Effects of Anions on Nanostructuring of Cationic Amphiphilic Peptides

Anion Effects on the Supramolecular Self-Assembly of Cationic Phenylalanine Derivatives

Supramolecular hydrogels have emerged as a class of promising biomaterials for applications such as drug delivery and tissue engineering. Self-assembling peptides have been well studied for such applications, but low molecular weight (LMW) amino acid-derived gelators have attracted interest as low-cost alternatives with similar emergent properties. Fluorenylmethyloxycarbonyl-phenylalanine (Fmoc-Phe) is one such privileged motif often chosen due to its inherent self-assembly potential. Previously, we developed cationic Fmoc-Phe-DAP gelators that assemble into hydrogel networks in aqueous NaCl solutions of sufficient ionic strength. The chloride anions in these solutions screen the cationic charge of the gelators to enable self-assembly to occur. Herein, we report the effects of varying the anions of sodium salts on the gelation potential, nanoscale morphology, and hydrogel viscoelastic properties of Fmoc-Phe-DAP and two of its fluorinated derivatives, Fmoc-3F-Phe-DAP and Fmoc-F₅-Phe-DAP. It was observed that both the anion identity and gelator structure had a significant impact on the self-assembly and gelation properties of these derivatives. Changing the anion identity resulted in significant polymorphism of the nanoscale morphology of the assembled states that was dependent on the chemical structure of the gelator. The emergent viscoelastic character of the hydrogel networks was also found to be reliant on the anion identity and gelator structure. These results demonstrate the complex interplay between the gelator and environment that have a profound and often unpredictable impact on both self-assembly properties and emergent viscoelasticity in supramolecular hydrogels formed by LMW compounds. This work also illustrates the current lack of understanding that limits the rational design of potential biomaterials that will be in contact with complex biological fluids and provides motivation for additional research to correlate the chemical structure of LMW gelators with the structure and emergent properties of the resulting supramolecular assemblies as a function of environment.

Mechanistic Investigation of a Self-Assembling Peptide against Escherichia coli

Because of their distinctive mode of action in targeting bacterial cell membranes, antimicrobial peptides (AMPs) are increasingly regarded as a potential candidate for the development of novel antibiotics to combat the wide spread of bacterial resistance. To date, understanding of the exact molecular process by which AMPs act on the real bacterial envelope remains challenging. Simultaneously, the aggregated state of AMPs upon interaction with bacterial envelopes is still elusive. Previously, we have demonstrated that the potent antibacterial activity of a designed surfactant-like peptide Ac-A₉K-NH₂ benefited greatly from its high self-assembling ability and appropriate self-assembled morphologies and sizes. By using high-resolution atomic force microscopy, we here not only follow the variations of the Escherichia coli cell envelope in the presence of Ac-A₉K-NH₂ but also characterize the peptide aggregates on the bacterial surface as well as on the substrate surface. The results, together with those from fluorescence, zeta potential, circular dichroism, and scanning electron microscopy measurements, indicate that both the positively charged peptide monomers and self-assembled nanostructures can directly act on the negatively charged bacterial surface, followed by their insertion into the bacterial membrane, the formation of surface nanopores, and membrane lysis. The mechanism of Ac-A₉K-NH₂ against E. coli is thus consistent with the detergent-like mode of action. This work enhances our mechanistic understanding of the antibacterial behaviors of self-assembling peptides that will be valuable in exploring their biomedical applications.

Active Modulation of States of Prestress in Self-Assembled Short Peptide Gels

Peptide hydrogels are excellent candidates for medical therapeutics due to their tuneable viscoelastic properties, however, in vivo they will be subject to various osmotic pressures, temperature changes, and biological co-solutes, which could alter their performance. Peptide hydrogels formed from the synthetic peptide I3K have a temperature-induced hardening of their shear modulus by a factor of 2. We show that the addition of uncross-linked poly( N-isopropylacrylamide) chains to the peptide gels increases the gels' temperature sensitivity by 3 orders of magnitude through the control of osmotic swelling and cross-linking. Using machine learning combined with single-molecule fluorescence microscopy, we measured the modulation of states of prestress in the gels on the level of single peptide fibers. A new self-consistent mixture model was developed to simultaneously quantify the energy and the length distributions of the states of prestress. Switching the temperature from 20 to 40 °C causes 6-fold increases in the number of states of prestress. At the higher temperature, many of the fibers experience constrained buckling with characteristic small wavelength oscillations in their curvature.

Effect of the Chloride Anions on the Formation of Self-Assembled Diphenylalanine Peptide Nanotubes

Self-assembled peptide nanostructures are being intensively investigated due to their potential applications such as biosensors, piezotransducers, and microactuators. It was predicted that their formation and hence piezoelectric property strongly depend on the water content and acidity of the stock solution. In this paper, simple diphenylalanine (FF) tubular structures were fabricated from the solutions with added hydrochloric acid in order to understand the influence of chloride ions on the self-assembly process and resulting piezoelectricity. Low-frequency Raman scattering, atomic, and piezoresponse force microscopies were used to characterize both the morphology and piezoelectric properties of the grown samples. The mechanism of chloride anions' effect on the formation of self-assembled peptide nanostructures is discussed based on the acquired Raman data and quantum-chemical modeling. It is shown that the addition of chloride anions causes a significant reduction of the dipole moments of FF tubes accompanied with the concomitant decrease of tube dimensions and apparent shear piezoelectric coefficients.

Construction of supramolecular hydrogels using photo-generated nitric oxide radicals

Photo-generated nitric oxide radicals (NO˙) derived from sodium nitroprusside dihydrate (SNP) are employed for the construction of supramolecular hydrogels based on an amino acid derivative precursor, N-fluorenylmethyloxycarbonyl tyrosine phosphate (FYP), which through dephosphorylation produces the gelator, N-fluorenylmethyloxycarbonyl tyrosine (FY). Self-assembly of the amphiphilic gelator yields high-aspect ratio nanofilaments that entangle to form self-supporting, viscoelastic hydrogels. The presence of photolyzed SNP yields periodically twisted nanofilaments with opposite chirality to filaments formed through conventional hydrogelation routes.

Evaluation of the antibacterial and antibiofilm activities of novel CRAMP-vancomycin conjugates with diverse linkers

We report the design, synthesis and antibacterial activity analysis of conjugates of vancomycin and cathelicidin-related antimicrobial peptides (CRAMP). Vancomycin inhibits the nascent peptidoglycan synthesis and is highly active against Gram-positive bacteria, whereas Gram-negative bacteria are generally insensitive due to a protective outer membrane. CRAMP is known to translocate across the Gram-negative outer membrane by a self-promoted uptake mechanism. Vancomycin-CRAMP conjugates were synthesized using click chemistry with diverse hydrophilic and hydrophobic linkers, with CRAMP functioning as a carrier peptide for the transfer of vancomycin through the outer membrane. Small hydrophobic linkers with an aromatic group result in the most active conjugates against planktonic Gram-negative bacteria, while maintaining the high activity of vancomycin against Gram-positive bacteria. These conjugates thus show a broad-spectrum activity, which is absent in CRAMP or vancomycin alone, and which is strongly improved compared to an equimolar mixture of CRAMP and vancomycin. In addition, these conjugates also show a strong inhibitory activity against S. Typhimurium biofilm formation.

A water-soluble metal-organic complex array as a multinuclear heterometallic peptide amphiphile that shows unconventional anion dependency in its self-assembly

Water-soluble metal-organic complex array 1, bearing Ru(II), Pt(II) and Rh(III) complexes at the side residues of the short peptide, exhibits anion and pH-responsive self-assembly behaviours in aqueous media. NaCl-induced aggregation of 1 at neutral pH was suppressed in phosphate buffered saline containing a mixture of Cl(-), HPO4(2-) and H2PO4(-), which is unconventional for a peptide amphiphile.

Modeling the self-assembly of peptide amphiphiles into fibers using coarse-grained molecular dynamics

We have studied the self-assembly of peptide amphiphiles (PAs) into a cylindrical micelle fiber starting from a homogeneous mixture of PAs in water using coarse-grained molecular dynamics simulations. Nine independent 16 μs runs all show spontaneous fiber formation in which the PA molecules first form spherical micelles, and then micelles form a three-dimensional network via van der Waals interactions. As the hydrophobic core belonging to the different micelles merge, the three-dimensional network disappears and a fiber having a diameter of ∼80 Å appears. In agreement with atomistic simulation results, water molecules are excluded from the hydrophobic core and penetrate to ∼15 Å away from the axis of fiber. About 66% of the surface of fiber is covered with the IKVAV epitope, and ∼92% of the epitope is exposed to water molecules.