Antivesiculation and Complete Unbinding of Tail-Tethered Lipids.

We report the effect of tail-tethering on vesiculation and complete unbinding of bilayered membranes. Amphiphilic molecules of a bolalipid, resembling the tail-tethered molecular structure of archaeal lipids, with two identical zwitterionic phosphatidylcholine headgroups self-assemble into a large flat lamellar membrane, in contrast to the multilamellar vesicles (MLVs) observed in its counterpart, monopolar nontethered zwitterionic lipids. The antivesiculation is confirmed by small-angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cyro-TEM). With the net charge of zero and higher bending rigidity of the membrane (confirmed by neutron spin echo (NSE) spectroscopy), the current membrane theory would predict that membranes should stack with each other (aka "bind") due to dominant van der Waals attraction, while the outcome of the nonstacking ("unbinding") membrane suggests that the theory needs to include entropic contribution for the nonvesicular structures. This report pioneers an understanding of how the tail-tethering of amphiphiles affects the structure, enabling better control over the final nanoscale morphology.

Head on Comparison of Self- and Nano-assemblies of Gamma Peptide Nucleic Acid Amphiphiles.

Peptide nucleic acids (PNAs) are nucleic acid analogs with superior hybridization properties and enzymatic stability than deoxyribonucleic acid (DNA). In addition to gene targeting applications, PNAs have garnered significant attention as bio-polymer due to the Watson-Crick -based molecular recognition and flexibility of synthesis. Here, we engineered PNA amphiphiles using chemically modified gamma PNA (8 mer in length) containing hydrophilic diethylene glycol units at the gamma position and covalently conjugated lauric acid (C12) as a hydrophobic moiety. Gamma PNA (γPNA) amphiphiles self-assemble into spherical vesicles. Further, we formulate nano-assemblies using the amphiphilic γPNA as a polymer via ethanol injection-based protocols. We perform comprehensive head-on comparison of the physicochemical and cellular uptake properties of PNA derived self- and nano-assemblies. Small-angle neutron scattering (SANS) and small-angle X-ray scattering (SAXS) analysis reveal ellipsoidal morphology of γPNA nano-assemblies that results in superior cellular delivery compate to the spherical self-assembly. Next, we compare the functional activities of γPNA self-and nano-assemblies in lymphoma cells via multiple endpoints, including gene expression, cell viability, and apoptosis-based assays. Overall, we establish that γPNA amphiphile is a functionally active bio-polymer to formulate nano-assemblies for a wide range of biomedical applications.

Dual-Modality Poly-l-histidine Nanoparticles to Deliver Peptide Nucleic Acids and Paclitaxel for In Vivo Cancer Therapy.

Cationic polymeric nanoformulations have been explored to increase the transfection efficiency of small molecules and nucleic acid-based drugs. However, an excessive positive charge density often leads to severe cell and tissue-based toxicity that restricts the clinical translation of cationic polymeric nanoformulations. Herein, we investigate a series of cationic poly(lactic-co-glycolic acid) (PLGA)-histidine-based nanoformulations for enhanced cytoplasmic delivery with minimal toxicity. PLGA/poly-l-histidine nanoparticles show promising physico-biochemical features and transfection efficiency in a series of in vitro and cell culture-based studies. Further, the use of acetone/dichloromethane as a solvent mixture during the formulation process significantly improves the morphology and size distribution of PLGA/poly-l-histidine nanoparticles. PLGA/poly-l-histidine nanoformulations undergo clathrin-mediated endocytosis. A contrast-matched small-angle neutron scattering experiment confirmed poly-l-histidine's distribution on the PLGA nanoformulations. PLGA/poly-l-histidine formulations containing paclitaxel as a small molecule-based drug and peptide nucleic acids targeting microRNA-155 as nucleic acid analog are efficacious in in vitro and in vivo studies. PLGA/poly-l-histidine NPs significantly decrease tumor growth in PNA-155 (∼6 fold) and paclitaxel (∼6.5 fold) treatment groups in a lymphoma cell line derived xenograft mice model without inducing any toxicity. Hence, PLGA/poly-l-histidine nanoformulations exhibit substantial transfection efficiency and are safe to deliver reagents ranging from small molecules to synthetic nucleic acid analogs and can serve as a novel platform for drug delivery.

Bicelles Rich in both Sphingolipids and Cholesterol and Their Use in Studies of Membrane Proteins.

How the distinctive lipid composition of mammalian plasma membranes impacts membrane protein structure is largely unexplored, partly because of the dearth of isotropic model membrane systems that contain abundant sphingolipids and cholesterol. This gap is addressed by showing that sphingomyelin and cholesterol-rich (SCOR) lipid mixtures with phosphatidylcholine can be cosolubilized by n-dodecyl-ß-melibioside to form bicelles. Small-angle X-ray and neutron scattering, as well as cryo-electron microscopy, demonstrate that these assemblies are stable over a wide range of conditions and exhibit the bilayered-disc morphology of ideal bicelles even at low lipid-to-detergent mole ratios. SCOR bicelles are shown to be compatible with a wide array of experimental techniques, as applied to the transmembrane human amyloid precursor C99 protein in this medium. These studies reveal an equilibrium between low-order oligomer structures that differ significantly from previous experimental structures of C99, providing an example of how ordered membranes alter membrane protein structure.

Correlation of the hierarchical structure with rheological behavior of polypseudorotaxane gel composed of pluronic and ß-cyclodextrin.

We have identified the hierarchical (primary, secondary, tertiary and quaternary) structures of a polypseudorotaxane (PPR) gel composed of the Pluronic F108 and ß-cyclodextrin system to be ß-cyclodextrin crystalline, lamellar sheets, lamellar stacks and "grains", respectively. The correlation between the rheological properties and the proposed structures under shear flows was rationalized. Alignment of lamellar stacks and reorganization of grain boundaries under shear flows were investigated by rheo-SANS, small angle X-ray scattering and small-angle light scattering. The relaxation of highly aligned lamellar stacks is slow (>2 h) after flow cessation compared to that of the regrouped grains (a few minutes). The main contribution to thixotropic behavior is likely from the faster relaxation of the reorganized grains containing highly oriented lamellar stacks. The comprehensive understanding of structure-function relationship of the PPR gel will facilitate the rational design for its applications.

DNA-Mediated Step-Growth Polymerization of Bottlebrush Macromonomers.

Herein, we report the DNA-mediated self-assembly of bivalent bottlebrush polymers, a process akin to the step-growth polymerization of small molecule monomers. In these "condensation reactions", the polymer serves as a steric guide to limit DNA hybridization in a fixed direction, while the DNA serves as a functional group equivalent, connecting complementary brushes to form well-defined, one-dimensional nanostructures. The polymerization was studied using spectroscopy, microscopy, and scattering techniques and was modeled numerically. The model made predictions of the degree of polymerization and size distribution of the assembled products, and suggested the potential for branching at hybridization junctions, all of which were confirmed experimentally. This study serves as a theoretical basis for the polymer-assembly approach which has the potential to open up new possibilities for suprapolymers with controlled architecture, macromonomer sequence, and end-group functionalities.

Enzyme-mimetic self-catalyzed polymerization of polypeptide helices.

Enzymes provide optimal three-dimensional structures for substrate binding and the subsequent accelerated reaction. Such folding-dependent catalytic behaviors, however, are seldom mechanistically explored with reduced structural complexity. Here, we demonstrate that the α-helix, a much simpler structural motif of enzyme, can facilitate its own growth through the self-catalyzed polymerization of N-carboxyanhydride (NCA) in dichloromethane. The reversible binding between the N terminus of α-helical polypeptides and NCAs promotes rate acceleration of the subsequent ring-opening reaction. A two-stage, Michaelis-Menten-type kinetic model is proposed by considering the binding and reaction between the propagating helical chains and the monomers, and is successfully utilized to predict the molecular weights and molecular-weight distributions of the resulting polymers. This work elucidates the mechanism of helix-induced, enzyme-mimetic catalysis, emphasizes the importance of solvent choice in the discovery of new reaction type, and provides a route for rapid production of well-defined synthetic polypeptides by taking advantage of self-accelerated ring-opening polymerizations.

Chemically Controlled Helical Polymorphism in Protein Tubes by Selective Modulation of Supramolecular Interactions.

Polymorphism has been the subject of investigation across different research disciplines. In biology, polymorphism could be interpreted in such a way that discrete biomacromolecules can adopt diversiform specific conformations/packing arrangement, and this polymorph-dependent property is essential for many biochemical processes. For example, bacterial flagellar filament, composed of flagellin, switches between different supercoiled state allowing the bacteria to swim and tumble. However, in artificial supramolecular systems, it is often challenging to achieve polymorph control and prediction, and in most cases, two or more concomitant polymorphs of similar formation energies coexist. Here, we show that a tetrameric protein with properly oriented binding sites on its surface can arrange into diverse protein tubes with distinct helical parameters by adding specifically designed inducing ligands. We examined several parameters of the ligand that would influence the protein tube formation and found that the flexibility of the ligand linker and the dimerization pose of the ligand complex is critical for the successful production of the tubes and eventually influence the specific helical polymorphs of the formed tubes. A surface lattice accommodation model was further developed to rationalize the geometrical relationship between each helical tube type. Molecular simulation was used to elucidate the interactions between ligands and SBA and molecular basis for polymorphic switching of the protein tubes. Moreover, the kinetics of structural formation was studied and the ligand design was found that can affect the kinetics of the protein polymerization pathway. In short, our designed protein tubes serves as an enlightening system for understanding how a protein polymer composed of a single protein switches among different helical states.

Directed Polymorphism and Mechanofluorochromism of Conjugated Materials through Weak Non-Covalent Control.

Understanding and manipulating crystal polymorphism can provide novel strategies for materials discovery in organic optoelectronics. In this paper, a series of seven ester-terminated three-ring phenylene ethynylenes (PEs) exhibit structure-dependent polymorphism wherein alkyl chain length modulates the propensity to form violet or green fluorescent solid phases, as well as tunable thermal and mechanofluorochromic (MFC) transitions. These compounds harness "soft" non-covalent control to achieve polymorphism: the electronic substituent effect of the ester groups weakens the fluoroarene-arene (ArF-ArH) interactions that typically direct crystal packing of this class of compounds, increasing competitiveness of other interactions. Small structural modifications tip this balance and shift the prevalence of violet- or green-emitting polymorphs. Compounds with short alkyl chain lengths show both violet and various green fluorescent polymorphs, while the violet fluorescent form dominates with alkyl lengths longer than butyl. Further, thermally induced green-to-violet fluorescent crystal-to-crystal transitions occur for single crystals of CO2-1 and CO2-3. Finally, the PEs show reversible violet-to-green mechanofluorochromism (MFC), with temperature required for reversion of this MFC decreasing with alkyl chain length. We therefore present this design of directional but weak interactions as a strategy to access polymorphs and tunable stimuli-responsive behavior in solids.

What causes the anomalous aggregation in pluronic aqueous solutions?

Pluronic (PL) block copolymers have been widely used as delivery carriers, molecular templates for porous media, and process additives for affecting rheological behavior. Unlike most surfactant systems, where unimer transforms into micelle with increased surfactant concentration, anomalous large PL aggregates below the critical micelle concentration (CMC) were found throughout four types of PL (F108, F127, F88 and P84). We characterized their structures using dynamic light scattering and small-angle X-ray/neutron scattering. Molecular dynamics simulations suggest that the PPO segments, though weakly hydrophobic interaction (insufficient to form micelles), promote the formation of large aggregates. Addition of acid or base (e.g. citric acid, acetic acid, HCl and NaOH) in F108 solution significantly suppresses the aggregate formation for up to 20 days due to the repulsion force from the attached H3O+ molecules on the EO segment in both PEO and PL and the reduction of CMC through the salting out effect, respectively.

Polymorphism in Benzene-1,3,5-tricarboxamide Supramolecular Assemblies in Water: A Subtle Trade-off between Structure and Dynamics.

In biology, polymorphism is a well-known phenomenon by which a discrete biomacromolecule can adopt multiple specific conformations in response to its environment. The controlled incorporation of polymorphism into noncovalent aqueous assemblies of synthetic small molecules is an important step toward the development of bioinspired responsive materials. Herein, we report on a family of carboxylic acid functionalized water-soluble benzene-1,3,5-tricarboxamides (BTAs) that self-assemble in water to form one-dimensional fibers, membranes, and hollow nanotubes. Interestingly, one of the BTAs with the optimized position of the carboxylic group in the hydrophobic domain yields nanotubes that undergo reversible temperature-dependent dynamic reorganizations. SAXS and Cryo-TEM data show the formation of elongated, well-ordered nanotubes at elevated temperatures. At these temperatures, increased dynamics, as measured by hydrogen-deuterium exchange, provide enough flexibility to the system to form well-defined nanotube structures with apparently defect-free tube walls. Without this flexibility, the assemblies are frozen into a variety of structures that are very similar at the supramolecular level, but less defined at the mesoscopic level.

Reversible mechanofluorochromism of aniline-terminated phenylene ethynylenes.

Seven three-ring phenylene-ethynylene (PE) structural analogs, differing only in the lengths of alkyl chains on terminal aniline substituents, show 50-62 nm bathochromic shifts in emission maxima in response to mechanical force (mechanofluorochromism, MC). These shifts are fully reversible with heat or solvent fuming. Shearing of these solids yields a transition from green-emitting crystalline phases to orange-emitting amorphous phases as established by differential scanning calorimetry and X-ray diffraction. Molecules with shorter alkyl chain lengths required higher temperatures to recover the hypsochromically shifted crystalline phases after grinding, while the recovery with chain lengths longer than butyl occurred at room temperature. In addition to this structure-dependent thermochromism, these compounds retain their MC properties in polymer hosts to various extents. The crystalline phases of these materials have PE chromophores that are twisted due to non-covalent perfluoroarene-arene (ArF-ArH) interactions involving perfluorophenyl pendants and the terminal rings of the PE chromophore, resulting in interrupted conjugation and an absence of chromophore aggregation. The MC behavior of an analog without the perfluoroarene rings is severely attenuated. This work demonstrates the general utility of twisted PEs as stimuli-responsive moieties and reveals clear structure-property relationships regarding the effects of alkyl chain length on these materials.

Supramolecular Assembly of Comb-like Macromolecules Induced by Chemical Reactions that Modulate the Macromolecular Interactions In Situ.

Supramolecular polymerization or assembly of proteins or large macromolecular units by a homogeneous nucleation mechanism can be quite slow and require specific solution conditions. In nature, protein assembly is often regulated by molecules that modulate the electrostatic interactions of the protein subunits for various association strengths. The key to this regulation is the coupling of the assembly process with a reversible or irreversible chemical reaction that occurs within the constituent subunits. However, realizing this complex process by the rational design of synthetic molecules or macromolecules remains a challenge. Herein, we use a synthetic polypeptide-grafted comb macromolecule to demonstrate how the in situ modulation of interactions between the charged macromolecules affects their resulting supramolecular structures. The kinetics of structural formation was studied and can be described by a generalized model of nucleated polymerization containing secondary pathways. Basic thermodynamic analysis indicated the delicate role of the electrostatic interactions between the charged subunits in the reaction-induced assembly process. This approach may be applicable for assembling a variety of ionic soft matters that are amenable to chemical reactions in situ.

Modulation of polypeptide conformation through donor-acceptor transformation of side-chain hydrogen bonding ligands.

Synthetic polypeptides have received increasing attention due to their ability to form higher ordered structures similar to proteins. The control over their secondary structures, which enables dynamic conformational changes, is primarily accomplished by tuning the side-chain hydrophobic or ionic interactions. Herein we report a strategy to modulate the conformation of polypeptides utilizing donor-acceptor interactions emanating from side-chain H-bonding ligands. Specifically, 1,2,3-triazole groups, when incorporated onto polypeptide side-chains, serve as both H-bond donors and acceptors at neutral pH and disrupt the α-helical conformation. When protonated, the resulting 1,2,3-triazolium ions lose the ability to act as H-bond acceptors, and the polypeptides regain their α-helical structure. The conformational change of triazole polypeptides in response to the donor-acceptor pattern was conclusively demonstrated using both experimental-based and simulation-based methods. We further showed the utility of this transition by designing smart, cell-penetrating polymers that undergo acid-activated endosomal escape in living cells.Hydrogen bonding plays a major role in determining the tridimensional structure of biopolymers. Here, the authors show that control over a polypeptide conformation can be achieved by altering the donor-acceptor properties of side-chain triazole units via protonation-deprotonation.

Sulfoethylated nanofibrillated cellulose: Production and properties.

Sulfoethylated nanofibrillated cellulose (NFCSulf) was produced by an industrially relevant process. The properties of the NFCSulf were compared with those of carboxymethylated nanofibrillated cellulose (NFCCarb), which has been identified as an attractive NFC for several industrial applications. The investigations revealed that NFCSulf is characterized by a higher degree of fibrillation and has superior redispersion properties. Furthermore, NFCSulf displays higher stability in varying pH values as compared to NFCCarb. Hence, NFCSulf may be a more attractive alternative than NFCCarb in applications such as rheological modifiers or adsorbing components in personal care products, in which the performance of NFC must remain unaffected in varying ambient conditions. The superior properties of NFCSulf compared to NFCCarb were proposed to be due to the combination of the unique chemical characteristics of the sulfoethylated reagent, and the larger size of the sulfonate group compared to the carboxymethyl group.

Fine structures of self-assembled beta-cyclodextrin/Pluronic in dilute and dense systems: a small angle X-ray scattering study.

The evolution of the fine structures of self-assembled polypseudorotaxane (PPR) in Pluronic (PL F108) solutions containing dilute to dense beta-cyclodextrin (ß-CD) was illustrated for the first time by small angle X-ray scattering (SAXS). Dense ß-CD (∼19 w/v%) was found feasible to be dispersed in 24% citric acid solution. 5% of PL F108 formed cylindrical micelles of 1 nm in radius and 8 nm in length in the presence of 24% citric acid through the dehydration of citric acid and citrate. PPR was formed through host-guest interaction between PL F108 and ß-CD. In dilute ß-CD system (1%), the single chains of PPR with separated ß-CD stacks on PL F108 were formed. The numbers of ß-CD in each stack increased from 1 to 4 on increasing ß-CD concentration to 9%. In a dense ß-CD system, PPR condensed to correlated structures majorly composed of two unit blocks through the hydrogen bonds between PPRs. Two distinguishable correlated domains with correlation lengths of 50 nm (marked α-phase) and 46 nm (marked ß-phase) along the chains, but without fine periodic structure within each individual domain, were identified in the 10% ß-CD solution. Periodic stacking of ß-CD in the domains developed in the 12% solution. As ß-CD concentration increased from 12 to 19%, the correlated heights of α and ß phases reduced from 41 and 32 nm to 30 and 10 nm, respectively. There were 48 ß-CDs that stabilized on each PL F108 chain in the 19% ß-CD system, which is in good agreement with stoichiometry.