Transport behavior of a single Ca(2+), K(+), and Na(+) in a water-filled transmembrane cyclic peptide nanotube

Strengthening the Self-Assembly of Supramolecular Polymeric Nanotubes in Water via the Introduction of Hydrophobic Moieties

Supramolecular polymeric nanotubes based on the self-assembling cyclic peptide-polymer conjugates are a promising class of materials, showing great potential in various biological applications. Herein, we present a novel strategy to promote nanotube assembly through effectively shielding the cyclic peptides from water, via the introduction of varying hydrophobic groups. As determined by a combination of SANS, TEM, and SLS, hydrophobic interactions, π-π stacking, and multiple hydrogen bonding interactions cooperate in the self-assembly of the cyclic peptide-polymer conjugates, allowing for the construction of supramolecular nanotubes that are longer than expected in water. This approach offers an effective pathway toward the design of organic nanotubes of hundreds of nanometers in water.

Exploring Cyclic Peptide Nanotube Stability Across Diverse Lipid Bilayers and Unveiling Water Transport Dynamics

Cyclic Peptide Nanotubes (CPNTs) have emerged as compelling candidates for various applications, particularly as nanochannels within lipid bilayers. In this study, the stability of two CPNTs, namely 8 × [(Cys-Gly-Met-Gly)2] and 8 × [(Gly-Leu)4], are comprehensively investigated across different lipid bilayers, including 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), a mixed model membrane (POPE/POPG), and a realistic yeast model membrane. The results demonstrate that both CPNTs maintain their tubular structures in all lipid bilayers, with [(Cys-Gly-Met-Gly)2] showing increased stability over an extended period in these lipid membranes. The insertion of CPNTs shows negligible impact on lipid bilayer properties, including area per lipid, volume per lipid, and bilayer thickness. The study demonstrates that the CPNT preserves its two-line water movement pattern within all the lipid membranes, reaffirming their potential as water channels. The MSD curves further reveal that the dynamics of water molecules inside the nanotube are similar for all the bilayer systems with minor differences that arise due to different lipid environments.

Molecular Dynamics Simulation Study of the Protonation State Dependence of Glutamic Acid Transport through a Cyclic Peptide Nanotube

The effect of the protonation state of glutamic acid on its translocation through cyclic peptide nanotubes (CPNs) was assessed by using molecular dynamics (MD) simulations. Anionic (GLU-), neutral zwitterionic (GLU0), and cationic (GLU+) forms of glutamic acid were selected as three different protonation states for an analysis of energetics and diffusivity for acid transport across a cyclic decapeptide nanotube. Based on the solubility-diffusion model, permeability coefficients for the three protonation states of the acid were calculated and compared with experimental results for CPN-mediated glutamate transport through CPNs. Potential of mean force (PMF) calculations reveal that, due to the cation-selective nature of the lumen of CPNs, GLU-, so-called glutamate, shows significantly high free energy barriers, while GLU+ displays deep energy wells and GLU0 has mild free energy barriers and wells inside the CPN. The considerable energy barriers for GLU- inside CPNs are mainly attributed to unfavorable interactions with DMPC bilayers and CPNs and are reduced by favorable interactions with channel water molecules through attractive electrostatic interactions and hydrogen bonding. Unlike the distinct PMF curves, position-dependent diffusion coefficient profiles exhibit comparable frictional behaviors regardless of the charge status of three protonation states due to similar confined environments imposed by the lumen of the CPN. The calculated permeability coefficients for the three protonation states clearly demonstrate that glutamic acid has a strong protonation state dependence for its transport through CPNs, as determined by the energetics rather than the diffusivity of the protonation state. In addition, the permeability coefficients also imply that GLU- is unlikely to pass through a CPN due to the high energy barriers inside the CPN, which is in disagreement with experimental measurements, where a considerable amount of glutamate permeating through the CPN was detected. To resolve the discrepancy between this work and the experimental observations, several possibilities are proposed, including a large concentration gradient of glutamate between the inside and outside of lipid vesicles and bilayers in the experiments, the glutamate activity difference between our MD simulations and experiments, an overestimation of energy barriers due to the artifacts imposed in MD simulations, and/or finally a transformation of the protonation state from GLU- to GLU0 to reduce the energy barriers. Overall, our study demonstrates that the protonation state of glutamic acid has a strong effect on the transport of the acid and suggests a possible protonation state change for glutamate permeating through CPNs.

Influence of salt and temperature on the self-assembly of cyclic peptides in water: a molecular dynamics study

It is found in the literature that cyclic peptides (CPs) are able to self-assemble in water to form cyclic peptide nanotubes (CPNTs) and are used extensively in the field of nanotechnology. Several factors influence the formation and stability of these nanotubes in water. However, an extensive study of the contribution of several important factors is still lacking. The purpose of this study is to explore the effect of temperature and salt (NaCl) on the association tendency of CPs. Furthermore, the self-association behavior of CPs in aqueous solutions at various temperatures is also thoroughly discussed. Cyclo-[(Asp-D-Leu-Lys-D-Leu)₂] is considered for this study and a series of classical molecular dynamics (MD) simulations at three different temperatures, viz. 280 K, 300 K, and 320 K, both in pure water and in NaCl solutions of different concentrations are carried out. The calculations of radial distribution functions, preferential interaction parameters, cluster formation and hydrogen bonding properties suggest a strong influence of NaCl concentration on the association propensity of CPs. Low NaCl concentration hinders CP association while high NaCl concentration facilitates the association of CPs. Besides this, the association of CPs is found to be enhanced at low temperature. Furthermore, the thermodynamics of CP association is predominantly found to be enthalpy driven in both the presence and absence of salt. No crossover between enthalpy and entropy in CP association is observed. In addition, the MM-GBSA method is used to investigate the binding free energies of the CP rings that self-assembled to form nanotube like structures at all three temperatures.

Molecular Self-Assembly and Supramolecular Chemistry of Cyclic Peptides

This Review focuses on the establishment and development of self-assemblies governed by the supramolecular interactions between cyclic peptides. The Review first describes the type of cyclic peptides able to assemble into tubular structures to form supramolecular cyclic peptide nanotubes. A range of cyclic peptides have been identified to have such properties, including α-peptides, β-peptides, α,γ-peptides, and peptides based on δ- and ε-amino acids. The Review covers the design and functionalization of these cyclic peptides and expands to a recent advance in the design and application of these materials through their conjugation to polymer chains to generate cyclic peptide-polymer conjugates nanostructures. The Review, then, concentrates on the challenges in characterizing these systems and presents an overview of the various analytical and characterization techniques used to date. This overview concludes with a critical survey of the various applications of the nanomaterials obtained from supramolecular cyclic peptide nanotubes, with a focus on biological and medical applications, ranging from ion channels and membrane insertion to antibacterial materials, anticancer drug delivery, gene delivery, and antiviral applications.

Study on the Assembly Mechanisms and Transport Properties of Transmembrane End-Charged Cyclic Peptide Nanotubes

In this work, two end-charged cyclic peptide nanotubes (CPNTs) embedded in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) were designed to simulate transmembrane ion channels. Density functional theory (DFT) computations at the level of M06-2X/6-31G give different assembling modes of the negatively charged ELWL-CPNT and positively charged RLWL-CPNT as (L-L)(D-L)(D-D)(L-L)(D-D)(L-L)(D-D) and (D-D)(L-L)(D-D)(L-L)(D-D)(L-L)(D-D), respectively. Molecular dynamics (MD) simulations indicate that a charge at a CPNT end obviously affects the structure of the channel water chain and the diffusion behavior of K⁺. The regions with the highest probability of H-bond defects in the channel water chains are gap5 and gap2 in ELWL/POPE-CPNT and RLWL/POPE-CPNT, respectively. K⁺ can easily enter either CPNT by desolvation, and behaves more actively in RLWL/POPE-CPNT, shuttling rapidly and frequently between an α-plane zone and an adjacent midplane region. Results of this work reveal that a charge at the end of an ionic channel may significantly alter the transport characteristics of the channel.

Construction of a web-based nanomaterial database by big data curation and modeling friendly nanostructure annotations

Modern nanotechnology research has generated numerous experimental data for various nanomaterials. However, the few nanomaterial databases available are not suitable for modeling studies due to the way they are curated. Here, we report the construction of a large nanomaterial database containing annotated nanostructures suited for modeling research. The database, which is publicly available through http://www.pubvinas.com/, contains 705 unique nanomaterials covering 11 material types. Each nanomaterial has up to six physicochemical properties and/or bioactivities, resulting in more than ten endpoints in the database. All the nanostructures are annotated and transformed into protein data bank files, which are downloadable by researchers worldwide. Furthermore, the nanostructure annotation procedure generates 2142 nanodescriptors for all nanomaterials for machine learning purposes, which are also available through the portal. This database provides a public resource for data-driven nanoinformatics modeling research aimed at rational nanomaterial design and other areas of modern computational nanotechnology.

A molecular dynamics investigation on transporting mechanism of glucose through a cyclic peptide nanotube

Cyclic peptide nanotubes (CPNTs) are open-ended, hollow, and tubular structures that are made of several cyclic peptide rings. These structures can act as a transmembrane channel and transport ions or small molecules. In this work, we studied the stability of a cyclic peptide nanotube 8 × [(Trp-D-Leu)4-Gln-D-Leu] into a fully hydrated membrane composed of DPPC/POPC/POPS/cholesterol and also the transport mechanism of β-D-glucose through this nanotube was investigated. Our findings revealed that the CPNT was stable in the lipid bilayer during the simulation time and non-bonded interactions, especially hydrogen bonding have an important role to form a stable CPNT in the membrane. The glucose jumps from a Cα-region into the mid-Cα region and spends more time in this region because of its more desirable interactions with water molecules and the CPNT. In the transport pathway, non-bonded interactions between glucose, water molecules and the CPNT facilitate the transport of the glucose through the CPNT. The collaboration of hydrogen bonds, electrostatic and van der Waals interactions change the pulling force and lead to transport glucose through the CPNT. Potential of mean force (PMF) calculations revealed that the glucose has a minimum value of binding free energy and maximum configurational entropy in MPR regions. These findings can be used to design more CPNTs with different goals such as drug delivery.Communicated by Ramaswamy H. Sarma.

Applications of cyclic peptide nanotubes (cPNTs)

Self-assembled cyclic peptide nanotubes (cPNTs) have recently drawn particular attention as one of the most intriguing nanostructures in the field of nanotechnology. Given their unique features including high surface area, increased drug loading, environmental stability, enhanced permeation, and modifiable drug release, these hollow tubular structures can be constructed with cyclic di-, tri-, tetra-, hexa-, octa-, and decapeptides with various amino acid sequences, enantiomers, and functionalized side chains and can be applied for antiviral and antibacterial drugs, drug delivery and gene delivery vectors, organic electronic devices, and ionic or molecular channels. Recent publications have presented promising results regarding the use of cPNTs as drugs or biomedical devices. However, there is an urgent need for the further in vivo nanotoxicity and safety testing of these nanotubes to evaluate their suitability in different fields.

Dynamic behavior and selective adsorption of a methanol/water mixture inside a cyclic peptide nanotube

Present molecular dynamics simulations indicate that the methanol component in a methanol/water mixture is more likely to be trapped in a cyclic peptide nanotube (CPNT), while water molecules tend to be present at the channel mouths as transient guests. Channel water resides mainly between methanol and the CPNT wall, resulting in a distinct decrease in the H-bond number per channel methanol. Six designed CPNTs with different channel diameters and outer surface characteristics all possess distinct selectivity to methanol over water. Of these, the amphipathic 8 × (AQ)₄-CPNT exhibits the best performance. Results in this study provide basic information for the application of a CPNT to enrich methanol from a methanol/water mixture. Graphical Abstract Typical overview of water and methanol molecular distribution in cyclic peptide nanotubes.

Ion Permeation Mechanism in Epithelial Calcium Channel TRVP6

Calcium is the most abundant metal in the human body that plays vital roles as a cellular electrolyte as well as the smallest and most frequently used signaling molecule. Calcium uptake in epithelial tissues is mediated by tetrameric calcium-selective transient receptor potential (TRP) channels TRPV6 that are implicated in a variety of human diseases, including numerous forms of cancer. We used TRPV6 crystal structures as templates for molecular dynamics simulations to identify ion binding sites and to study the permeation mechanism of calcium and other ions through TRPV6 channels. We found that at low Ca²⁺ concentrations, a single calcium ion binds at the selectivity filter narrow constriction formed by aspartates D541 and allows Na⁺ permeation. In the presence of ions, no water binds to or crosses the pore constriction. At high Ca²⁺ concentrations, calcium permeates the pore according to the knock-off mechanism that includes formation of a short-lived transition state with three calcium ions bound near D541. For Ba²⁺, the transition state lives longer and the knock-off permeation occurs slower. Gd³⁺ binds at D541 tightly, blocks the channel and prevents Na⁺ from permeating the pore. Our results provide structural foundations for understanding permeation and block in tetrameric calcium-selective ion channels.

Molecular Dynamics Simulations on the Behaviors of Hydrophilic/Hydrophobic Cyclic Peptide Nanotubes at the Water/Hexane Interface

In this work, nine kinds of amino acid residues, i.e., alanine (A), leucine (L), valine (V), isoleucine (I), tryptophan (W), glutamine (Q), threonine (T), serine (S), and cysteine (C), were selected to construct seven cyclic peptide nanotubes (CPNTs) with diverse hydrophilic/hydrophobic external surfaces, which were further separately inserted at the water/hexane interface to investigate their microstructures and interfacial properties. Molecular dynamics (MD) simulations reveal that all the CPNTs except the QT- and VL-CPNTs have different degrees of tilt, fracture, and shedding at the interface. The end-CPs are more susceptible to the effect of the surroundings than the mid-CPs. The interactions of individual CP subunits with the neighborings disclose the firmness of the mid-CPs and the dissociation of the end-CPs. The results indicate that a hydrophobic CPNT is prone to stay at the interface, while a hydrophilic CPNT easily enters the water phase, resulting in many H-bonds with water. Results in this work enrich the dynamic properties of a hydrophilic/hydrophobic CPNT at the biphase interface at the atomic level.

Conformational changes during permeation of Na+ through a modified cyclic peptide nanotube promote energy landscape roughness

Using a metadynamics approach, we investigate the potential of mean force for Na⁺ permeation inside a cyclic peptide nanotube (CPN) with modified interior as a function of ion position, coordination number, and lumen chemistry. We show that functionalizing the lumen of a CPN with a methyl-benzoic acid group introduces non-periodic variations in the internal energy of the nanotube, which dictate the overall free energy roughness during the permeation of Na⁺. These non-periodic variations arise from the structural dynamics of the functional group, where changes in the dihedral angles induced by the proximity of the ion give rise to conformational changes that increase landscape roughness and thereby decrease transport rate. Our computational framework emphasizes the advantages of using the coordination number as a collective variable to investigate the available conformations during ion permeation through CPNs, and reveals new structure-function relations for chemically tunable CPNs, paving the way for rational design of nano-porous systems with tunable selectivity and flux.