Hybrid Peptides: Direct Transformation of α/α, β-Unsaturated γ-Hybrid Peptides to α/γ-Hybrid Peptide 12-Helices

Proteolytically Stable ααγ-Hybrid Peptides Inhibit the Aggregation and Cytotoxicity of Aβ42

The recent approval of antibody-based therapy for targeting the clearance of amyloid plaques fuels the research in designing small molecules and peptide inhibitors to target the aggregation of Aβ-peptides. Here, we report that the 15-residue ααγ-hybrid peptide not only inhibits the aggregation of soluble Aβ42 into fibrils but also disintegrates the aggregated Aβ42 fibrils into smaller assemblies. Further, the hybrid peptide completely rescues neuronal cells from the toxicity of Aβ42 at equimolar concentrations. The shorter 10- and 12-mer peptides showed weak aggregation inhibition activity, while the fully hydrophobic 15-mer ααγ-hybrid peptide analogue showed no aggregation inhibition activity. Further, the 15-mer ααγ-hybrid peptide showed resistance against trypsin digestion and also nontoxic to the neuronal cells. The CD revealed that the peptide upon interaction induces a helix-type conformation in the Aβ42. This is in sharp contrast to the β-sheet conformation of Aβ42 upon incubation. The two-dimensional-NMR (2D-NMR) analysis revealed a large perturbation in the chemical shifts of residues at the N-terminus. The presence of 15-mer peptide at an equimolar concentration of Aβ42 showed less tendency for aggregation and also exhibited nontoxicity to the neuronal cells. The results reported here may be useful in designing new therapeutics for Alzheimer's disease.

Exploring Helical Peptides and Foldamers for the Design of Metal Helix Frameworks: Current Trends and Future Perspectives

Flexible and biocompatible metal peptide frameworks (MPFs) derived from short and ultra-short peptides have been explored for the storage of greenhouse gases, molecular recognition, and chiral transformations. In addition to short flexible peptides, peptides with specifically folded conformations have recently been utilized to fabricate a variety of metal helix frameworks (MHFs). The secondary structures of the peptides govern the structure-assembly relationship and thereby control the formation of three-dimensional (3D)-MHFs. Particularly, the hierarchical structural organization of peptide-based MHFs has not yet been discussed in detail. Here, we describe the recent progress of metal-driven folded peptide assembly to construct 3D porous structures for use in future energy storage, chiral recognition, and biomedical applications, which could be envisioned as an alternative to the conventional metal-organic frameworks (MOFs).

α,β-Unsaturated γ-Peptide Foldamers

Despite their concomitant emergence in the 1990s, γ-peptide foldamers have not developed as fast as β-peptide foldamers and to date, only a few γ-oligomer structures have been reported, and with sparse applications. Among these examples, sequences containing α,β-unsaturated γ-amino acids have recently drawn attention since the Z/E configurations of the double bond provide opposite planar restrictions leading to divergent conformational behaviors, from helix to extended structures. In this Review, we give a comprehensive overview of the developments of γ-peptide foldamers containing α,β-unsaturated γ-amino acids with examples of applications for health and catalysis, as well as materials science.

Structural Investigation of Hybrid Peptide Foldamers Composed of α-Dipeptide Equivalent β-Oxy-δ5 -amino Acids

Due to their equivalent lengths, δ-amino acids can serve as surrogates of α-dipeptides. However, δ-amino acids with proteinogenic side chains have not been well studied because of synthetic difficulties and because of their insolubility in organic solvents. Recently we reported the spontaneous supramolecular gelation of δ-peptides composed of β(O)-δ⁵ -amino acids. Here, we report the incorporation of β(O)-δ⁵ -amino acids as guests into the host α-helix, α,γ-hybrid peptide 12-helix and their single-crystal conformations. In addition, we studied the solution conformations of hybrid peptides composed of 1:1 alternating α and β(O)-δ⁵ -amino acids. In contrast to the control α-helix structures, the crystal structure of peptides with β(O)-δ⁵ -amino acids exhibit α-helical conformations consisting of both 13- and 10-membered H-bonds. The α,δ-hybrid peptide adopted mixed 13/11-helix conformation in solution with alternating H-bond directionality. Crystal-structure analysis revealed that the α,γ⁴ -hybrid peptide accommodated the guest β(O)-δ⁵ -amino acid without significant deviation to the overall helix folding. The results reported here emphasize that β(O)-δ⁵ -amino acids with proteinogenic side chains can be accommodated into regular α-helix or 12-helix as guests without much deviation of the overall helix folding of the peptides.

Direct Transformation of N-Protected α,β-Unsaturated γ-Amino Amides into γ-Lactams through a Base-Mediated Molecular Rearrangement

Here, we are reporting a single-step transformation of N-protected α,β-unsaturated γ-amino amides into 5,5-disubstituted γ-lactams through a base-mediated new molecular rearrangement. In contrast to the known N- to C(O) cyclization of saturated γ-amino acids into corresponding γ-lactams, the new rearrangement involves the cyclization between N-terminal C^γ- to C-terminal amide N. The cyclization process was initiated by the migration of double bond from α,β → β,γ position. The enamine-imine tautomerization of the new β,γ-double bond and subsequent 5-exo-trig cyclization of terminal amide leads to the formation of N-protected 5,5-disubstituted γ-lactam. The structures of various γ-lactams obtained from the rearrangement were studied in single crystals. Overall, the results reported here demonstrate the facile and single-step transformation of N-protected α,β-unsaturated γ-amino amides into γ-lactams and provided an excellent opportunity to construct small-molecule peptidomimetics.

Impact of Backbone Pattern and Residue Substitution on Helicity in α/β/γ-Peptides

We have evaluated the impact of changes in the chemical structure of peptidic oligomers containing α-, β-, and γ-amino acid residues (α/β/γ-peptides) on the propensities of these oligomers to adopt helical conformations in aqueous and alcoholic solutions. These studies were inspired by our previous discovery that α/β/γ-peptides containing a regular αγααβα hexad repeat adopt an α-helix-like conformation in which the β and γ residues are aligned in a stripe along one side, and the remainder of the helix surface is defined by the α residues. This helix was found to be most stable when the β and γ residues were rigidified with specific cyclic constraints. Relaxation of the β residue constraints caused profound conformational destabilization, but relaxation of the γ residue constraints led to only a moderate drop in helicity. The new work more broadly characterizes the effect of γ residue substitution on helix stability, based on circular dichroism and two-dimensional NMR measurements. We find that even a fully unsubstituted γ residue (derived from γ-aminobutyric acid) supports a moderate helical propensity, which is surprising in light of the strong destabilizing effect of glycine residues on α-helix stability. Additional studies examine the effects of altering sequence in terms of amino acid type, by comparing a prototype with the αγααβα hexad pattern to isomers with irregular arrangements of the α, β, and γ residues along the backbone. The data indicate that the strong helix-forming propensity previously discovered for α/β/γ-peptide 12-mers is retained when sequence is varied, with small variations detected across diverse α-β-γ placements. These structural findings suggest that α/β/γ-peptide scaffolds represent versatile scaffolds for the design of peptidic foldamers that display specific functions.

Artificial β-Double Helices from Achiral γ-Peptides

Double helices are not common in polypeptides and proteins except in the peptide antibiotic gramicidin A and analogous l,d-peptides. In contrast to natural polypeptides, remarkable β-double-helical structures from achiral γ-peptides built from α,β-unsaturated γ-amino acids have been observed. The crystal structures suggest that they adopted parallel β-double helical structures and these structures are stabilized by the interstrand backbone amide H-bonds. Furthermore, both NMR spectroscopy and fluorescence studies support the existence of double-helical conformations in solution. Although a variety of folded architectures featuring distinct H-bonds have been discovered from the β- and γ-peptide foldamers, this is the first report to show that achiral γ-peptides can spontaneously intertwine into β-double helical structures.

Impact of γ-Amino Acid Residue Preorganization on α/γ-Peptide Foldamer Helicity in Aqueous Solution

α/γ-Peptide foldamers containing either γ(4)-amino acid residues or ring-constrained γ-amino acid residues have been reported to adopt 12-helical secondary structure in nonpolar solvents and in the solid state. These observations have engendered speculation that the seemingly flexible γ(4) residues have a high intrinsic helical propensity and that residue-based preorganization may not significantly stabilize the 12-helical conformation. However, the prior studies were conducted in environments that favor intramolecular H-bond formation. Here, we use 2D-NMR to compare the ability of γ(4) residues and cyclic γ residues to support 12-helix formation in more challenging environments, methanol and water. Both γ residue types support 12-helical folding in methanol, but only the cyclically constrained γ residues promote helicity in water. These results demonstrate the importance of residue-based preorganization strategies for achieving stable folding among short foldamers in aqueous solution.

Structural features and molecular aggregations of designed triple-stranded β-sheets in single crystals

Design, synthesis, single-crystal conformations and molecular aggregations of hybrid triple-stranded β-sheets, and their structural analogy with protein structures are reported.

Can Helical Peptides Unwind One Turn at a Time? - Controlled Conformational Transitions in α,β(2,3)-Hybrid Peptides

Unfolding of helical trans-β(2,3) -hybrid peptides with (α-β)n α composition, when executed by increasing solvent polarity or temperature, proceeded in a systematic manner with the turns unwinding sequentially; C-terminal region of these peptides were first to unwind and the process propagated towards N terminus with more and more β residues equilibrating from the gauche to the anti rotameric state across Cα-Cβ . This is evidenced by clear change in their Cβ H signal splitting, (3)JCαH-CβH values, and sequential disappearance of i,i+2 NOEs.

Engineering polypeptide folding through trans double bonds: transformation of miniature β-meanders to hybrid helices

Utilization of conjugated double bonds to engineer the novel folded miniature β-meander type structures, transformation of miniature β-meanders into 10/12-helices using catalytic hydrogenation, their solution and single crystal conformations are reported.

An efficient one pot ipso-nitration: structural transformation of a dipeptide by N-terminus modification

CuSO₄ catalyzed one pot ipso-nitration of rigid dipeptide leads to structural transformation from anti parallel to parallel β-sheet.

HBTU mediated 1-hydroxybenzotriazole (HOBt) conjugate addition: synthesis and stereochemical analysis of β-benzotriazole N-oxide substituted γ-amino acids and hybrid peptides

Synthesis and conformational analysis of β-benzotriazole N-oxide (β-BtO) substituted γ-amino acids by direct conjugate addition of HOBt to E-vinylogous γ-amino acids are reported.

Cyclic constraints on conformational flexibility in γ-peptides: conformation specific IR and UV spectroscopy

Single-conformation spectroscopy has been used to study two cyclically constrained and capped γ-peptides: Ac-γACHC-NHBn (hereafter γACHC, Figure 1a), and Ac-γACHC-γACHC-NHBn (γγACHC, Figure 1b), under jet-cooled conditions in the gas phase. The γ-peptide backbone in both molecules contains a cyclohexane ring incorporated across each Cβ-Cγ bond and an ethyl group at each Cα. This substitution pattern was designed to stabilize a (g+, g+) torsion angle sequence across the Cα-Cβ-Cγ segment of each γ-amino acid residue. Resonant two-photon ionization (R2PI), infrared-ultraviolet hole-burning (IR-UV HB), and resonant ion-dip infrared (RIDIR) spectroscopy have been used to probe the single-conformation spectroscopy of these molecules. In both γACHC and γγACHC, all population is funneled into a single conformation. With RIDIR spectra in the NH stretch (3200-3500 cm(-1)) and amide I/II regions (1400-1800 cm(-1)), in conjunction with theoretical predictions, assignments have been made for the conformations observed in the molecular beam. γACHC forms a single nearest-neighbor C9 hydrogen-bonded ring whereas γγACHC takes up a next-nearest-neighbor C14 hydrogen-bonded structure. The gas-phase C14 conformation represents the beginning of a 2.614-helix, suggesting that the constraints imposed on the γ-peptide backbone by the ACHC and ethyl groups already impose this preference in the gas-phase di-γ-peptide, in which only a single C14 H-bond is possible, constituting one full turn of the helix. A similar conformational preference was previously documented in crystal structures and NMR analysis of longer γ-peptide oligomers containing the γACHC subunit [Guo, L., et al. Angew. Chem. Int. Ed. 2011, 50, 5843-5846]. In the gas phase, the γACHC-H2O complex was also observed and spectroscopically interrogated in the molecular beam. Here, the monosolvated γACHC retains the C9 hydrogen bond observed in the bare molecule, with the water acting as a bridge between the C-terminal carbonyl and the π-cloud of the UV chromophore. This is in contrast to the unconstrained γ-peptide-H2O complex, which incorporates H2O into both C9 and amide-stacked conformations.

Self-assembly to function: design, synthesis, and broad spectrum antimicrobial properties of short hybrid E-vinylogous lipopeptides

Nonribosomal E-vinylogous γ-amino acids are widely present in many peptide natural products and have been exploited as inhibitors for serine and cysteine proteases. Here, we are reporting the broad spectrum antimicrobial properties and self-assembled nanostructures of various hybrid lipopeptides composed of 1:1 alternating α- and E-vinylogous residues. Analysis of the results revealed that self-assembled nanostructures also play a significant role in the antimicrobial and hemolytic activities. In contrast to the α-peptide counterparts, vinylogous hybrid peptides displayed excellent antimicrobial properties against various bacterial and fungal strains. Peptides that adopted nanofiber structures displayed less hemolytic activity, while peptides that adopted nanoneedle structures displayed the highest hemolytic activity.