which contributes to superior bioactive molecular delivery. Our results identify specific molecular mechanisms that drive FBRs in the CNS and have important implications for developing effective biomaterials for CNS applications.