Helix Handedness Research Articles

Low-voltage and wide-band tuning of lasing in a dye-doped liquid-crystal sandwich structure.

This paper reports the electrical tuning of a lasing in a liquid crystal (LC) sandwich structure. A dye-doped nematic LC (NLC) layer is sandwiched between two CLC layers to act as a phase retarder with the CLC layers acting as cavity mirrors, for the selective reflection of light in the photonic band with the same sense of helix handedness as that of the CLC layers. The transmittance spectrum of the sandwich cell provides a large range of modulation due to the wavelength dependent nature of phase retardation between the optical eigenmodes in the NLC layer. Lasing occurs at wavelengths corresponding to the maximum transmittance within the reflection band of the CLC layers. The application of voltage to the NLC layer makes it possible to shift the wavelengths of maximum transmittance, thereby tuning the wavelength of lasing. In these experiments, an applied voltage of 1.25 V was sufficient to shift the lasing peak wavelength by approximately 47 nm.

Rotatable Diffraction Gratings Based on Cholesteric Liquid Crystals with Phototunable Helix Pitch

A novel type of rotatable diffraction volume gratings based on cholesteric liquid crystals with phototunable helix pitch is elaborated. For this purpose the cholesteric liquid crystalline mixture with high helix pitch tunability and reversible photoinduced helix handedness inversion containing azobenzene‐derivative of the chiral‐photochromic dopant is developed. It was shown that the prepared cholesteric mixture under the hybrid (homeotropic – planar) boundary conditions spontaneously forms a well‐oriented striped domain texture which acts as a phase grating. Under the effect of light the changes of helix pitch take place due to the molecular photoreaction of chiral‐photochromic dopant causing the variation of the period of striped domain structure and its rotation. The clockwise and counterclockwise rotation of the obtained gratings can be achieved by switching between UV and visible light, correspondingly. The maximum angle of the continuous rotation is ≈690°, while the total rotation caused by UV light illumination reaches ≈1220°. The creation of a variety of complex‐organized structures by local light exposure of the cholesteric domain texture is demonstrated. The observed advanced photooptical effects have potentially wide and important applications from scientific and applied points of view.

In situ helicity inversion of self-assembled nano-helices.

The handedness of nanometrical helices based on surfactant assemblies was inverted when these helices were in contact with an excess solution of chiral anions with opposite enantiomers. An important difference in the kinetics of chirality inversion at the molecular level and mesoscopic level was observed.

Formation of Helical Phases in Achiral Block Copolymers by Simple Addition of Small Chiral Additives

Helical superstructures were induced in poly(ethylene oxide)-b-poly(tert-butyl acrylate) (PEO-b-PtBA) achiral diblock copolymers (BCPs) through the simple addition of pure enantiomers of tartaric acid. Hydrogen bond interactions between tartaric acid and poly(ethylene oxide) (PEO) block not only enhance the phase segregation strength of the PEO-based block copolymer but also transfer the chiral information from the additive into the achiral backbone to induce the conformational chirality. The helical phase was formed after thermal annealing with a pitch of ∼25 nm and confirmed by transmission electron microscopy (TEM) and TEM tomography. The handedness of helices can be easily selected by choice of the corresponding enantioisomer of tartaric acid.

Controlling Helix Handedness in Water‐Soluble Quinoline Oligoamide Foldamers

AbstractFor biological applications, the control of the helix handedness of water‐soluble quinoline‐based oligoamide foldamers has been investigated by the installation of chiral end groups at either the C or N terminus. This has resulted in the development of monomer units capable of unequivocally inducing helical sense without impacting the aqueous solubility. Furthermore, we showed that very slow helix handedness inversion in water can be exploited. The incorporation of a chiral moiety with no handedness‐induction properties allows the chromatographic separation of P and M helices as diastereoisomers with kinetically locked handedness.

Left or right cholesterics? A matter of helix handedness and curliness.

Using an Onsager-like theory, we have investigated the relationship between the morphology of hard helical particles and the features (pitch and handedness) of the cholesteric phase that they form. We show that right-handed helices can assemble into right- (R) and left-handed (L) cholesterics, depending on their curliness, and that the cholesteric pitch is a non-monotonic function of the intrinsic pitch of particles. The theory leads to the definition of a hierarchy of pseudoscalars, which quantify the difference in the average excluded volume between pair configurations of helices having (R) and (L)-skewed axes. The predictions of the Onsager-like theory are supported by Monte Carlo simulations of the isotropic phase of hard helices, showing how the cholesteric organization, which develops on scales longer than hundreds of molecular sizes, is encoded in the short-range chiral correlations between the helical axes.

Helix foldamers of γ-peptides based on 2-aminocyclohexylacetic acid: a computational study

The conformational preferences of helix foldamers have been studied for oligo-γ-peptides of 2-aminocyclohexylacetic acid (γAc6a) with a cyclohexyl constraint on the Cβ–Cγ bond using density functional methods. For the di-γAc6a peptide with the homochiral (1S,2S) configurations, the right-handed (P) 9-helical structure is found to be the most preferred in the gas phase, whereas the (P) 14-helical structure is favored by ∼0.2 kcal mol−1 over the former in water. As the peptide sequence becomes longer, the (P)-2.514-helices are the most preferred for tetra- to octa-γAc6a peptides both in the gas phase and in water, which is due to the favored conformational energies. The calculated mean backbone torsion angles and helical parameters of the 14-helix foldamers of oligo-γAc6a peptides are consistent with those of the X-ray structures of Cα-ethyl-γAc6a peptides. Although the 14-helix foldamers of γAc6a and 2-(aminomethyl)cyclohexanecarboxylic acid (γAmc6) oligopeptides have similar mean backbone torsion angles and helical parameters, there are some differences in structure and/or helix handedness for other helix foldamers, which can be ascribed to the different substitutions of the cyclohexyl constraint on Cβ–Cγ or Cα–Cβ bonds. Therefore, the conformational preferences of the oligo-γAc6a peptides obtained here are expected to provide useful information for structure-based designs of biologically active γ-peptides with specific functions.

Structure of a Complex Formed by a Protein and a Helical Aromatic Oligoamide Foldamer at 2.1 Å Resolution

AbstractIn the search of molecules that could recognize sizeable areas of protein surfaces, a series of ten helical aromatic oligoamide foldamers was synthesized on solid phase. The foldamers comprise three to five monomers carrying various proteinogenic side chains, and exist as racemic mixtures of interconverting right‐handed and left‐handed helices. Functionalization of the foldamers by a nanomolar ligand of human carbonic anhydrase II (HCA) ensured that they would be held in close proximity to the protein surface. Foldamer–protein interactions were screened by circular dichroism (CD). One foldamer displayed intense CD bands indicating that a preferred helix handedness is induced upon interacting with the protein surface. The crystal structure of the complex between this foldamer and HCA could be resolved at 2.1 Å resolution and revealed a number of unanticipated protein–foldamer, foldamer–foldamer, and protein–protein interactions.

Effect of the amyloid β hairpin’s structure on the handedness of helices formed by its aggregates

Various structural models for amyloid β fibrils have been derived from a variety of experimental techniques. However, these models cannot differentiate between the relative position of the two arms of the β hairpin called the stagger. Amyloid fibrils of various hierarchical levels form left-handed helices composed of β sheets. However it is unclear if positive, negative and zero staggers all form the macroscopic left-handed helices. To address this issue we have conducted extensive molecular dynamics simulations of amyloid β sheets of various staggers and shown that only negative staggers lead to the experimentally observed left-handed helices while positive staggers generate the incorrect right-handed helices. This result suggests that the negative staggers are physiologically relevant structure of the amyloid β fibrils.

Chiral properties of structure and magnetism in Mn(1-x)Fe(x)Ge compounds: when the left and the right are fighting, who wins?

Magnetic susceptibility measurements have shown that the compounds Mn(1-x)Fe(x)Ge are magnetically ordered through the whole range of concentrations x = [0.0,1.0]. Small-angle neutron scattering reveals the helical nature of the spin structure with a wave vector, which changes from its maximum (|k| = 2.3 nm(-1)) for pure MnGe, through its minimum (|k| → 0) at x(c) ≈ 0.75, to the value of |k| = 0.09 nm(-1) for pure FeGe. The macroscopic magnetic measurements confirm the ferromagnetic nature of the compound with x = x(c). The observed transformation of the helix structure to the ferromagnet at x = x(c) is explained by different signs of chirality for the compounds with x > x(c) and x<x(c). We used x-ray diffraction and polarized neutron scattering to evaluate the crystallographic chirality Γ(c) and the magnetic chirality γ(m) of the FeGe single crystals. Similar to previous observations for FeSi-based compounds, FeGe demonstrates left- (right-)handed crystalline chirality acompained by right (left) handedness of the magnetic helix (Γ(c) γ(m) = -1). At variance, MnSi related compounds show the opposite behavior (Γ(c)γ(m) = 1). Since the magnetic chirality γ(m) relates to the sign of the Dzyaloshinskii-Moriya interaction (DMI), for the same geometrical arrangement (Γ(c)) the sign of DMI can be set by the proper choice of the transition metal.

Porphyrin–Peptoid Conjugates: Face-to-Face Display of Porphyrins on Peptoid Helices

Distance, orientation, and number controlled porphyrin-peptoid conjugates (PPCs) were efficiently synthesized. Cofacial (1, 2, and 4), slipped-cofacial (3), and unstructured (5) arrangements of porphyrins provided distinct optical and electronic properties characterized by UV-vis and circular dichroism spectroscopy. In addition, ECCD spectra confirmed the handedness of peptoid helices.

Mean-Field Interactions between Nucleic-Acid-Base Dipoles can Drive the Formation of a Double Helix

A proposed coarse-grained model of nucleic acids demonstrates that average interactions between base dipoles, together with chain connectivity and excluded-volume interactions, are sufficient to form double-helical structures of DNA and RNA molecules. Additionally, local interactions determine helix handedness and direction of strand packing. This result, and earlier research on reduced protein models, suggests that mean-field multipole-multipole interactions are the principal factors responsible for the formation of regular structure of biomolecules.

A left-handed building block self-assembles into right- and left-handed helices

Self-assembly, the process by which building blocks spontaneously form an ordered structure, has become a key concept in soft matter science. In self-assembly, the physical properties of the building blocks determine the structure of the assembly. For example, the pitch and handedness of a self-assembled helix depend on the interactions between its constituent particles. There are many nanoscale building blocks of a single handedness that self-assemble into both left- and right-handed structures, but there is currently no model system that explains this ambiguous relationship between building block chirality and assembly chirality. Here we report the discovery of a family of left-handed building blocks that self-assemble into helices whose handedness can be tuned by a single building block parameter. In simulations, pairs of Paramonov–Yaliraki ellipsoids rigidly bound in a biphenyl-like arrangement self-assemble into helices. Adjusting the dihedral angle between the ellipsoids without altering the building block handedness was sufficient to produce both right- and left-handed helices. This model system should provide design insights for supramolecular assemblies of nanoscale structures with adjustable internal dihedral angles.

A novel equilibrium relating to the helix handedness in G-quadruplexes formed by heterochiral oligonucleotides with an inversion of polarity site

Investigations of heterochiral oligodeoxynucleotides 5'-TD1GD2GD3-3'-3'-GL3GL2TL1-5' (L33) and 3'-TD1GD2GD3-5'-5'-GL3GL2TL1-3' (L55) forming quadruplex structures are reported. Data indicate the presence of enantiomeric left- and right-handed quadruplex helices. In the case of L55, NMR experiments point to an unusual equilibrium between them.

Identification of a Foldaxane Kinetic Byproduct during Guest-Induced Single to Double Helix Conversion

An aromatic oligoamide sequence was designed and synthesized to fold in a single helix having a large cavity and to behave as a host for a dumbbell-shaped guest derived from tartaric acid. NMR, molecular modeling, and circular dichroism (CD) evidence demonstrated the rapid formation of this 1:1 host-guest complex and induction of the helix handedness of the host by the guest. This complex was found to be a long-lived kinetic supramolecular byproduct, as it slowly transformed into a 2:2 host-guest complex with two guest molecules bound at the extremities of a double helix formed by the host, as shown by NMR and CD spectroscopy and a solid-state structure. The guest also induced the handedness of the double helical host, but with an opposite bias. The chiroptical properties of the system were thus found to revert with time as the 1:1 complex formed first, followed by the 2:2 complex.

Spin and gravity give a helping hand

The handedness of supramolecular helices formed from achiral monomers has been controlled by applying rotational and gravitational forces, but at the start of the assembly process only. This demonstrates that a falsely chiral influence is able to induce absolute enantioselection.

Helicity controlled by the chirality of amino acid: two novel enantiopure chiral 3D architectures containing fivefold interwoven helices

Two novel three-dimensional (3D) chiral coordination polymers, namely [Zn4(btc)2(Hbtc)(L-His)2(H2O)4]·1.5H2O (L-1) and [Zn4(btc)2(Hbtc)(D-His)2(H2O)4]·1.5H2O (D-1), have been assembled from zinc salt with mixed ligands 1,3,5-benzenetricarboxylic acid (H3btc) and chiral histidine (His). Their structures were determined by single-crystal X-ray diffraction analyses, elemental analyses, infrared (IR) spectra, X-ray powder diffraction, circular dichroism (CD) spectra, and thermogravimetric (TG) analysis. Compounds L-1 and D-1 are enantiomers and exhibit unique 3D chiral frameworks containing fivefold interwoven helices, which represent the first examples of chiral metal–amino acid complexes containing n-fold interwoven helices. Furthermore, the helices in L-1 are left-handed, while those in D-1 are right-handed, which indicate that the handedness of helices can be controlled by the chirality of His bound to the metal. More wonderfully, the 3D framework can be simplified to a (3,4)-connected chiral net with an unprecedented (4·92)(4·94·11) topology. Moreover, the luminescent properties of compound L-1 are discussed.

Optically Active Helical Substituted Polyacetylenes Showing Reversible Helix Inversion in Emulsion and Solution State

This Communication reports two substituted polyacetylenes that can adopt helical structures of preferential screw sense in both emulsion (nanoparticle) and solution; however, the handedness of the macromolecular helices is just opposite in the two states. More interestingly, the helical screw sense of the polymers demonstrated a reversible transition between the two states. The unprecedented findings are of significant importance for acquiring new insights into helical polymers and for developing novel advanced chiral materials.

Handedness of two-fold helices and chiral space groups

Handedness of two-fold helices and chiral space groups

Coupled linear and rotary motion in supramolecular helix handedness inversion

We report on a remarkable pathway for the inversion of handedness in a model supramolecular helix that exhibits repeated extension and contraction coupled with correlated rotation. The pathway involves a boundary between two segments of opposite handedness, which propagates along the helix through a hopping mechanism, causing periodic opening and closing of a gap between the two segments, while they rotate cooperatively in opposite directions. While the hopping mechanism is likely to have wider implications for reversal of helix handedness in general, the existence of such a pathway suggests the possibility of designing a novel class of nanoscale machines exploiting helix handedness inversion.