Nucleation and stability of hydrogen-bond surrogate-based alpha-helices

Bioinformatics leading to conveniently accessible, helix enforcing, bicyclic ASX motif mimics (BAMMs)

Helix mimicry provides probes to perturb protein-protein interactions (PPIs). Helical conformations can be stabilized by joining side chains of non-terminal residues (stapling) or via capping fragments. Nature exclusively uses capping, but synthetic helical mimics are heavily biased towards stapling. This study comprises: (i) creation of a searchable database of unique helical N-caps (ASX motifs, a protein structural motif with two intramolecular hydrogen-bonds between aspartic acid/asparagine and following residues); (ii) testing trends observed in this database using linear peptides comprising only canonical L-amino acids; and, (iii) novel synthetic N-caps for helical interface mimicry. Here we show many natural ASX motifs comprise hydrophobic triangles, validate their effect in linear peptides, and further develop a biomimetic of them, Bicyclic ASX Motif Mimics (BAMMs). BAMMs are powerful helix inducing motifs. They are synthetically accessible, and potentially useful to a broad section of the community studying disruption of PPIs using secondary structure mimics.

Dual-Capped Helical Interface Mimics

Disruption of protein-protein interactions is medicinally important. Interface helices may be mimicked in helical probes featuring enhanced rigidities, binding to protein targets, stabilities in serum, and cell uptake. This form of mimicry is dominated by stapling between side chains of helical residues: there has been less progress on helical N-caps, and there were no generalizable C-caps. Conversely, in natural proteins, helicities are stabilized and terminated by C- and N-caps but not staples. Bicyclic caps previously introduced by us enable interface helical mimicry featuring rigid synthetic caps at both termini in this work. An unambiguously helical dual-capped system proved to be conformationally stable, binding cyclins A and E, and showed impressive cellular uptake. In addition, the dual-capped mimic was completely resistant to proteolysis in serum over an extended period when compared with "gold standard" hydrocarbon-stapled controls. Dual-capped peptidomimetics are a new, generalizable paradigm for helical interface probe design.

Modulating Secondary Structure Motifs Through Photo-Labile Peptide Staples

Peptide-protein interactions (PPIs) are facilitated by the well-defined three-dimensional structure of bioactive peptides, interesting compounds for the development of new therapeutic agents. Their secondary structure and thus their propensity to engage in PPIs can be influenced by the introduction of peptide staples on the side chains. In particular, light-controlled staples based on azobenzene photoswitches and their structural influence on helical peptides have been studied extensively. In contrast, photolabile staples bearing photocages as a structural key motif, have mainly been used to block supramolecular interactions. Their influence on the secondary structure of the target peptide is under-investigated. Thus, in this study we use a combination of spectroscopic techniques and in silico simulations to systematically study a series of helical peptides with varying length of the photo-labile staple to obtain a detailed insight into the structure-property relationship in such photoresponsive biomolecules.

Synthetic helical peptide capping strategies

Relatively small mimics of interface secondary structures can be used to disrupt protein-protein interactions (PPIs). This strategy is valuable because many PPIs are pivotal in cell biology and contemporary medicinal chemistry. Small peptides tend to have random coil conformations in solution, so the entropy costs are high for them to order into states binding protein receptors. Consequently, peptides constrained in conformations resembling interface secondary structures are favored for enhanced affinities to PPI components. Helices are commonly found at PPI interfaces. The two general strategies that have emerged for imposing helical constraints in probes, capping and stapling, are often confused because both involve formation of macrocyclic rings. This review considers parameters that distinguish capping from stapling. Capping motifs terminate helices and project the adjacent peptide units in non-helical orientations, but stapling enforces helical motifs in ways that enable adjacent peptide fragments to extend helices. There is no evidence that stapling is more effective than capping for helix mimicry, but stapling is used more frequently. This imbalance may be because no strategies have emerged for synthetic C-capping with compact unit; if convenient and effective C-capping strategies were available then capping strategies should be more widely used.

PTHG2 Reduces Bone Loss in Ovariectomized Mice by Directing Bone Marrow Mesenchymal Stem Cell Fate

Teriparatide, also known as 1-34 parathyroid hormone (PTH (1-34)), is commonly used for the treatment of osteoporosis in postmenopausal women. But its therapeutic application is restricted by poor metabolic stability, low bioavailability, and rapid clearance. Herein, PTHG2, a glycosylated teriparatide derivative, is designed and synthesized to improve PTH stability and exert more potent antiosteoporosis effect. Surface plasmon resonance (SPR) analysis shows that PTHG2 combines to PTH 1 receptor. Additional acetylglucosamine covalent bonding in the first serine at the N terminal of PTH (1-34) improves stability and increases protein hydrolysis resistance. Intermittent administration of PTHG2 preserves bone quality in ovariectomy- (OVX-) induced osteoporosis mice model, along with increased osteoblastic differentiation and bone formation, and reduced marrow adipogenesis. In vitro, PTHG2 inhibits adipogenic differentiation and promotes osteoblastic differentiation of bone marrow mesenchymal stem cells (BMSCs). For molecular mechanism, PTHG2 directs BMSCs fate through stimulating the cAMP-PKA signaling pathway. Blocking PKA abrogates the pro-osteogenic effect of PTHG2. In conclusion, our study reveals that PTHG2 can accelerate osteogenic differentiation of BMSCs and inhibit adipogenic differentiation of BMSCs and show a better protective effect than PTH (1-34) in the treatment of osteoporosis.

Hydrogen bond surrogate helices as minimal mimics of protein α-helices

Examination of complexes of proteins with biomolecular ligands reveals that proteins tend to interact with partners via folded sub-domains, in which the backbone possesses secondary structure. α-Helices comprising the largest class of protein secondary structures, play fundamental roles in a multitude of highly specific protein-protein and protein-nucleic acid interactions. We have demonstrated a unique strategy for stabilization of the α-helical conformation that involves replacement of one of the main chain i and i+4 hydrogen bonds in the target α-helix with a covalent bond. We termed this synthetic strategy a hydrogen bond surrogate (HBS) approach. Two salient features of this approach are: (1) the internal placement of the crosslink allows development of helices such that none of the solvent-exposed surfaces are blocked by the constraining element, i.e., all side chains of the constrained helices remain available for molecular recognition. (2) This approach can be deployed to constrain very short peptides (<10 amino acid residues) into highly stable α-helices. This chapter presents the biophysical basis for the development of the hydrogen bond surrogate approach, as well as methods for the synthesis and conformational analysis of the artificial helices.

H-Bond Surrogate-Stabilized Shortest Single-Turn α-Helices: sp2 Constraints and Residue Preferences for the Highest α-Helicities

Short α-helical sequences of proteins fail to maintain their native conformation when taken out of their protein context. Several covalent constraints have been designed, including the covalent H-bond surrogate (HBS)-where a peptide backbone i + 4 → i H-bond is replaced by a covalent surrogate-to nucleate α-helix in short sequences (>7 < 15 amino acids). But constraining the shortest sequences (four amino acids) into a single α-helical turn is still a significant challenge. Here, we introduce an HBS model that can be placed in unstructured tetrapeptides without excising any of its residues, and that biases them predominantly into remarkably stable single α-helical turns in varying solvents, pH values, and temperatures. Circular dichroism (CD), Fourier transform infrared (FT-IR) absorption, one-dimensional (1D)-NMR, two-dimensional (2D)-NMR spectral and computational analyses of the HBS-constrained tetrapeptide analogues reveal that (a) the number of sp² atoms in the HBS-constrained backbone influences their predominance and rigidity in the α-helical conformation; and (b) residue preferences at the unnatural HBS-constrained positions influence their α-helicities, with Moc[GFA]G-OMe (1a) showing the highest known α-helicity (θ_n→π*MRE ∼-25.3 × 10³ deg cm² dmol^-1 at 228 nm) for a single α-helical turn. Current findings benefit chemical biological applications desiring predictable access to single α-helical turns in tetrapeptides.

Hydrogen Bond Surrogate Stabilization of β-Hairpins

Peptide secondary and tertiary structure motifs frequently serve as inspiration for the development of protein-protein interaction (PPI) inhibitors. While a wide variety of strategies have been used to stabilize or imitate α-helices, similar strategies for β-sheet stabilization are more limited. Synthetic scaffolds that stabilize reverse turns and cross-strand interactions have provided important insights into β-sheet stability and folding. However, these templates occupy regions of the β-sheet that might impact the β-sheet's ability to bind at a PPI interface. Here, we present the hydrogen bond surrogate (HBS) approach for stabilization of β-hairpin peptides. The HBS linkage replaces a cross-strand hydrogen bond with a covalent linkage, conferring significant conformational and proteolytic resistance. Importantly, this approach introduces the stabilizing linkage in the buried β-sheet interior, retains all side chains for further functionalization, and allows efficient solid-phase macrocyclization. We anticipate that HBS stabilization of PPI β-sheets will enhance the development of β-sheet PPI inhibitors and expand the repertoire of druggable PPIs.

Stabilized helical peptides: overview of the technologies and its impact on drug discovery

INTRODUCTION

Protein-protein interactions are predominant in the workings of all cells. Until now, there have been a few successes in targeting protein-protein interactions with small molecules. Peptides may overcome some of the challenges of small molecules in disrupting protein-protein interactions. However, peptides present a new set of challenges in drug discovery. Thus, the study of the stabilization of helical peptides has been extensive. Areas covered: Several technological approaches to helical peptide stabilization have been studied. In this review, stapled peptides, foldamers, and hydrogen bond surrogates are discussed. Issues regarding design principles are also discussed. Furthermore, this review introduces select computational techniques used to aid peptide design and discusses clinical trials of peptides in a more advanced stage of development. Expert opinion: Stabilized helical peptides hold great promise in a wide array of diseases. However, the field is still relatively new and new design principles are emerging. The possibilities of peptide modification are quite extensive and expanding, so the design of stabilized peptides requires great attention to detail in order to avoid a large number of failed lead peptides. The start of clinical trials with stapled peptides is a promising sign for the future.

Stapled Voltage-Gated Calcium Channel (CaV) α-Interaction Domain (AID) Peptides Act As Selective Protein-Protein Interaction Inhibitors of CaV Function

For many voltage-gated ion channels (VGICs), creation of a properly functioning ion channel requires the formation of specific protein–protein interactions between the transmembrane pore-forming subunits and cystoplasmic accessory subunits. Despite the importance of such protein–protein interactions in VGIC function and assembly, their potential as sites for VGIC modulator development has been largely overlooked. Here, we develop meta-xylyl (m-xylyl) stapled peptides that target a prototypic VGIC high affinity protein–protein interaction, the interaction between the voltage-gated calcium channel (Ca_V) pore-forming subunit α-interaction domain (AID) and cytoplasmic β-subunit (Ca_Vβ). We show using circular dichroism spectroscopy, X-ray crystallography, and isothermal titration calorimetry that the m-xylyl staples enhance AID helix formation are structurally compatible with native-like AID:Ca_Vβ interactions and reduce the entropic penalty associated with AID binding to Ca_Vβ. Importantly, electrophysiological studies reveal that stapled AID peptides act as effective inhibitors of the Ca_Vα₁:Ca_Vβ interaction that modulate Ca_V function in an Ca_Vβ isoform-selective manner. Together, our studies provide a proof-of-concept demonstration of the use of protein–protein interaction inhibitors to control VGIC function and point to strategies for improved AID-based Ca_V modulator design.

An optimal hydrogen-bond surrogate for α-helices

Substitution of a main chain i → i + 4 hydrogen bond with a covalent bond can nucleate and stabilize the α-helical conformation in peptides. Herein we describe the potential of different alkene isosteres to mimic intramolecular hydrogen bonds and stabilize α-helices in diverse peptide sequences.

ff14SB: Improving the Accuracy of Protein Side Chain and Backbone Parameters from ff99SB

Molecular mechanics is powerful for its speed in atomistic simulations, but an accurate force field is required. The Amber ff99SB force field improved protein secondary structure balance and dynamics from earlier force fields like ff99, but weaknesses in side chain rotamer and backbone secondary structure preferences have been identified. Here, we performed a complete refit of all amino acid side chain dihedral parameters, which had been carried over from ff94. The training set of conformations included multidimensional dihedral scans designed to improve transferability of the parameters. Improvement in all amino acids was obtained as compared to ff99SB. Parameters were also generated for alternate protonation states of ionizable side chains. Average errors in relative energies of pairs of conformations were under 1.0 kcal/mol as compared to QM, reduced 35% from ff99SB. We also took the opportunity to make empirical adjustments to the protein backbone dihedral parameters as compared to ff99SB. Multiple small adjustments of φ and ψ parameters were tested against NMR scalar coupling data and secondary structure content for short peptides. The best results were obtained from a physically motivated adjustment to the φ rotational profile that compensates for lack of ff99SB QM training data in the β-ppII transition region. Together, these backbone and side chain modifications (hereafter called ff14SB) not only better reproduced their benchmarks, but also improved secondary structure content in small peptides and reproduction of NMR χ1 scalar coupling measurements for proteins in solution. We also discuss the Amber ff12SB parameter set, a preliminary version of ff14SB that includes most of its improvements.

Constraining cyclic peptides to mimic protein structure motifs

Many proteins exert their biological activities through small exposed surface regions called epitopes that are folded peptides of well-defined three-dimensional structures. Short synthetic peptide sequences corresponding to these bioactive protein surfaces do not form thermodynamically stable protein-like structures in water. However, short peptides can be induced to fold into protein-like bioactive conformations (strands, helices, turns) by cyclization, in conjunction with the use of other molecular constraints, that helps to fine-tune three-dimensional structure. Such constrained cyclic peptides can have protein-like biological activities and potencies, enabling their uses as biological probes and leads to therapeutics, diagnostics and vaccines. This Review highlights examples of cyclic peptides that mimic three-dimensional structures of strand, turn or helical segments of peptides and proteins, and identifies some additional restraints incorporated into natural product cyclic peptides and synthetic macrocyclic peptidomimetics that refine peptide structure and confer biological properties.

Bicyclic tetrapeptides as potent HDAC inhibitors: effect of aliphatic loop position and hydrophobicity on inhibitory activity

Several histone deacetylase (HDAC) inhibiting bicyclic tetrapeptides have been designed and synthesized through intramolecular ring-closing metathesis (RCM) reaction and peptide cyclization. We designed bicyclic tetrapeptides based on CHAP31, trapoxin B and HC-toxin I. The HDAC inhibitory and p21 promoter assay results showed that the aliphatic loop position as well as the hydrophobicity plays an important role toward the activity of the bicyclic tetrapeptide HDAC inhibitors.

Synthesis of hydrogen-bond surrogate α-helices as inhibitors of protein-protein interactions

The α-helix is a prevalent secondary structure in proteins and is critical in mediating protein-protein interactions (PPIs). Peptide mimetics that adopt stable helices have become powerful tools for the modulation of PPIs in vitro and in vivo. Hydrogen-bond surrogate (HBS) α-helices utilize a covalent bond in place of an N-terminal i to i+4 hydrogen bond and have been used to target and disrupt PPIs that become dysregulated in disease states. These compounds have improved conformational stability and cellular uptake as compared to their linear peptide counterparts. The protocol presented here describes current methodology for the synthesis of HBS α-helical mimetics. The solid-phase synthesis of HBS helices involves solid-phase peptide synthesis with three key steps involving incorporation of N-allyl functionality within the backbone of the peptide, coupling of a secondary amine, and a ring-closing metathesis step.

Truncated and helix-constrained peptides with high affinity and specificity for the cFos coiled-coil of AP-1

Protein-based therapeutics feature large interacting surfaces. Protein folding endows structural stability to localised surface epitopes, imparting high affinity and target specificity upon interactions with binding partners. However, short synthetic peptides with sequences corresponding to such protein epitopes are unstructured in water and promiscuously bind to proteins with low affinity and specificity. Here we combine structural stability and target specificity of proteins, with low cost and rapid synthesis of small molecules, towards meeting the significant challenge of binding coiled coil proteins in transcriptional regulation. By iteratively truncating a Jun-based peptide from 37 to 22 residues, strategically incorporating i→i+4 helix-inducing constraints, and positioning unnatural amino acids, we have produced short, water-stable, α-helical peptides that bind cFos. A three-dimensional NMR-derived structure for one peptide (24) confirmed a highly stable α-helix which was resistant to proteolytic degradation in serum. These short structured peptides are entropically pre-organized for binding with high affinity and specificity to cFos, a key component of the oncogenic transcriptional regulator Activator Protein-1 (AP-1). They competitively antagonized the cJun–cFos coiled-coil interaction. Truncating a Jun-based peptide from 37 to 22 residues decreased the binding enthalpy for cJun by ∼9 kcal/mol, but this was compensated by increased conformational entropy (TΔS ≤7.5 kcal/mol). This study demonstrates that rational design of short peptides constrained by α-helical cyclic pentapeptide modules is able to retain parental high helicity, as well as high affinity and specificity for cFos. These are important steps towards small antagonists of the cJun-cFos interaction that mediates gene transcription in cancer and inflammatory diseases.

Structure–activity relationship studies of miniproteins targeting the androgen receptor–coactivator interaction

Miniproteins featuring a stable α-helical motif allow exploring point mutations in and around FXXLF motifs to improve androgen receptor affinity.

Nucleation effects in peptide foldamers

Oligomers composed of β(3)-amino acid residues and a mixture of α- and β(3)-residues have emerged as proteolytically stable structural mimics of α-helices. An attractive feature of these oligomers is that they adopt defined conformations in short sequences. In this manuscript, we evaluate the impact of β(3)-residues as compared to their α-amino acid analogs in prenucleated helices. Our hydrogen-deuterium exchange results suggest that heterogeneous sequences composed of "αααβ" repeats are conformationally more rigid than the corresponding homogeneous α-peptide helices, with the macrocycle templating the helical conformation having a significant influence.

End-Capped α-Helices as Modulators of Protein Function

Examination of complexes of proteins with other biomolecules reveals that proteins tend to interact with partners via folded sub-domains, in which the backbone possesses secondary structure. α-Helices, the largest class of protein secondary structures, play fundamental roles in a multitude of highly specific protein-protein and protein-nucleic acids interactions. Herein, we describe the potential of a helix nucleation strategy to afford modulators of protein-protein interactions.

Design, synthesis and protein-targeting properties of thioether-linked hydrogen bond surrogate helices

Appropriately-placed hydrogen bond surrogates have been demonstrated to efficiently nucleate helical conformations. Herein we describe an efficient method for the synthesis of thioether-based hydrogen bond surrogate (teHBS) helices. A teHBS helix is shown to adopt a stable conformation and target its cognate protein receptor with high affinity.

Reversible α-helix formation controlled by a hydrogen bond surrogate

Strategically placed covalent linkages have been shown to stabilize helical conformations in short peptide sequences. Here we report the synthesis of a stabilized α-helix that utilizes an internal disulfide linkage. Structural analysis indicates that the dynamic nature of the disulfide bridge allows for the reversible formation of an α-helix through oxidation and reduction reactions.

Stabilized helical peptides: overview of the technologies and therapeutic promises

INTRODUCTION

Helical structures in proteins and naturally occurring peptides play a major role in a variety of biological processes by mediating interactions with proteins and other macromolecules such as nucleic acids and lipid membranes. The use of short synthetic peptides encompassing helical segments to modulate or disrupt such interactions, when associated with human diseases, represents great pharmacological interest.

AREAS COVERED

Multiple chemical approaches have been developed to increase the conformational and metabolic stabilities of helical peptides and to improve their biomedical potential. After a brief overview of these technologies and the most recent developments, this review will focus on the main therapeutic areas and targets and will discuss their promise.

EXPERT OPINION

Potential benefits associated with increased helix stability extend beyond simple affinity enhancement. Some peptidomimetic helices are being endowed with features desirable for cellular activity such as increased resistance to proteolysis and/or cell permeability. Recent advances in the field of peptide and related peptidomimetic helices are not just conceptual, but are likely to be of practical utility in the process of optimizing peptides as clinical candidates, and developing medium-size therapeutics.

Mini review: protein-protein interactions in transcription: a fertile ground for helix mimetics

Designed ligands that inhibit protein-protein interactions involved in gene expression are valuable as reagents for genomics research and as leads for drug discovery efforts. Selective modulation of protein-protein interactions has proven to be a daunting task for synthetic ligands; however, the last decade has seen significant advances in inhibitor design, especially for helical protein interfaces. This review discusses examples of transcriptional complexes targeted by designer helices.

Structural and pharmacological effects of ring-closing metathesis in peptides

Applications of ring-closing alkene metathesis (RCM) in acyclic α- and β-peptides and closely related systems are reviewed, with a special emphasis on the structural and pharmacological effects of cyclization by RCM.

Peptoid macrocycles: making the rounds with peptidomimetic oligomers

Macrocyclic constraints are often employed to rigidify the conformation of flexible oligomeric systems. This approach has recently been used to organize the structure of peptoid oligomers, which are peptidomimetics composed of chemically diverse N-substituted glycine monomer units. In this review, we describe advances in the synthesis and characterization of cyclic peptoids. We evaluate how the installation of covalent constraints between the oligomer termini or side chains has been effective in defining peptoid conformations. We also discuss the potential applications for this promising family of macrocyclic peptidomimetics.

A hydrogen bond surrogate approach for stabilization of short peptide sequences in alpha-helical conformation

Alpha-helices constitute the largest class of protein secondary structures and play a major role in mediating protein-protein interactions. Development of stable mimics of short alpha-helices would be invaluable for inhibition of protein-protein interactions. This Account describes our efforts in developing a general approach for constraining short peptides in alpha-helical conformations by a main-chain hydrogen bond surrogate (HBS) strategy. The HBS alpha-helices feature a carbon-carbon bond derived from a ring-closing metathesis reaction in place of an N-terminal intramolecular hydrogen bond between the peptide i and i + 4 residues. Our approach is centered on the helix-coil transition theory in peptides, which suggests that the energetically demanding organization of three consecutive amino acids into the helical orientation inherently limits the stability of short alpha-helices. The HBS method affords preorganized alpha-turns to overcome this intrinsic nucleation barrier and initiate helix formation. The HBS approach is an attractive strategy for generation of ligands for protein receptors because placement of the cross-link on the inside of the helix does not block solvent-exposed molecular recognition surfaces of the molecule. Our metathesis-based synthetic strategy utilizes standard Fmoc solid phase peptide synthesis methodology, resins, and reagents and provides HBS helices in sufficient amounts for subsequent biophysical and biological analyses. Extensive conformational analysis of HBS alpha-helices with 2D NMR, circular dichroism spectroscopies and X-ray crystallography confirms the alpha-helical structure in these compounds. The crystal structure indicates that all i and i + 4 C=O and NH hydrogen-bonding partners fall within distances and angles expected for a fully hydrogen-bonded alpha-helix. The backbone conformation of HBS alpha-helix in the crystal structure superimposes with an rms difference of 0.75 A onto the backbone conformation of a model alpha-helix. Significantly, the backbone torsion angles for the HBS helix residues fall within the range expected for a canonical alpha-helix. Thermal and chemical denaturation studies suggest that the HBS approach provides exceptionally stable alpha-helices from a variety of short sequences, which retain their helical conformation in aqueous buffers at exceptionally high temperatures. The high degree of thermal stability observed for HBS helices is consistent with the theoretical predictions for a nucleated helix. The HBS approach was devised to afford internally constrained helices so that the molecular recognition surface of the helix and its protein binding properties are not compromised by the constraining moiety. Notably, our preliminary studies illustrate that HBS helices can target their expected protein receptors with high affinity.

Contemporary strategies for the stabilization of peptides in the alpha-helical conformation

Herein we review contemporary synthetic and protein design strategies to stabilize the alpha-helical motif in short peptides and miniature proteins. Advances in organometallic catalyst design, specifically for the olefin metathesis reaction, enable the use of hydrocarbon bridges to either crosslink side chains of specific residues or mimic intramolecular hydrogen bonds with carbon-carbon bonds. The resulting hydrocarbon-stapled and hydrogen bond surrogate alpha-helices provide unique synthetic ligands for targeting biomolecules. In the protein design realm, several classes of miniature proteins that display stable helical domains have been engineered and manipulated with powerful in vitro selection technologies to yield libraries of sequences that retain their helical folds. Rational re-design of these scaffolds provide distinctive reagents for the modulation of protein-protein interactions.

Trapping a folding intermediate of the alpha-helix: stabilization of the pi-helix

We report the design, synthesis, and characterization of a short peptide trapped in a pi-helix configuration. This high-energy conformation was nucleated by a preorganized pi-turn, which was obtained by replacing an N-terminal intramolecular main chain i and i + 5 hydrogen bond with a carbon-carbon bond. Our studies highlight the nucleation parameter as a key factor contributing to the relative instability of the pi-helix and allow us to estimate fundamental helix-coil transition parameters for this conformation.

Design and synthesis of alpha-helical peptides and mimetics

The alpha-helix is the most abundant secondary structural element in proteins and is an important structural domain for mediating protein-protein and protein-nucleic acid interactions. Strategies for the rational design and synthesis of alpha-helix mimetics have not matured as well as other secondary structure mimetics such as strands and turns. This perspective will focus on developments in the design, synthesis and applications of alpha-helices and mimetics, particularly in the last 5 years. Examples where synthetic compounds have delivered promising biological results will be highlighted as well as opportunities for the design of mimetics of the type I alpha-helical antifreeze proteins.