Left-handed Twist Research Articles

Programming liquid crystal elastomers for multistep ambidirectional deformability.

Ambidirectionality, which is the ability of structural elements to move beyond a reference state in two opposite directions, is common in nature. However, conventional soft materials are typically limited to a single, unidirectional deformation unless complex hybrid constructs are used. We exploited the combination of mesogen self-assembly, polymer chain elasticity, and polymerization-induced stress to design liquid crystalline elastomers that exhibit two mesophases: chevron smectic C (cSmC) and smectic A (SmA). Inducing the cSmC-SmA-isotropic phase transition led to an unusual inversion of the strain field in the microstructure, resulting in opposite deformation modes (e.g., consecutive shrinkage or expansion and right-handed or left-handed twisting and tilting in opposite directions) and high-frequency nonmonotonic oscillations. This ambidirectional movement is scalable and can be used to generate Gaussian transformations at the macroscale.

Torques within and outside the human spindle balance twist at anaphase.

At each cell division, nanometer-scale motors and microtubules give rise to the micron-scale spindle. Many mitotic motors step helically around microtubules in vitro, and most are predicted to twist the spindle in a left-handed direction. However, the human spindle exhibits only slight global twist, raising the question of how these molecular torques are balanced. Here, we find that anaphase spindles in the epithelial cell line MCF10A have a high baseline twist, and we identify factors that both increase and decrease this twist. The midzone motors KIF4A and MKLP1 are together required for left-handed twist at anaphase, and we show that KIF4A generates left-handed torque in vitro. The actin cytoskeleton also contributes to left-handed twist, but dynein and its cortical recruitment factor LGN counteract it. Together, our work demonstrates that force generators regulate twist in opposite directions from both within and outside the spindle, preventing strong spindle twist during chromosome segregation.

Temperature-Induced Sign Inversion of Circularly Polarized Luminescence of Binaphthyl-Bridged Tetrathiapyrenophanes.

D2-symmetric (R)-binaphthyl-bridged pyrenophanes containing thioether bonds were synthesized. The pyrenophanes exhibited the temperature-induced sign inversion of circularly polarized luminescence (CPL) while maintaining the emission wavelength and reversibility. The Δglum value reached 0.02, and the FL quenching by heat was negligible. The sign inversion of CPL originates from the inversion of intramolecular excimer chirality associated with excitation dynamics. The two pyrenes form a kinetically trapped left-handed twist excimer at low temperatures, while they form a thermodynamically favored right-handed twist excimer at high temperatures. The thioether linkers can impart flexibility suitable for the inversion of chirality of the excimers.

Raman Optical Activity of Retinal Chromophore in Sensory Rhodopsin II.

Raman optical activity (ROA) spectroscopy was used to study the conformation of the retinal chromophore in sensory rhodopsin II (SRII), which is a blue-green light sensor of microbes. The ROA spectrum consisted of the negative vibrational bands of the chromophore, whose relative intensities are similar to those of the parent Raman spectrum. This spectral feature was explained by the left-handed helical twist of the retinal chromophore on the basis of quantum chemical calculations. On the other hand, we found that the chromophore conformation based on the crystal structures of SRII has a right-handed helical twist, which does not agree with the observation. This specific result suggests that the consistency with chiro-optical properties can be a key criterion for the accurate prediction and/or evaluation of chromophore conformation in retinal-binding proteins.

Cellulose assembles into helical bundles of uniform handedness in cell walls with abnormal pectin composition.

Plant cells and organs grow into a remarkable diversity of shapes, as directed by cell walls composed primarily of polysaccharides such as cellulose and multiple structurally distinct pectins. The properties of the cell wall that allow for precise control of morphogenesis are distinct from those of the individual polysaccharide components. For example, cellulose, the primary determinant of cell morphology, is a chiral macromolecule that can self-assemble in vitro into larger-scale structures of consistent chirality, and yet most plant cells do not display consistent chirality in their growth. One interesting exception is the Arabidopsis thaliana rhm1 mutant, which has decreased levels of the pectin rhamnogalacturonan-I and causes conical petal epidermal cells to grow with a left-handed helical twist. Here, we show that in rhm1 the cellulose is bundled into large macrofibrils, unlike the evenly distributed microfibrils of the wild type. This cellulose bundling becomes increasingly severe over time, consistent with cellulose being synthesized normally and then self-associating into macrofibrils. We also show that in the wild type, cellulose is oriented transversely, whereas in rhm1 mutants, the cellulose forms right-handed helices that can account for the helical morphology of the petal cells. Our results indicate that when the composition of pectin is altered, cellulose can form cellular-scale chiral structures in vivo, analogous to the helicoids formed in vitro by cellulose nano-crystals. We propose that an important emergent property of the interplay between rhamnogalacturonan-I and cellulose is to permit the assembly of nonbundled cellulose structures, providing plants flexibility to orient cellulose and direct morphogenesis.

Defect loops in three-dimensional active nematics as active multipoles.

We develop a description of defect loops in three-dimensional active nematics based on a multipole expansion of the far-field director and show how this leads to a self-dynamics dependent on the loop's geometric type. The dipole term leads to active stresses that generate a global self-propulsion for splay and bend loops. The quadrupole moment is nonzero only for nonplanar loops and generates a net "active torque," such that defect loops are both self-motile and self-orienting. Our analysis identifies right- and left-handed twist loops as the only force- and torque-free geometries, suggesting a mechanism for generating an excess of twist loops. Finally, we determine the Stokesian flows created by defect loops and describe qualitatively their hydrodynamics.

Binaphthyl-Bridged Pyrenophanes: Intense Circularly Polarized Luminescence Based on a D2 Symmetry Strategy.

A series of D2 -symmetric macrocycles composed of alternately linked pyrene and binaphthyl moieties (binaphthyl-bridged pyrenophanes) have been synthesized. Among them, a pyrenophane possessing ether linkers at the 2,7-positions of the pyrenes exhibited intense circularly polarized luminescence (CPL) with a |glum | value of 0.053. This value is by far the highest for excimers and was not sensitive to temperature, solvent, or concentration. The CPL originated from a twisting pyrene excimer, with the (R)-binaphthyl moieties producing a left-handed twist excimer, which exhibited (-)-CPL. The electric and magnetic transition dipole moments are perfectly parallel, which is the best relationship for strong CPL.

The chirality of the mitotic spindle provides a mechanical response to forces and depends on microtubule motors and augmin

SummaryForces produced by motor proteins and microtubule dynamics within the mitotic spindle are crucial for proper chromosome segregation. In addition to linear forces, rotational forces or torques are present in the spindle, which are reflected in the left-handed twisted shapes of microtubule bundles that make the spindle chiral. However, the biological role and molecular origins of spindle chirality are unknown. By developing methods for measuring the spindle twist, we show that spindles are most chiral near the metaphase-to-anaphase transition. To assess the role of chirality in spindle mechanics, we compressed the spindles along their axis. This resulted in a stronger left-handed twist, suggesting that the twisted shape allows for a mechanical response to forces. Inhibition or depletion of motor proteins that perform chiral stepping, Eg5/kinesin-5, Kif18A/kinesin-8, MKLP1/kinesin-6, and dynein, decreased the left-handed twist or led to right-handed twist, implying that these motors regulate the twist by rotating microtubules within their antiparallel overlaps or at the spindle pole. A right-handed twist was also observed after the depletion of the microtubule nucleator augmin, indicating its contribution to the twist through the nucleation of antiparallel bridging microtubules. The uncovered switch from left-handed to right-handed twist reveals the existence of competing mechanisms that promote twisting in opposite directions. As round spindles are more twisted than the elongated ones are, we infer that bending and twisting moments are generated by similar molecular mechanisms and propose a physiological role for spindle chirality in allowing the spindle to absorb mechanical load.Video abstract

Induction of compression wood inhibits development of spiral grain in radiata pine

Summary Spiral grain refers to the helical patterns formed by the wood grain in the trunks of many tree species. In most gymnosperms, grain near the pith is vertical but wood formed after several years of growth has a slight to pronounced left-handed twist. Grain changes presumably involve the slow rotation of cells within the vascular cambium, but the mechanisms that allow this reorientation to occur remain unclear. Understanding this process is, however, important as the presence of strong spiral grain within the corewood of gymnosperms is a major wood quality issue devaluing cut timber. In this study, we measured wood grain in stems of Pinus radiata (radiata pine) saplings through reconstructions of resin canals that follow the grain, visualised by serial sectioning and scanning with circularly polarised light, and through X-ray computed microtomography (μCT) and image analysis in ImageJ. Vertical trees retained a symmetrical grain pattern that was weakly right-handed near the pith, but which became progressively more left-handed during the first eight months of growth. In tilted trees, however, the development of left-handed grain was inhibited by the formation of compression wood on the lower side of the tree whereas the wood on the upper side of the tree developed increasingly more left-handed grain as in the vertical controls. These results demonstrate that a previously unidentified link exists between compression wood formation and the inhibition of grain development.

Opposing motors provide mechanical and functional robustness in the human spindle.

At each cell division, the spindle self-organizes from microtubules and motors. In human spindles, the motors dynein and Eg5 generate contractile and extensile stress, respectively. Inhibiting dynein or its targeting factor NuMA leads to unfocused, turbulent spindles, and inhibiting Eg5 leads to monopoles; yet, bipolar spindles form when both are inhibited together. What, then, are the roles of these opposing motors? Here, we generate NuMA/dynein- and Eg5-doubly inhibited spindles that not only attain a typical metaphase shape and size but also undergo anaphase. However, these spindles have reduced microtubule dynamics and are mechanically fragile, fracturing under force. Furthermore, they exhibit lagging chromosomes and a dramatic left-handed twist at anaphase. Thus, although these opposing motors are not required for spindle shape, they are essential to its mechanical and functional robustness. This work suggests a design principle whereby opposing active stresses provide robustness to force-generating cellular structures.

A Comprehensive Landscape for Fibril Association Behaviors Encoded Synergistically by Saccharides and Peptides.

Nature provides us a panorama of fibrils with tremendous structural polymorphism from molecular building blocks to hierarchical association behaviors. Despite recent achievements in creating artificial systems with individual building blocks through self-assembly, molecularly encoding the relationship from model building blocks to fibril association, resulting in controlled macroscopic properties, has remained an elusive goal. In this paper, by employing a designed set of glycopeptide building blocks and combining experimental and computational tools, we report a library of controlled fibril polymorphism with elucidation from molecular packing to fibril association and the related macroscopic properties. The growth of the fibril either axially or radially with right- or left-handed twisting is determined by the subtle trade-off of oligosaccharide and oligopeptide components. Meanwhile, visible evidence for the association process of double-strand fibrils has been experimentally and theoretically proposed. Finally the fibril polymorphs demonstrated significant different macroscopic properties on hydrogel formation and cellular migration control.

Supramolecular Copolymerization of Bichromophoric Chiral and Achiral Perylenediimide Dyes

Propagation and amplification of chirality are considered to play an important role in the chemical evolution of biological homochirality. Stereochemical communications have been demonstrated to have a significant effect on the formation of chiral hierarchical structures in helical polymers, surface assemblies and supramolecular polymers. The formation of supramolecular copolymers based on chiral and achiral bichromophoric perylenediimide (PDI) dyes having a binaphtyl- and biphenyl-core-bridging unit, respectively, was investigated in terms of chiral amplification and propagation. The biphenyl-bridged PDI dye was expected to perform as a prochiral component to adopt both right- and left-handed twisting structures with the free rotation over the phenyl-phenyl linkage upon partnered with the chiral binaphtly PDI dye in the coassemblies. The coassemblies between the chiral and achiral PDI dyes with dissimilar core units demonstrated the composition dependent control in the length of supramolecular nanofibers as well as amplification of optical activity.

Zn(II)-Coordination Polymers with a Right- and Left-Handed Twist: Multifunctional Metal–Organic Hybrid for Dye Adsorption and Drug Delivery

A relatively less explored bis-pyridyl-bis-amide ligand (L1) having a diphenylmethane backbone was exploited to generate a new series of coordination polymers as potential multifunctional materials...

Metal ions confinement defines the architecture of G-quartet, G-quadruplex fibrils and their assembly into nematic tactoids

G-quadruplex, assembled from a square array of guanine (G) molecules, is an important structure with crucial biological roles in vivo but also a versatile template for ordered functional materials. Although the understanding of G-quadruplex structures is the focus of numerous studies, little is known regarding the control of G-quartet stacking modes and the spontaneous orientation of G-quadruplex fibrils. Here, the effects of different metal ions and their concentrations on stacking modes of G-quartets are elucidated. Monovalent cations (typically K+) facilitate the formation of G-quadruplex hydrogels with both heteropolar and homopolar stacking modes, showing weak mechanical strength. In contrast, divalent metal ions (Ca2+, Sr2+, and Ba2+) at given concentrations can control G-quartet stacking modes and increase the mechanical rigidity of the resulting hydrogels through ionic bridge effects between divalent ions and borate. We show that for Ca2+ and Ba2+ at suitable concentrations, the assembly of G-quadruplexes results in the establishment of a mesoscopic chirality of the fibrils with a regular left-handed twist. Finally, we report the discovery of nematic tactoids self-assembled from G-quadruplex fibrils characterized by homeotropic fibril alignment with respect to the interface. We use the Frank-Oseen elastic energy and the Rapini-Papoular anisotropic surface energy to rationalize two different configurations of the tactoids. These results deepen our understanding of G-quadruplex structures and G-quadruplex fibrils, paving the way for their use in self-assembly and biomaterials.

Taylor State Merging at SSX: Experiment and Simulation

We describe experiments and simulations of dynamical merging with two Taylor state plasmas in a Swarthmore Spheromak Experiment (SSX) device. Taylor states are formed by magnetized plasma guns at opposite ends of the device. We performed experiments with Taylor states of both senses of magnetic helicity (right-handed twist or left-handed twist). We present results of both counter-helicity merging (one side left-handed, the other right-handed) and co-helicity merging (both sides left-handed). Experiments show significant ion heating, consistent with magnetic reconnection. We suggest that the merged, warm state could be a suitable target for future magneto-inertial fusion experiments. Magnetohydrodynamic simulations of these experiments reveal the structure of the final relaxed, merged state.

Special classes of optical vector vortex beams are Majorana-like photons

Majorana-like photons are introduced in this paper, which are attributed to the polarization and wavefront of special function class of optical vector vortex beams. A Majorana photon is a photon that is identical to its anti-photon. It has within itself both chirality, right and left-handed twist in polarization and wavefront. A theory is presented which reveals that certain types of cylindrical vector vortex photons that are spin–orbit coupled beams – radial, and azimuthal Laguerre-Gaussian, hybrid π-vector beams, and Airy beams – are Majorana-like based on their SAM (polarization) and OAM (wavefront) modes Majorana-like vector photons may play an important role in free-space fiber communication, propagation, quantum computing, optical computing, and imaging in turbid and bio-media as non-separable entangled polarized photons— a quantum mechanical entity.

Chirally Twisted Ultrathin Polydopamine Nanoribbons: Synthesis and Spontaneous Assembly of Silver Nanoparticles on Them.

Polydopamine (PDA) is a synthetic polymeric material with immense potential in biomedical and surface functionalization applications. Herein, we have screened self-assemblies formed by Phenylalanine-based amphiphiles (Phe-AMPs) as soft templates for preparing chiral PDA nanostructures. Our study revealed that the amphiphile 2 endowed with a primary amine residue afforded chirally-twisted ultrathin nanoribbons of PDA under optimized conditions. The chirality at the Phe residue of 2 modulated the twist-chirality of the PDA nanoribbons; the l-2 resulted in nanoribbons with right-handed twist, whereas the d-2 induced a left-handed twist to the ribbons. The racemic mixture of these two amphiphiles produced flat, achiral tapes. The PDA ribbon thickness was ≈5.86±0.40 nm, whereas its width and length were ≈133.5±3.2 nm and >5000 nm, respectively. Upon dialysis, hollow PDA nanotubes were obtained due to curling of the PDA nanoribbons. These PDA-nanoarchitectures were employed to spontaneously form and assemble Ag-nanoparticles along the edges of the PDA nanoribbons. In this work we are reporting chirality controlled synthesis of PDA nanostructures for the first time.

Structures of major pilins in Clostridium perfringens demonstrate dynamic conformational change.

Pili in Gram-positive bacteria are flexible rod proteins associated with the bacterial cell surface, and they play important roles in the initial adhesion to host tissues and colonization. The pilus shaft is formed by the covalent polymerization of major pilins, catalyzed by sortases, a family of cysteine transpeptidases. Here, X-ray structures of the major pilins from Clostridium perfringens strains 13 and SM101 and of sortase from strain SM101 are presented with biochemical analysis to detect the formation of pili in vivo. The major pilin from strain 13 adopts an elongated structure to form noncovalently linked polymeric chains in the crystal, yielding a practical model of the pilus fiber structure. The major pilin from strain SM101 adopts a novel bent structure and associates to form a left-handed twist like an antiparallel double helix in the crystal, which is likely to promote bacterial cell-cell interactions. A modeling study showed that pilin with a bent structure interacts favorably with sortase. The major pilin from strain SM101 was considered to be in an equilibrium state between an elongated and a bent structure through dynamic conformational change, which may be involved in pili-mediated colonization and sortase-mediated polymerization of pili.

Liquid Crystal Templates Combined with Photolithography Enable Synthesis of Chiral Twisted Polymeric Microparticles.

Liquid crystals (LC), when combined with photolithography, enable synthesis of microparticles with 2D and 3D shapes and internal complexities. Films of nematic LCs are prepared using mixtures of reactive (RM257) and non-reactive mesogens with controlled alignment of LCs at the confining surfaces, photo-polymerized the RM257 using a photomask, and then extracted the unreacted mesogens to yield the polymeric microparticles. The extraction results in a controlled anisotropic shrinkage amount dependent on the RM257 content and the direction dependent on LC alignment. Control over the aspect ratio, size, and thickness of the microparticles are obtained with a coefficient of variance less than 2%. In addition, non-parallel LC anchoring at the two surfaces results in a controllable right- or left-handed twisting of microparticles. These methods may find substantial use in applications including drug delivery, emulsions, separations, and sensors, besides their potential in revealing new fundamental concepts in self-assembly and colloidal interactions.

A Minimal Sequence for Left‐Handed G‐Quadruplex Formation

Recently, we observed the first example of a left-handed G-quadruplex structure formed by natural DNA, named Z-G4. We analysed the Z-G4 structure and inspected its primary 28-nt sequence in order to identify motifs that convey the unique left-handed twist. Using circular dichroism spectroscopy, NMR spectroscopy, and X-ray crystallography, we revealed a minimal sequence motif of 12 nt (GTGGTGGTGGTG) for formation of the left-handed DNA G-quadruplex, which is found to be highly abundant in the human genome. A systematic analysis of thymine loop mutations revealed a moderate sequence tolerance, which would further broaden the space of sequences prone to left-handed G-quadruplex formation.