Exploring Macroscopic Dipoles of Designed Cyclic Peptide Ordered Assemblies to Harvest Piezoelectric Properties.

Crystalline materials exhibiting non-centrosymmetry and possessing substantial surface dipole moments play a critical role in piezoelectricity. Designing biocompatible self-assembled materials with these attributes is particularly challenging when compared to inorganic materials and ceramics. In this study, we elucidate the crystal conformations of novel cyclic peptides that exhibit self-assembly into tubular structures characterized by unidirectional hydrogen bonding and piezoelectric properties. Unlike cyclic peptides derived from alternating L- and D-amino acids, those derived from new δ-amino acids demonstrate the formation of self-assembled tubes with unidirectional hydrogen bonds. Further, the tightly packed tubular assemblies and higher macrodipole moments result in superior piezoelectric coefficients compared to peptides with lower macrodipole moments. Our findings underscore the potential for designing cyclic peptides with unidirectional hydrogen bonds, thereby paving the way for their application in design of biocompatible piezo- and ferroelectric materials.

Design of Chiral ß-Double Helices from γ-Peptide Foldamers.

Chirality is ubiquitous in nature, and homochirality is manifested in many biomolecules. Although ß-double helices are rare in peptides and proteins, they consist of alternating L- and D-amino acids. No peptide double helices with homochiral amino acids have been observed. Here, we report chiral ß-double helices constructed from γ-peptides consisting of alternating achiral (E)-α,ß-unsaturated 4,4-dimethyl γ-amino acids and chiral (E)-α,ß-unsaturated γ-amino acids in both single crystals and in solution. The two independent strands of the same peptide intertwine to form a ß-double helix structure, and it is stabilized by inter-strand hydrogen bonds. The peptides with chiral (E)-α,ß-unsaturated γ-amino acids derived from α-L-amino acids adopt a (P)-ß-double helix, whereas peptides consisting of (E)-α,ß-unsaturated γ-amino acids derived from α-D-amino acids adopt an (M)-ß-double helix conformation. The circular dichroism (CD) signature of the (P) and (M)-ß-double helices and the stability of these peptides at higher temperatures were examined. Furthermore, ion transport studies suggested that these peptides transport ions across membranes. Even though the structural analogy suggests that these new ß-double helices are structurally different from those of the α-peptide ß-double helices, they retain ion transport activity. The results reported here may open new avenues in the design of functional foldamers.

Anion Tuned Structural Modulation and Nonlinear Optical Effects of Metal-Ion Directed 310 -Helix Networks.

Metals play an important role in the structure and functions of various proteins. The combination of metal ions and peptides have been emerging as an attractive field to create advanced structures and biomaterials. Here, we are reporting the anion-influenced, silver ion coordinated diverse networks of designed short tripeptide 310 -helices with terminal pyridyl groups. The short peptides adopted classical right-handed, left-handed and 310 EL -helical conformations in the presence of different silver salts. The peptides have displayed conformational flexibility to accommodate different sizes and interactions of anions to yield a variety of metal-coordinated networks. The complexes of metal ions and peptides have shown different porous networks, right- and left-handed helical polymers, transformation of helix into superhelix and 2 : 2 metal-peptide macrocycles. Further, the metal-peptide crystals with inherent dipoles of helical peptides gave striking second harmonic generation response. The optical energy upconversion from NIR to red and green light is demonstrated. Overall, we have shown the utilization of short 310 -helices for the construction of diverse metal-coordinated helical networks and notable non-linear optical effects.

Proteolytically Stable ααγ-Hybrid Peptides Inhibit the Aggregation and Cytotoxicity of Aß42.

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 Aß-peptides. Here, we report that the 15-residue ααγ-hybrid peptide not only inhibits the aggregation of soluble Aß42 into fibrils but also disintegrates the aggregated Aß42 fibrils into smaller assemblies. Further, the hybrid peptide completely rescues neuronal cells from the toxicity of Aß42 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 Aß42. This is in sharp contrast to the ß-sheet conformation of Aß42 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 Aß42 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.

Foldamer Nanotubes Mediated Label-Free Detection of Protein-Small Molecule Interactions.

Development of miniaturized lab-on-chip devices for the detection of rapid and specific small molecule-protein binding interactions at very low concentrations holds significant importance in drug discovery and biomedical applications. Here, the label-free detection of small molecule-protein interactions is reported on the surface functionalizable nanotubes of α,γ-hybrid peptide helical foldamers using nanoscale capacitance and impedance spectroscopy. The 12-helix conformation of the α,γ-hybrid peptide observed in the single crystals, self-assembled into nanotubes in an aqueous environment with exposed cysteine thiols for small molecule conjugation. The binding of streptavidin to the covalently linked biotin on the surface of nanotubes was detected at the picomolar concentrations. No change in the capacitance and impedance were observed in the absence of either immobilized biotin or protein streptavidin. The functionalizable hybrid peptide nanotubes reported here pave the way for the label-free detection of various small molecule protein interactions at very low concentrations.

Cyclization of N-Boc-(E)-α,ß-unsaturated γ-amino acid active esters into N-Boc-(Z)-α,ß-unsaturated γ-lactams through E → Z isomerization.

Here, we are reporting the spontaneous transformation of the active esters of N-Boc protected E-α,ß-unsaturated γ-amino acids into the corresponding Z-α,ß-unsaturated γ-lactams with concomitant E → Z isomerization in the presence of a weak base. No cyclization was observed in the absence of the base. Analysis revealed that amide γ-NH is crucial for both lactamization and E → Z isomerization. This mild transformation provides easy access to the synthetically challenging α,ß-unsaturated γ-lactams and also gives new insights into the E → Z isomerization of double bonds.

Stereoselective synthesis of backbone extended π-conjugated amino esters.

Utilization of the Wittig reaction to synthesize conjugative multiple double bonds is rare. We examined the utility of the Wittig reaction to construct conjugative two and three carbon-carbon double bonds on the N-protected amino acid backbone. The ethyl esters of N-Boc amino acids with multiple carbon-carbon double bonds in the backbone were isolated in excellent yields with exceptional E-selectivity of the double bonds. The allylic alcohols of α,ß-unsaturated γ-amino esters were selectively synthesized from the DIBAL-H and BF3·OEt2. The allylic alcohols were transformed into aldehydes using IBX oxidation. Using this protocol, we synthesized ethyl esters of N-Boc-(E,E)-α,ß,γ,δ-unsaturated ε-amino acids with various side-chain functionalities and ethyl esters of N-Boc-(E,E,E)-α,ß,γ,δ,ε,ζ-unsaturated η-amino acids with excellent yields. We speculated the exceptional E-selectivity is probably due to the stabilization of the planar transition state of the Wittig reaction with the double bond p-orbitals. No racemization was observed in the synthesis of amino acids. The reported process may serve as an excellent route to synthesize multiple conjugative carbon-carbon double bonds.

A cationic amphiphilic peptide chaperone rescues Aß42 aggregation and cytotoxicity.

Directing Aß42 to adopt a conformation that is free from aggregation and cell toxicity is an attractive and viable strategy to design therapeutics for Alzheimer's disease. Over the years, extensive efforts have been made to disrupt the aggregation of Aß42 using various types of inhibitors but with limited success. Herein, we report the inhibition of aggregation of Aß42 and disintegration of matured fibrils of Aß42 into smaller assemblies by a 15-mer cationic amphiphilic peptide. The biophysical analysis comprising thioflavin T (ThT) mediated amyloid aggregation kinetic analysis, dynamic light scattering, ELISA, AFM, and TEM suggested that the peptide effectively disrupts Aß42 aggregation. The circular dichroism (CD) and 2D-NMR HSQC analysis reveal that upon interaction, the peptide induces a conformational change in Aß42 that is free from aggregation. Further, the cell assay experiments revealed that this peptide is non-toxic to cells and also rescues the cells from the toxicity of Aß42. Peptides with a shorter length displayed either weak or no inhibitory effect on Aß42 aggregation and cytotoxicity. These results suggest that the 15-residue cationic amphiphilic peptide reported here may serve as a potential therapeutic candidate for Alzheimer's disease.

Structural Investigation of Hybrid Peptide Foldamers Composed of α-Dipeptide Equivalent ß-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 ß(O)-δ5 -amino acids. Here, we report the incorporation of ß(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 ß(O)-δ5 -amino acids. In contrast to the control α-helix structures, the crystal structure of peptides with ß(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 ß(O)-δ5 -amino acid without significant deviation to the overall helix folding. The results reported here emphasize that ß(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.