Heterogeneous-Backbone Foldamer Mimics of Zinc Finger Tertiary Structure

Markov State Models Reveal How Folding Kinetics Influence Absorption Spectra of Foldamers

Self-assembly of platinum(II) complex foldamers is an essential approach to fabricate advanced luminescent materials. However, a comprehensive understanding of folding kinetics and their absorption spectra remains elusive. By constructing Markov state models (MSMs) from large-scale molecular dynamics simulations, we reveal that two largely similar dinuclear alknylplatinum(II) terpyridine foldamers, Pt-PEG and Pt-PE with slightly different bridges, exhibit distinctive folding kinetics. Particularly, Pt-PEG bears bridge-dominant, plane-dominant, and cooperative pathways, while Pt-PE only prefers the plane-dominant pathway. Such preference originates from their difference in intrabridge electrostatic interactions, leading to contrastive distributions of metastable states. We also found that the bridge-dominant pathway for Pt-PEG becomes more favorable when lowering the temperature. Interestingly, based on the comprehensive conformation ensembles from our MSMs, we reveal the conformation-dependent absorption spectra of Pt-PEG and Pt-PE. Our theoretical spectra not only align with experimental results but also reveal the contributions of diverse conformations to the overall absorption bands explicitly, facilitating the rational design of stimuli-responsive smart luminescent molecules.

Design and application of Cd2+ polypeptide fluorescent probes based on Aggregation Induced Emission (AIE)

Cadmium is a toxic heavy metal, which is both an environmental pollutant, and a threat to human health. A fluorescent probe was developed to detect Cd2+ selectively, sensitively, and quickly. This study reports the successful development of a polypeptide fluorescent probe TPE-HC (TPE-His-Pro-Gly-Cys) which selectively detects Cd2+ by Aggregation-Induced Emission effect. After fluorescence excitation, Cd2+ can be effectively detected based on the change of fluorescence intensity. The detection limit of Cd2+ in buffer solution was determined to be 151 nM (R2 = 0.9933). This probe exhibits high sensitivity, high cell permeabilit y, and low biological toxicity, and can perform live cell imaging under biological conditions. This study indicates that TPE-HC can detect Cd2+ in biological environments.

Backbone Modification in a Protein Hydrophobic Core

Targeted protein backbone modification can recreate tertiary structures reminiscent of folds found in nature on artificial scaffolds with improved biostability. Incorporation of altered monomers in such entities is typically limited to sites distant from the hydrophobic core to avoid potential disruptions to folding. This is limiting, as it is advantageous in some applications to incorporate artificial connectivity at buried sites. Here, we report an examination of protein backbone modification targeted specifically to hydrophobic core positions and its impacts on tertiary folded structure and fold stability. Different artificial monomer types are placed at core, core-flanking, or solvent-exposed positions in a compact three-helix protein. Effects on structure and folding energetics are assessed by NMR spectroscopy and biophysical methods. Results show that artificial residues can be well accommodated in the hydrophobic core of a defined tertiary fold, with effects on stability only modestly larger than identical changes at solvent-exposed sites. Collectively, these results provide new insights into folding behavior of protein-like artificial chains as well as strategies for the design of such molecules.

Ligand reaction-based fluorescent peptide probes for the detection of Cu2+ and glutathione

Copper is a critical element in both human and animal metabolic processes. Its role includes supporting connective tissue cross-linking, as well as iron and lipid metabolism; at the same time, copper is also a toxic heavy metal that can cause harm to both the environment and human health. Glutathione (GSH) is a tripeptide composed of glutamic acid, cysteine, and glycine combined with sulfhydryl groups. Its properties include acting as an antioxidant and facilitating integrative detoxification. GSH is present in both plant and animal cells and has a fundamental role in maintaining living organisms. GSH is the most abundant thiol antioxidant in the human body. It exists in reduced and oxidized forms within cells and provides significant biochemical functions, such as regulating vitamins such as vitamins D, E, and C, and facilitating detoxification. A fluorescent probe has been developed to detect copper ions selectively, sensitively, and rapidly. This report outlines the successful work on creating a peptide probe, TGN (TPE-Trp-Pro-Gly-Cln-His-NH2 ), with specific Cu2+ detection capabilities, and a significant fluorescence recovery occurred with the addition of GSH. This indicates that the probe can detect Cu2+ and GSH concurrently. The detection limit for Cu2+ in the buffer solution was 264 nM (R2  = 0.9992), and the detection limit for GSH using the TGN-Cu2+ complex was 919 nM (R2  = 0.9917). The probe exhibits high cell permeability and low biotoxicity that make it ideal for live cell imaging in biological conditions. This peptide probe has the capability to detect Cu2+ and GSH in biological cells.

Unnatural helical peptidic foldamers as protein segment mimics

Unnatural helical peptidic foldamers have attracted considerable attention owing to their unique folding behaviours, diverse artificial protein binding mechanisms, and promising applications in chemical, biological, medical, and material fields. Unlike the conventional α-helix consisting of molecular entities of native α-amino acids, unnatural helical peptidic foldamers are generally comprised of well-defined backbone conformers with unique and unnatural structural parameters. Their folded structures usually arise from unnatural amino acids such as N-substituted glycine, N-substituted-β-alanine, β-amino acid, urea, thiourea, α-aminoxy acid, α-aminoisobutyric acid, aza-amino acid, aromatic amide, γ-amino acid, as well as sulfono-γ-AA amino acid. They can exhibit intriguing and predictable three-dimensional helical structures, generally featuring superior resistance to proteolytic degradation, enhanced bioavailability, and improved chemodiversity, and are promising in mimicking helical segments of various proteins. Although it is impossible to include every piece of research work, we attempt to highlight the research progress in the past 10 years in exploring unnatural peptidic foldamers as protein helical segment mimics, by giving some representative examples and discussing the current challenges and future perspectives. We expect that this review will help elucidate the principles of structural design and applications of existing unnatural helical peptidic foldamers in protein segment mimicry, thereby attracting more researchers to explore and generate novel unnatural peptidic foldamers with unique structural and functional properties, leading to more unprecedented and practical applications.

Chemical Shifts of Artificial Monomers Used to Construct Heterogeneous-Backbone Protein Mimetics in Random Coil and Folded States

The construction of protein-sized synthetic chains that blend natural amino acids with artificial monomers to create so-called heterogeneous-backbones is a powerful approach to generate complex folds and functions from bio-inspired agents. A variety of techniques from structural biology commonly used to study natural proteins have been adapted to investigate folding in these entities. In NMR characterization of proteins, proton chemical shift is a straightforward to acquire, information-rich metric that bears directly on a variety of properties related to folding. Leveraging chemical shift to gain insight into folding requires a set of reference chemical shift values corresponding to each building block type (i.e., the 20 canonical amino acids in the case of natural proteins) in a random coil state and knowledge of systematic changes in chemical shift associated with particular folded conformations. Although well documented for natural proteins, these issues remain unexplored in the context of protein mimetics. Here, we report random coil chemical shift values for a library of artificial amino acid monomers frequently used to construct heterogeneous-backbone protein analogues as well as a spectroscopic signature associated with one monomer class, β3-residues bearing proteinogenic side chains, adopting a helical folded conformation. Collectively, these results will facilitate the continued utilization of NMR for the study of structure and dynamics in protein-like artificial backbones.

Two-Helix Supramolecular Proteomimetic Binders Assembled via PNA-Assisted Disulfide Crosslinking

Peptidic motifs folded in a defined conformation are able to inhibit protein-protein interactions (PPIs) covering large interfaces and as such they are biomedical molecules of interest. Mimicry of such natural structures with synthetically tractable constructs often requires complex scaffolding and extensive optimization to preserve the fidelity of binding to the target. Here, we present a novel proteomimetic strategy based on a 2-helix binding motif that is brought together by hybridization of peptide nucleic acids (PNA) and stabilized by a rationally positioned intermolecular disulfide crosslink. Using a solid phase synthesis approach (SPPS), the building blocks are easily accessible and such supramolecular peptide-PNA helical hybrids could be further coiled using precise templated chemistry. The elaboration of the structural design afforded high affinity SARS CoV-2 RBD (receptor binding domain) binders without interference with the underlying peptide sequence, creating a basis for a new architecture of supramolecular proteomimetics.

Non-Conserved Amino Acid Residues Modulate the Thermodynamics of Zn(II) Binding to Classical ββα Zinc Finger Domains

Classical zinc fingers domains (ZFs) bind Zn(II) ion by a pair of cysteine and histidine residues to adopt a characteristic and stable ββα fold containing a small hydrophobic core. As a component of transcription factors, they recognize specific DNA sequences to transcript particular genes. The loss of Zn(II) disrupts the unique structure and function of the whole protein. It has been shown that the saturation of ZFs under cellular conditions is strictly related to their affinity for Zn(II). High affinity warrants their constant saturation, while medium affinity results in their transient structurization depending on cellular zinc availability. Therefore, there must be factors hidden in the sequence and structure of ZFs that impact Zn(II)-to-protein affinities to control their function. Using molecular dynamics simulations and experimental spectroscopic and calorimetric approaches, we showed that particular non-conserved residues derived from ZF sequences impact hydrogen bond formation. Our in silico and in vitro studies show that non-conserved residues can alter metal-coupled folding mechanisms and overall ZF stability. Furthermore, we show that Zn(II) binding to ZFs can also be entropically driven. This preference does not correlate either with Zn(II) binding site or with the extent of the secondary structure but is strictly related to a reservoir of interactions within the second coordination shell, which may loosen or tighten up the structure. Our findings shed new light on how the functionality of ZFs is modulated by non-coordinating residues diversity under cellular conditions. Moreover, they can be helpful for systematic backbone alteration of native ZF ββα scaffold to create artificial foldamers and proteins with improved stability.

Design and Engineering of Miniproteins

The potential of miniproteins in the biological and chemical sciences is constantly increasing. Significant progress in the design methodologies has been achieved over the last 30 years. Early approaches based on propensities of individual amino acid residues to form individual secondary structures were subsequently improved by structural analyses using NMR spectroscopy and crystallography. Consequently, computational algorithms were developed, which are now highly successful in designing structures with accuracy often close to atomic range. Further perspectives include construction of miniproteins incorporating non-native secondary structures derived from sequences with units other than α-amino acids. Noteworthy, miniproteins with extended structures, which are now feasibly accessible, are excellent scaffolds for construction of functional molecules.

Heterogeneous-Backbone Proteomimetic Analogues of Lasiocepsin, a Disulfide-Rich Antimicrobial Peptide with a Compact Tertiary Fold

The emergence of resistance to clinically used antibiotics by bacteria presents a significant problem in public health. Natural antimicrobial peptides (AMPs) are a valuable source of antibiotics that act by a mechanism less prone to the evolutionary development of resistance. In an effort to realize the promise of AMPs while overcoming limitations such as poor biostability, researchers have sought sequence-defined oligomers with artificial amide-based backbones that show membrane-disrupting functions similar to natural agents. Most of this precedent has focused on short peptidomimetic analogues of unstructured chains or secondary folds; however, the natural antimicrobial arsenal includes a number of small- and medium-sized proteins that act via an ordered tertiary structure. Generating proteomimetic analogues of these scaffolds poses a challenge due to the increased complexity of the target for mimicry. Here, we report the development of heterogeneous-backbone variants of lasiocepsin, a 27-residue disulfide-rich AMP found in bee venom that adopts a compact tertiary fold. Iterative cycles of design, synthesis, and biological evaluation yielded analogues of the natural domain with ∼30 to 40% artificial backbone content, comparable antibacterial activity, reduced host cell toxicity, and improved stability to proteolytic degradation. High-resolution structures determined for several variants by NMR provide insights into the interplay among backbone composition, tertiary fold, and biological properties. Collectively, the results reported here broaden the scope of protein functional mimicry by artificial backbone analogues of tertiary folding patterns and suggest protein backbone engineering as a means to tune protein function by exerting site-specific control over protein folded structure.

Implications of the unfolded state in the folding energetics of heterogeneous-backbone protein mimetics

Sequence-encoded folding is the foundation of protein structure and is also possible in synthetic chains of artificial chemical composition. In natural proteins, the characteristics of the unfolded state are as important as those of the folded state in determining folding energetics. While much is known about folded structures adopted by artificial protein-like chains, corresponding information about the unfolded states of these molecules is lacking. Here, we report the consequences of altered backbone composition on the structure, stability, and dynamics of the folded and unfolded states of a compact helix-rich protein. Characterization through a combination of biophysical experiments and atomistic simulation reveals effects of backbone modification that depend on both the type of artificial monomers employed and where they are applied in sequence. In general, introducing artificial connectivity in a way that reinforces characteristics of the unfolded state ensemble of the prototype natural protein minimizes the impact of chemical changes on folded stability. These findings have implications in the design of protein mimetics and provide an atomically detailed picture of the unfolded state of a natural protein and artificial analogues under non-denaturing conditions.

Biophysical experiments and atomistic simulation reveal impacts of protein backbone alteration on the ensemble that defines the unfolded state. These effects have implications on folded stability of protein mimetics.

High five! Methyl probes at five ring positions of phenylalanine explore the hydrophobic core dynamics of zinc finger miniproteins

The elucidation of internal dynamics in proteins is essential for the understanding of their stability and functionality. Breaking the symmetry of the degenerate rotation of the phenyl side chain provides additional structural information and allows a detailed description of the dynamics. Based on this concept, we propose a combination of synthetic and computational methods, to study the rotational mobility of the Phe ring in a sensitive zinc finger motif. The systematic methyl hopping around the phenylalanine ring yields o-, m-, p-tolyl and xylyl side chains that provide a vast array of additional NOE contacts, allowing the precise determination of the orientation of the aromatic ring. MD simulations and metadynamics complement these findings and facilitate the generation of free energy profiles for each derivative. Previous studies used a wide temperature window in combination with NMR spectroscopy to elucidate the side chain mobility of stable proteins. The zinc finger moiety exhibits a limited thermodynamic stability in a temperature range of only 40 K, making this approach impractical for this compound class. Therefore, we have developed a method that can be applied even to thermolabile systems and facilitates the detailed investigation of protein dynamics.

Applications of isothermal titration calorimetry in pure and applied research from 2016 to 2020

The last 5 years have seen a series of advances in the application of isothermal titration microcalorimetry (ITC) and interpretation of ITC data. ITC has played an invaluable role in understanding multiprotein complex formation including proteolysis-targeting chimeras (PROTACS), and mitochondrial autophagy receptor Nix interaction with LC3 and GABARAP. It has also helped elucidate complex allosteric communication in protein complexes like trp RNA-binding attenuation protein (TRAP) complex. Advances in kinetics analysis have enabled the calculation of kinetic rate constants from pre-existing ITC data sets. Diverse strategies have also been developed to study enzyme kinetics and enzyme-inhibitor interactions. ITC has also been applied to study small molecule solvent and solute interactions involved in extraction, separation, and purification applications including liquid-liquid separation and extractive distillation. Diverse applications of ITC have been developed from the analysis of protein instability at different temperatures, determination of enzyme kinetics in suspensions of living cells to the adsorption of uremic toxins from aqueous streams.

Analysis of folded structure and folding thermodynamics in heterogeneous-backbone proteomimetics

Recent years have seen a growing number of examples of designed oligomeric molecules with artificial backbone connectivity that are capable of adopting complex folded tertiary structures analogous to those seen in natural proteins. A range of experimental techniques from structural biology and biophysics have been brought to bear in the study of these proteomimetic agents. Here, we discuss some considerations encountered in the characterization of high-resolution folded structure as well as folding thermodynamics of protein-like artificial backbones. We provide an overview of the use of X-ray crystallography and NMR spectroscopy in such systems and review example applications of these methods in the primary literature. Further, we provide detailed protocols for two experiments that have proved useful in our prior and ongoing efforts to compare folding thermodynamics between natural protein domains and heterogeneous-backbone counterparts.

Constrained beta-amino acid-containing miniproteins

The impact of constrained beta-amino acid substitutions on the conformational stability of two model miniproteins was evaluated.

Metal-Binding Foldamers

Sequence-defined oligomeric molecules with discrete folding propensities, termed foldamers, are a versatile source of agents with tailored structure and function. An inspiration for the development of the foldamer paradigm are natural biomacromolecules, the sequence-encoded folding of which is the basis of life. Metal ions and clusters are common features in proteins, where the role of metal varies from supporting structure to enabling function. The ubiquity of metals in natural systems suggests promise for metals in the context of folded artificial backbones. In this Minireview, we highlight efforts to realize this potential through a survey of published work on the design, synthesis, and characterization of metal-binding foldamers.

Effects of turn-structure on folding and entanglement in artificial molecular overhand knots

The length and constitution of spacers linking three 2,6-pyridinedicarboxamide units in a molecular strand influence the tightness of the resulting overhand (open-trefoil) knot that the strand folds into in the presence of lanthanide(iii) ions. The use of β-hairpin forming motifs as linkers enables a metal-coordinated pseudopeptide with a knotted tertiary structure to be generated. The resulting pseudopeptide knot has one of the highest backbone-to-crossing ratios (BCR)—a measure of knot tightness (a high value corresponding to looseness)—for a synthetic molecular knot to date. Preorganization in the crossing-free turn section of the knot affects aromatic stacking interactions close to the crossing region. The metal-coordinated pseudopeptide knot is compared to overhand knots with other linkers of varying tightness and turn preorganization, and the entangled architectures characterized by NMR spectroscopy, ESI-MS, CD spectroscopy and, in one case, X-ray crystallography. The results show how it is possible to program specific conformational properties into different key regions of synthetic molecular knots, opening the way to systems where knotting can be systematically incorporated into peptide-like chains through design.

Spacers linking 2,6-pyridinedicarboxamide units influence the tightness of the corresponding lanthanide-coordinated overhand knot. β-Hairpin forming motifs generate a metal-coordinated pseudopeptide with a knotted tertiary structure.

Proteomimetic surface fragments distinguish targets by function

The fragment-centric design promises a means to develop complex xenobiotic protein surface mimetics, but it is challenging to find locally biomimetic structures. To address this issue, foldameric local surface mimetic (LSM) libraries were constructed. Protein affinity patterns, ligand promiscuity and protein druggability were evaluated using pull-down data for targets with various interaction tendencies and levels of homology. LSM probes based on H14 helices exhibited sufficient binding affinities for the detection of both orthosteric and non-orthosteric spots, and overall binding tendencies correlated with the magnitude of the target interactome. Binding was driven by two proteinogenic side chains and LSM probes could distinguish structurally similar proteins with different functions, indicating limited promiscuity. Binding patterns displayed similar side chain enrichment values to those for native protein-protein interfaces implying locally biomimetic behavior. These analyses suggest that in a fragment-centric approach foldameric LSMs can serve as useful probes and building blocks for undruggable protein interfaces.

A twist in the road less traveled: The AMBER ff15ipq-m force field for protein mimetics

We present a new force field, AMBER ff15ipq-m, for simulations of protein mimetics in applications from therapeutics to biomaterials. This force field is an expansion of the AMBER ff15ipq force field that was developed for canonical proteins and enables the modeling of four classes of artificial backbone units that are commonly used alongside natural α residues in blended or "heterogeneous" backbones: chirality-reversed D-α-residues, the Cα-methylated α-residue Aib, homologated β-residues (β3) bearing proteinogenic side chains, and two cyclic β residues (βcyc; APC and ACPC). The ff15ipq-m force field includes 472 unique atomic charges and 148 unique torsion terms. Consistent with the AMBER IPolQ lineage of force fields, the charges were derived using the Implicitly Polarized Charge (IPolQ) scheme in the presence of explicit solvent. To our knowledge, no general force field reported to date models the combination of artificial building blocks examined here. In addition, we have derived Karplus coefficients for the calculation of backbone amide J-coupling constants for β3Ala and ACPC β residues. The AMBER ff15ipq-m force field reproduces experimentally observed J-coupling constants in simple tetrapeptides and maintains the expected conformational propensities in reported structures of proteins/peptides containing the artificial building blocks of interest-all on the μs timescale. These encouraging results demonstrate the power and robustness of the IPolQ lineage of force fields in modeling the structure and dynamics of natural proteins as well as mimetics with protein-inspired artificial backbones in atomic detail.

Retarded Translocation of Nucleic Acids through α-Hemolysin Nanopore in the Presence of a Calcium Flux

Electrophysiological measurement of molecular translocation through a nanopore is the fundamental basis of nanopore sensing. Free translocation of nucleic acids however is normally so fast that the identities of the compounds are not clearly resolvable. Inspired by recent progress in fluorescence imaging based nanopore sensing, we found that during electrophysiology measurements, translocation of nucleic acids is also retarded whenever a calcium flux around the pore vicinity is established. The residence time of nucleic acids has been extended to tens of milliseconds, a result of the strong coupling between nucleic acids and free calcium ions. The methodology presented here is applicable to both DNAs and RNAs and is able to clearly discriminate between different RNA homopolymers. This offers new insights for calcium imaging based nanopore sensing and suggests a new strategy of electrophysiology-based nanopore sensing aimed at a retarded motion of nucleic acids.

Proteomimetic Zinc Finger Domains with Modified Metal-binding β-Turns

The mimicry of protein tertiary folds by chains artificial in backbone chemical composition leads to proteomimetic analogues with potential utility as bioactive agents and as tools to shed light on biomacromolecule behavior. Notable successes toward such molecules have been achieved; however, as protein structural diversity is vast, design principles must be continually honed as they are applied to new prototype folding patterns. One specific structure where a gap remains in understanding how to effectively generate modified backbone analogues is the metal-binding β-turn found in zinc finger domains. Literature precedent suggests several factors that may act in concert, including the artificial moiety used to modify the turn, the sequence in which it is applied, and modifications present elsewhere in the domain. Here, we report efforts to gain insights into these issues and leverage these insights to construct a zinc finger mimetic with backbone modifications throughout its constituent secondary structures. We first conduct a systematic comparison of four turn mimetics in a common host sequence, quantifying relative efficacy for use in a metal-binding context. We go on to construct a proteomimetic zinc finger domain in which the helix, strands, and turn are simultaneously modified, resulting in a variant with 23% artificial residues, a tertiary fold indistinguishable from the prototype, and a folded stability comparable to the natural backbone on which the variant is based. Collectively, the results reported provide new insights into the effects of backbone modification on structure and stability of metal-binding domains and help inform the design of metalloprotein mimetics.

Prebiotic Peptides: Molecular Hubs in the Origin of Life

The fundamental roles that peptides and proteins play in today's biology makes it almost indisputable that peptides were key players in the origin of life. Insofar as it is appropriate to extrapolate back from extant biology to the prebiotic world, one must acknowledge the critical importance that interconnected molecular networks, likely with peptides as key components, would have played in life's origin. In this review, we summarize chemical processes involving peptides that could have contributed to early chemical evolution, with an emphasis on molecular interactions between peptides and other classes of organic molecules. We first summarize mechanisms by which amino acids and similar building blocks could have been produced and elaborated into proto-peptides. Next, non-covalent interactions of peptides with other peptides as well as with nucleic acids, lipids, carbohydrates, metal ions, and aromatic molecules are discussed in relation to the possible roles of such interactions in chemical evolution of structure and function. Finally, we describe research involving structural alternatives to peptides and covalent adducts between amino acids/peptides and other classes of molecules. We propose that ample future breakthroughs in origin-of-life chemistry will stem from investigations of interconnected chemical systems in which synergistic interactions between different classes of molecules emerge.

Proteomimetics as protein-inspired scaffolds with defined tertiary folding patterns

Proteins have evolved as a variable platform that provides access to molecules with diverse shapes, sizes and functions. These features have inspired chemists for decades to seek artificial mimetics of proteins with improved or novel properties. Such work has focused primarily on small protein fragments, often isolated secondary structures; however, there has lately been a growing interest in the design of artificial molecules that mimic larger, more complex tertiary folds. In this Perspective, we define these agents as 'proteomimetics' and discuss the recent advances in the field. Proteomimetics can be divided into three categories: protein domains with side-chain functionality that alters the native linear-chain topology; protein domains in which the chemical composition of the polypeptide backbone has been partially altered; and protein-like folded architectures that are composed entirely of non-natural monomer units. We give an overview of these proteomimetic approaches and outline remaining challenges facing the field.

Exploring the Functional Consequences of Protein Backbone Alteration in Ubiquitin through Native Chemical Ligation

Ubiquitin (Ub) plays critical roles in myriad protein degradation and signaling networks in the cell. We report herein Ub mimetics based on backbones that blend natural and artificial amino acid units. The variants were prepared by a modular route based on native chemical ligation. Biological assays show that some are enzymatically polymerized onto protein substrates, and that the resulting Ub tags are recognized for downstream pathways. These results advance the size and complexity of folded proteins mimicked by artificial backbones and expand the functional scope of such agents.

Advancing the Frontiers of Chemical Protein Synthesis-The 7th CPS Meeting, Haifa, Israel

The 7^th Chemical Protein Synthesis Meeting took place in September 2017 in Haifa, Israel, bringing together 100 scientists from 11 countries. The cutting-edge scientific program included new synthetic strategies and ligation auxiliaries, novel insights into protein signaling and post-translational modifications, and a range of promising therapeutic applications.

Heterogeneous-Backbone Foldamer Mimics of a Computationally Designed, Disulfide-Rich Miniprotein

Disulfide-rich peptides have found widespread use in the development of bioactive agents; however, low proteolytic stability and the difficulty of exerting synthetic control over chain topology present barriers to their application in some systems. Herein, we report a method that enables the creation of artificial backbone ("foldamer") mimics of compact, disulfide-rich tertiary folds. Systematic replacement of a subset of natural α-residues with various artificial building blocks in the context of a computationally designed prototype sequence leads to "heterogeneous-backbone" variants that undergo clean oxidative folding, adopt tertiary structures indistinguishable from that of the prototype, and enjoy proteolytic protection beyond that inherent to the topologically constrained scaffold. Collectively, these results demonstrate systematic backbone substitution to be a viable method to engineer the properties of disulfide-rich sequences and expands the repertoire of protein mimicry by foldamers to an exciting new structural class.

Flow-chemistry enabled efficient synthesis of β-peptides: backbone topology vs. helix formation

Enantiodiscriminative helix formation was observed for β-peptide H14 helices when enantiomers of bridged bicyclic residues were introduced.

Amino acid modifications for conformationally constraining naturally occurring and engineered peptide backbones: Insights from the Protein Data Bank

Extensive efforts invested in understanding the rules of protein folding are now being applied, with good effect, in de novo design of proteins/peptides. For proteins containing standard α-amino acids alone, knowledge derived from experimentally determined three-dimensional (3D) structures of proteins and biologically active peptides are available from the Protein Data Bank (PDB), and the Cambridge Structural Database (CSD). These help predict and design protein structures, with reasonable confidence. However, our knowledge of 3D structures of biomolecules containing backbone modified amino acids is still evolving. A major challenge in de novo protein/peptide design concerns the engineering of conformationally constrained molecules with specific structural elements and chemical groups appropriately positioned for biological activity. This review explores four classes of amino acid modifications that constrain protein/peptide backbone structure. Systematic analysis of peptidic molecule structures (eg, bioactive peptides, inhibitors, antibiotics, and designed molecules), containing these backbone-modified amino acids, found in the PDB and CSD are discussed. The review aims to provide structure-function insights that will guide future design of proteins/peptides.

Conformationally Programmable Chiral Foldamers with Compact and Extended Domains Controlled by Monomer Structure

Foldamers are an important class of abiotic macromolecules, with potential therapeutic applications in the disruption of protein–protein interactions. The majority adopt a single conformational motif such as a helix. A class of foldamer is now introduced where the choice of heterocycle within each monomer, coupled with a strong conformation‐determining dipole repulsion effect, allows both helical and extended conformations to be selected. Combining these monomers into hetero‐oligomers enables highly controlled exploration of conformational space and projection of side‐chains along multiple vectors. The foldamers were rapidly constructed via an iterative deprotection‐cross‐coupling sequence, and their solid‐ and solution‐phase conformations were analysed by X‐ray crystallography and NMR and CD spectroscopy. These molecules may find applications in protein surface recognition where the interface does not involve canonical peptide secondary structures.

Foldamer Tertiary Structure through Sequence-Guided Protein Backbone Alteration

The prospect of recreating the complex structural hierarchy of protein folding in synthetic oligomers with backbones that are artificial in covalent structure ("foldamers") has long fascinated chemists. Foldamers offer complex functions from biostable scaffolds and have found widespread applications in fields from biomedical to materials science. Most precedent has focused on isolated secondary structures or their assemblies. In considering the goal of complex protein-like tertiary folding patterns, a key barrier became apparent. How does one design a backbone with covalent connectivity and a sequence of side-chain functional groups that will support defined intramolecular packing of multiple artificial secondary structures? Two developments were key to overcoming this challenge. First was the recognition of the power of blending α-amino acid residues with monomers differing in backbone connectivity to create "heterogeneous-backbone" foldamers. Second was the finding that replacing some of the natural α-residues in a biological sequence with artificial-backbone variants can result in a mimic that retains both the fold and function of the native sequence and, in some cases, gains advantageous characteristics. Taken together, these precedents lead to a view of a protein as chemical entity having two orthogonal sequences: a sequence of side-chain functional groups and a separate sequence of backbone units displaying those functional groups. In this Account, we describe our lab's work over the last ∼10 years to leverage the above concept of protein sequence duality in order to develop design principles for constructing heterogeneous-backbone foldamers that adopt complex protein-like tertiary folds. Fundamental to the approach is the utilization of a variety of artificial building blocks (e.g., d-α-residues, Cα-Me-α-residues, N-Me-α-residues, β-residues, γ-residues, δ-residues, polymer segments) in concert, replacing a fraction of α-residues in a given prototype sequence. We provide an overview of the state-of-the-art in terms of design principles for choosing substitutions based on consideration of local secondary structure and retention of key side-chain functional groups. We survey high-resolution structures of backbone-modified proteins to illustrate how diverse artificial moieties are accommodated in tertiary fold contexts. We detail efforts to elucidate how backbone alteration impacts folding thermodynamics and describe how such data informs the development of improved design rules. Collectively, a growing body of results by our lab and others spanning multiple protein systems suggests there is a great deal of plasticity with respect to the backbone chemical structures upon which sequence-encoded tertiary folds can manifest. Moreover, these efforts suggest sequence-guided backbone alteration as a broadly applicable strategy for generating foldamers with complex tertiary folding patterns. We conclude by offering some perspective regarding the near future of this field, in terms of unanswered questions, technological needs, and opportunities for new areas of inquiry.

Evaluation of β-Amino Acid Replacements in Protein Loops: Effects on Conformational Stability and Structure

β-Amino acids have a backbone that is expanded by one carbon atom relative to α-amino acids, and β residues have been investigated as subunits in protein-like molecules that adopt discrete and predictable conformations. Two classes of β residue have been widely explored in the context of generating α-helix-like conformations: β³ -amino acids, which are homologous to α-amino acids and bear a side chain on the backbone carbon adjacent to nitrogen, and residues constrained by a five-membered ring, such the one derived from trans-2-aminocyclopentanecarboxylic acid (ACPC). Substitution of α residues with their β³  homologues within an α-helix-forming sequence generally causes a decrease in conformational stability. Use of a ring-constrained β residue, however, can offset the destabilizing effect of α→β substitution. Here we extend the study of α→β substitutions, involving both β³ and ACPC residues, to short loops within a small tertiary motif. We start from previously reported variants of the Pin1 WW domain that contain a two-, three-, or four-residue β-hairpin loop, and we evaluate α→β replacements at each loop position for each variant. By referral to the ϕ,ψ angles of the native structure, one can choose a stereochemically appropriate ACPC residue. Use of such logically chosen ACPC residues enhances conformational stability in several cases. Crystal structures of three β-containing Pin1 WW domain variants show that a native-like tertiary structure is maintained in each case.

Building Peptide Bonds in Haifa: The Seventh Chemical Protein Synthesis (CPS) Meeting

The power of CPS, live! More than 90 attendees from around the world came together in Haifa to present and hear about cutting-edge science in protein chemistry, from advances in synthetic methods to applications in biology and medicine. The meeting was a powerful demonstration that chemical protein synthesis can provide otherwise unattainable insights into protein structure and function.