Recipe for β-Sheets: Foldamers Containing Amyloidogenic Peptide Sequences

Fine-Tuning and Enhancement of pH-Dependent Membrane Permeation of Cyclic Peptides by Utilizing Noncanonical Amino Acids with Extended Side Chains

The development of cyclic peptides that exhibit pH-sensitive membrane permeation is a promising strategy for tissue-selective drug delivery. We investigated the pH-dependent interactions of designed cyclic peptides bearing noncanonical amino acids of long acidic side chains with lipid membranes, including surface binding, insertion, and translocation across the membrane. As the length of the side chain of acidic amino acid increased, the binding affinity of the peptides to phosphatidylcholine bilayer surfaces decreased, while the pH for the 50% insertion of the peptides into the bilayers increased. The pH for membrane permeation of the peptides increased with the side chain length, resulting in specific membrane permeation at pH ∼6.5. The longer side chain of acidic amino acids improved the maximum rate of membrane permeation at low pH, where both entropic and enthalpic contributions affected the permeation. Our peptide also showed intracellular delivery of cargo molecules into living cells in a pH-dependent manner.

Structures of synthetic helical filaments and tubes based on peptide and peptido-mimetic polymers

Synthetic peptide and peptido-mimetic filaments and tubes represent a diverse class of nanomaterials with a broad range of potential applications, such as drug delivery, vaccine development, synthetic catalyst design, encapsulation, and energy transduction. The structures of these filaments comprise supramolecular polymers based on helical arrangements of subunits that can be derived from self-assembly of monomers based on diverse structural motifs. In recent years, structural analyses of these materials at near-atomic resolution (NAR) have yielded critical insights into the relationship between sequence, local conformation, and higher-order structure and morphology. This structural information offers the opportunity for development of new tools to facilitate the predictable and reproduciblede novodesign of synthetic helical filaments. However, these studies have also revealed several significant impediments to the latter process – most notably, the common occurrence of structural polymorphism due to the lability of helical symmetry in structural space. This article summarizes the current state of knowledge on the structures of designed peptide and peptido-mimetic filamentous assemblies, with a focus on structures that have been solved to NAR for which reliable atomic models are available.

Macrocyclic Peptides Derived from Familial Alzheimer's Disease Mutants Show Charge-Dependent Oligomeric Assembly and Toxicity

This work probes the role of charge in the oligomeric assembly, toxicity, and membrane destabilization of a series of peptides derived from Aβ and the E22Q and E22K familial mutants. In the mutant Aβ peptides, an acidic residue (E) is replaced with either a neutral or basic residue (Q or K), thus altering the net charge of the peptide. Acetylation at peripheral positions permits modulation of charge of the peptides and allows investigation of the role of charge in their oligomeric assembly, cytotoxicity, and membrane disruption. Peptides with the same net charge generally behave similarly even if the amino acid residue at position 22 differs. As the net charge of the peptide decreases, so does the extent of assembly, cytotoxicity, and membrane destabilization, which were determined using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, lactate dehydrogenase (LDH)-release assays with SH-SY5Y cells, and dye leakage assays using liposomes. These findings suggest that the charge of the amino acid side chain, rather than its size or hydrophobicity, accounts for the differences in the oligomeric assembly and toxicity of the E22 familial mutants of Aβ.

Chiral, sequence-definable foldamer-derived macrocycles

Nature's oligomeric macromolecules have been a long-standing source of inspiration for chemists producing foldamers. Natural systems are frequently conformationally stabilised by macrocyclisation, yet this approach has been rarely adopted in the field of foldamer chemistry. Here we present a new class of chiral cyclic trimers and tetramers formed by macrocyclisation of open-chain foldamer precursors. Symmetrical products are obtained via a [2 + 2] self-assembly approach, while full sequence control is demonstrated through linear synthesis and cyclisation of an unsymmetrical trimer. Structural characterisation is achieved through a combined X-ray and DFT approach, which indicates the tetramers adopt a near-planar conformation, while the trimers adopt a shallow bowl-like shape. Finally, a proof-of-concept experiment is conducted to demonstrate the macrocycles' capacity for cation binding.

Dipole-controlled pre-organization enables the cyclization of sequence-defined foldamers into macrocycles. The structure and properties of trimeric and tetrameric macrocycles are explored, and their ability to bind cationic guests is demonstrated.

Synthesis and study of macrocyclic β-hairpin peptides for investigating amyloid oligomers

Chemically constrained peptides that self-assemble can be used to better understand the molecular basis of amyloid diseases. The formation of small assemblies of the amyloidogenic peptides and proteins, termed oligomers, is central to amyloid diseases. The use of chemical model systems can help provide insights into the structures and interactions of amyloid oligomers, which are otherwise difficult to study. This chapter describes the use of macrocyclic β-hairpin peptides as model systems to study amyloid oligomers. The first part of the chapter describes the chemical synthesis of the macrocyclic β-hairpin peptides and covalent assemblies thereof. The second part of the chapter describes the characterization of the oligomers formed by the macrocyclic β-hairpin peptides, focusing on SDS-PAGE, size-exclusion chromatography (SEC), and X-ray crystallography. The procedures provided focus on the β-amyloid peptide, but these strategies are applicable to a broad range of amyloid-derived peptides and proteins.

Visualizing and trapping transient oligomers in amyloid assembly pathways

Oligomers which form during amyloid fibril assembly are considered to be key contributors towards amyloid disease. However, understanding how such intermediates form, their structure, and mechanisms of toxicity presents significant challenges due to their transient and heterogeneous nature. Here, we discuss two different strategies for addressing these challenges: use of (1) methods capable of detecting lowly-populated species within complex mixtures, such as NMR, single particle methods (including fluorescence and force spectroscopy), and mass spectrometry; and (2) chemical and biological tools to bias the amyloid energy landscape towards specific oligomeric states. While the former methods are well suited to following the kinetics of amyloid assembly and obtaining low-resolution structural information, the latter are capable of producing oligomer samples for high-resolution structural studies and inferring structure-toxicity relationships. Together, these different approaches should enable a clearer picture to be gained of the nature and role of oligomeric intermediates in amyloid formation and disease.

Phenylalanine Mutation to Cyclohexylalanine Facilitates Triangular Trimer Formation by β-Hairpins Derived from Aβ

Oligomers of the β-amyloid peptide, Aβ, play a central role in the pathogenesis and progression of Alzheimer's disease. Trimers and higher-order oligomers composed of trimers are thought to be the most neurotoxic Aβ oligomers. To gain insights into the structure and assembly of Aβ oligomers, our laboratory has previously designed and synthesized macrocyclic peptides derived from Aβ17-23 and Aβ30-36 that fold to form β-hairpins and assemble to form trimers. In this study, we found that mutating Phe20 to cyclohexylalanine (Cha) in macrocyclic Aβ-derived peptides promotes crystallization of an Aβ-derived peptide containing the Aβ24-29 loop (peptide 3F20Cha) and permits elucidation of its structure and assembly by X-ray crystallography. X-ray crystallography shows that peptide 3F20Cha forms a hexamer. X-ray crystallography and SDS-PAGE further show that trimer 4F20Cha, a covalently stabilized trimer derived from peptide 3F20Cha, forms a dodecamer. Size exclusion chromatography shows that trimer 4F20Cha forms higher-order assemblies in solution. Trimer 4F20Cha exhibits cytotoxicity against the neuroblastoma cell line SH-SY5Y. These studies demonstrate the use of the F20Cha mutation to further stabilize oligomers of Aβ-derived peptides that contain more of the native sequence and thus better mimic the oligomers formed by full-length Aβ.

Oligo(5-amino-N-acylanthranilic acids): Amide Bond Formation without Coupling Reagent and Folding upon Binding Anions

Oligomers of 5-amino-N-acylanthranilic acid, previously unknown aromatic oligoamides that cannot be obtained with known amide coupling methods, are synthesized based on a new, highly efficient amide-bond formation strategy that takes advantage of the ring-opening of benzoxazinone derivatives. These oligoamides offer multiple backbone NH groups as H-bond donors which, in the presence of iodide or chloride ion, are convergently arranged and H-bonded, which enforces a folded, crescent conformation. These aromatic oligoamides provide a versatile platform based on which anion-dependent foldamers, or anion binders with tunable affinity and specificity, are being constructed.

Multiturn Hollow Helices: Synthesis and Folding of Long Aromatic Oligoamides

Aromatic oligoamides adopting helical conformations are synthesized by coupling carboxyl-terminated basic units having two, four, and eight residues to amine-terminated oligomer precursors. Coupling yields show no noticeable reduction with the size of the basic units or the final product. One- and two-dimensional NMR spectroscopy and computational studies demonstrate the reliable helical folding of these oligomers. The X-ray structure of 16mer 7 reveals a compact, multiturn helix having a 9 Å inner pore.

Interpenetrating Cubes in the X-ray Crystallographic Structure of a Peptide Derived from Medin19-36

Amyloidogenic peptides and proteins are rich sources of supramolecular assemblies. Sequences derived from well-known amyloids, including Aβ, human islet amyloid polypeptide, and tau have been found to assemble as fibrils, nanosheets, ribbons, and nanotubes. The supramolecular assembly of medin, a 50-amino acid peptide that forms fibrillary deposits in aging human vasculature, has not been heavily investigated. In this work, we present an X-ray crystallographic structure of a cyclic β-sheet peptide derived from the 19-36 region of medin that assembles to form interpenetrating cubes. The edge of each cube is composed of a single peptide, and each vertex is occupied by a divalent metal ion. This structure may be considered a metal-organic framework (MOF) containing a large peptide ligand. This work demonstrates that peptides containing Glu or Asp that are preorganized to adopt β-hairpin structures can serve as ligands and assemble with metal ions to form MOFs.

Peripheral cyclic β-amino acids balance the stability and edge-protection of β-sandwiches

Engineering water-soluble stand-alone β-sandwich mimetics is a current challenge because of the difficulties associated with tailoring long-range interactions. In this work, single cis-(1R,2S)-2-aminocyclohexanecarboxylic acid mutations were introduced into the edge strands of the eight-stranded β-sandwich mimetic structures from the betabellin family. Temperature-dependent NMR and CD measurements, together with thermodynamic analyses, demonstrated that the modified peripheral strands exhibited an irregular and partially disordered structure but were able to exert sufficient shielding on the hydrophobic core to retain the predominantly β-sandwich structure. Although the frustrated interactions decreased the free energy of unfolding, the temperature of the maximum stabilities increased to or remained at physiologically relevant temperatures. We found that the irregular peripheral strands were able to prevent edge-to-edge association and fibril formation in the aggregation-prone model. These findings establish a β-sandwich stabilization and aggregation inhibition approach, which does not interfere with the pillars of the peptide bond or change the net charge of the peptide.

Controlling the Oligomerization State of Aβ-Derived Peptides with Light

A key challenge in studying the biological and biophysical properties of amyloid-forming peptides is that they assemble to form heterogeneous mixtures of soluble oligomers and insoluble fibrils. Photolabile protecting groups have emerged as tools to control the properties of biomolecules with light. Blocking intermolecular hydrogen bonds that stabilize amyloid oligomers provides a general strategy to control the biological and biophysical properties of amyloid-forming peptides. In this paper we describe the design, synthesis, and characterization of macrocyclic β-hairpin peptides that are derived from amyloidogenic peptides and contain the N-2-nitrobenzyl photolabile protecting group. Each peptide contains two heptapeptide segments from Aβ_16-36 or Aβ_17-36 constrained into β-hairpins. The N-2-nitrobenzyl group is appended to the amide backbone of Gly₃₃ to disrupt the oligomerization of the peptides by disrupting intermolecular hydrogen bonds. X-ray crystallography reveals that N-2-nitrobenzyl groups can either block assembly into discrete oligomers or permit formation of trimers, hexamers, and dodecamers. Photolysis of the N-2-nitrobenzyl groups with long-wave UV light unmasks the amide backbone and alters the assembly and the biological properties of the macrocyclic β-hairpin peptides. SDS-PAGE studies show that removing the N-2-nitrobenzyl groups alters the assembly of the peptides. MTT conversion and LDH release assays show that decaging the peptides induces cytotoxicity. Circular dichroism studies and dye leakage assays with liposomes reveal that decaging modulates interactions of the peptides with lipid bilayers. Collectively, these studies demonstrate that incorporating N-2-nitrobenzyl groups into macrocyclic β-hairpin peptides provides a new strategy to probe the structures and the biological properties of amyloid oligomers.

Aromatic oligureas as hosts for anions and cations

Aromatic oligoureas 3 and 4 have urea moieties engaging in weak intramolecular H-bonding that constrains their backbones. The shorter 3a and 3b are able to bind chloride and acetate but not their corresponding counterion. The longer 4 binds both an anion and its counterion with the same affinity.

Effective formation of stable and versatile double-stranded β-sheets templated by a hydrogen-bonded duplex

An effective approach to construct stable and versatile double-stranded β-sheets composed of tetra- and penta-peptides through a hydrogen-bonded duplex template has been explored.

A Hexamer of a Peptide Derived from Aβ16-36

The absence of high-resolution structures of amyloid oligomers constitutes a major gap in our understanding of amyloid diseases. A growing body of evidence indicates that oligomers of the β-amyloid peptide Aβ are especially important in the progression of Alzheimer's disease. In many Aβ oligomers, the Aβ monomer components are thought to adopt a β-hairpin conformation. This paper describes the design and study of a macrocyclic β-hairpin peptide derived from Aβ16-36. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and size exclusion chromatography studies show that the Aβ16-36 β-hairpin peptide assembles in solution to form hexamers, trimers, and dimers. X-ray crystallography reveals that the peptide assembles to form a hexamer in the crystal state and that the hexamer is composed of dimers and trimers. Lactate dehydrogenase release assays show that the oligomers formed by the Aβ16-36 β-hairpin peptide are toxic toward neuronally derived SH-SY5Y cells. Replica-exchange molecular dynamics demonstrates that the hexamer can accommodate full-length Aβ. These findings expand our understanding of the structure, solution-phase behavior, and biological activity of Aβ oligomers and may offer insights into the molecular basis of Alzheimer's disease.

Stabilization, Assembly, and Toxicity of Trimers Derived from Aβ

Oligomers of the β-amyloid peptide Aβ have emerged as important contributors to neurodegeneration in Alzheimer’s disease. Mounting evidence suggests that Aβ trimers and higher-order oligomers derived from trimers have special significance in the early stages of Alzheimer’s disease. Elucidating the structures of these trimers and higher-order oligomers is paramount for understanding their role in neurodegeneration. This paper describes the design, synthesis, X-ray crystallographic structures, and biophysical and biological properties of two stabilized trimers derived from the central and C-terminal regions of Aβ. These triangular trimers are stabilized through three disulfide cross-links between the monomer subunits. The X-ray crystallographic structures reveal that the stabilized trimers assemble hierarchically to form hexamers, dodecamers, and annular porelike structures. Solution-phase biophysical studies reveal that the stabilized trimers assemble in solution to form oligomers that recapitulate some of the higher-order assemblies observed crystallographically. The stabilized trimers share many of the biological characteristics of oligomers of full-length Aβ, including toxicity toward a neuronally derived human cell line, activation of caspase-3 mediated apoptosis, and reactivity with the oligomer-specific antibody A11. These studies support the biological significance of the triangular trimer assembly of Aβ β-hairpins and may offer a deeper understanding of the molecular basis of Alzheimer’s disease.

Square channels formed by a peptide derived from transthyretin

High-resolution structures of peptide supramolecular assemblies are key to understanding amyloid diseases and designing peptide-based materials. This paper explores the supramolecular assembly of a macrocyclic β-sheet peptide derived from transthyretin (TTR). The peptide mimics the β-hairpin formed by the β-strands G and H of TTR, which form the interface of the TTR tetramer. X-ray crystallography reveals that the peptide does not form a tetramer, but rather assembles to form square channels. The square channels are formed by extended networks of β-sheets and pack in a "tilted windows" pattern. This unexpected structure represents an emergent property of the peptide and broadens the scope of known supramolecular assemblies of β-sheets.

X-ray Crystallographic Structures of a Trimer, Dodecamer, and Annular Pore Formed by an Aβ17-36 β-Hairpin

High-resolution structures of oligomers formed by the β-amyloid peptide Aβ are needed to understand the molecular basis of Alzheimer’s disease and develop therapies. This paper presents the X-ray crystallographic structures of oligomers formed by a 20-residue peptide segment derived from Aβ. The development of a peptide in which Aβ_17–36 is stabilized as a β-hairpin is described, and the X-ray crystallographic structures of oligomers it forms are reported. Two covalent constraints act in tandem to stabilize the Aβ_17–36 peptide in a hairpin conformation: a δ-linked ornithine turn connecting positions 17 and 36 to create a macrocycle and an intramolecular disulfide linkage between positions 24 and 29. An N-methyl group at position 33 blocks uncontrolled aggregation. The peptide readily crystallizes as a folded β-hairpin, which assembles hierarchically in the crystal lattice. Three β-hairpin monomers assemble to form a triangular trimer, four trimers assemble in a tetrahedral arrangement to form a dodecamer, and five dodecamers pack together to form an annular pore. This hierarchical assembly provides a model, in which full-length Aβ transitions from an unfolded monomer to a folded β-hairpin, which assembles to form oligomers that further pack to form an annular pore. This model may provide a better understanding of the molecular basis of Alzheimer’s disease at atomic resolution.

X-ray Crystallographic Structure of Oligomers Formed by a Toxic β-Hairpin Derived from α-Synuclein: Trimers and Higher-Order Oligomers

Oligomeric assemblies of the protein α-synuclein are thought to cause neurodegeneration in Parkinson’s disease and related synucleinopathies. Characterization of α-synuclein oligomers at high resolution is an outstanding challenge in the field of structural biology. The absence of high-resolution structures of oligomers formed by α-synuclein impedes understanding the synucleinopathies at the molecular level. This paper reports the X-ray crystallographic structure of oligomers formed by a peptide derived from residues 36–55 of α-synuclein. The peptide 1a adopts a β-hairpin structure, which assembles in a hierarchical fashion. Three β-hairpins assemble to form a triangular trimer. Three copies of the triangular trimer assemble to form a basket-shaped nonamer. Two nonamers pack to form an octadecamer. Molecular modeling suggests that full-length α-synuclein may also be able to assemble in this fashion. Circular dichroism spectroscopy demonstrates that peptide 1a interacts with anionic lipid bilayer membranes, like oligomers of full-length α-synuclein. LDH and MTT assays demonstrate that peptide 1a is toxic toward SH-SY5Y cells. Comparison of peptide 1a to homologues suggests that this toxicity results from nonspecific interactions with the cell membrane. The oligomers formed by peptide 1a are fundamentally different than the proposed models of the fibrils formed by α-synuclein and suggest that α-Syn_36–55, rather than the NAC, may nucleate oligomer formation.

Surprising impact of remote groups on the folding--unfolding and dimer-chain equilibria of bifunctional H-bonding unimers

Oligoamide 1, consisting of two H-bonding units linked by a trimethylene linker, was previously found to form a very stable, folded dimer. In this work, replacing the side chains and end groups of 1 led to derivatives that show the surprising impact of end groups on the folding and dimer-chain equilibria of the resultant molecules.

Comparison of backbone modification in protein β-sheets by α→γ residue replacement and α-residue methylation

The mimicry of protein tertiary structure by oligomers with unnatural backbones is a significant contemporary research challenge. Among common elements of secondary structure found in natural proteins, sheets have proven the most difficult to address. Here, we report the systematic comparison of different strategies for peptide backbone modification in β-sheets with the goal of identifying the best method for replacing a multi-stranded sheet in a protein tertiary fold. The most effective sheet modifications examined led to native-like tertiary folding behavior with a thermodynamic folded stability comparable to the prototype protein on which the modified backbones are based.

X-ray Crystallographic Structures of Oligomers of Peptides Derived from β2-Microglobulin

Amyloid diseases such as Alzheimer's disease, Parkinson's disease, and type II diabetes share common features of toxic soluble protein oligomers. There are no structures at atomic resolution of oligomers formed by full-length amyloidogenic peptides and proteins, and only a few structures of oligomers formed by peptide fragments. The paucity of structural information provides a fundamental roadblock to understanding the pathology of amyloid diseases and developing preventions or therapies. Here, we present the X-ray crystallographic structures of three families of oligomers formed by macrocyclic peptides containing a heptapeptide sequence derived from the amyloidogenic E chain of β2-microglobulin (β2m). Each macrocyclic peptide contains the heptapeptide sequence β2m63-69 and a second heptapeptide sequence containing an N-methyl amino acid. These peptides form β-sheets that further associate into hexamers, octamers, and dodecamers: the hexamers are trimers of dimers; the octamers are tetramers of dimers; and the dodecamers contain two trimer subunits surrounded by three pairs of β-sheets. These structures illustrate a common theme in which dimer and trimer subunits further associate to form a hydrophobic core. The seven X-ray crystallographic structures not only illustrate a range of oligomers that a single amyloidogenic peptide sequence can form, but also how mutation can alter the size and topology of the oligomers. A cocrystallization experiment in which a dodecamer-forming peptide recruits a hexamer-forming peptide to form mixed dodecamers demonstrates that one species can dictate the oligomerization of another. These findings should also be relevant to the formation of oligomers of full-length peptides and proteins in amyloid diseases.

Aromatic oligoamides with increased backbone flexibility: improved synthetic efficiencies, solvent-dependent folding and cooperative conformational transitions

A 15-residue aromatic oligoamide with a backbone of increased flexibility exhibits solvent- and temperature-dependent folding and highly cooperative conformational transition.

Environmentally responsive histidine-carboxylate zipper formation between proteins and nanoparticles

Interfacing synthetic materials with biomacromolecules provides new systems for biological applications. We report the creation of a reversible multivalent supramolecular "zipper" recognition motif between gold nanoparticles and proteins. In this assembly, carboxylate-functionalized nanoparticles interact strongly with oligohistidine tags. This interaction can be tuned through His-tag length, and offers unique binding profiles based on the pH and electrolyte concentration of the medium.

X-ray crystallographic structures of trimers and higher-order oligomeric assemblies of a peptide derived from Aβ(17-36)

A peptide derived from Aβ_17–36 crystallizes to form trimers that further associate to form higher-order oligomers. The trimers consist of three highly twisted β-hairpins in a triangular arrangement. Two trimers associate face-to-face in the crystal lattice to form a hexamer; four trimers in a tetrahedral arrangement about a central cavity form a dodecamer. These structures provide a working model for the structures of oligomers associated with neurodegeneration in Alzheimer’s disease.