Macroscopic Helix Research Articles

Macroscopic Helical Assembly of One-Dimensional Coordination Polymers: Helicity Inversion Triggered by Solvent Isomerism.

Assembling macroscopic helices with controllable chirality and understanding their formation mechanism are highly desirable but challenging tasks for artificial systems, especially coordination polymers. Here, we utilize solvents as an effective tool to induce the formation of macroscopic helices of chiral coordination polymers (CPs) and manipulate their helical sense. We chose the Ni/R-,S-BrpempH2 system with a one-dimensional tubular structure, where R-,S-BrpempH2 stands for R-,S-(1-(4-bromophenyl)ethylaminomethylphosphonic acid). The morphology of the self-assemblies can be controlled by varying the cosolvent in water, resulting in the formation of twisted ribbons of R-,S-Ni(Brpemp)(H2O)·H2O (R-,S-2T) in pure H2O; needle-like crystals of R-,S-Ni(Brpemp)(H2O)2·1/3CH3CN (R-,S-1C) in 20 vol % CH3CN/H2O; nanofibers of R-,S-Ni(Brpemp)(H2O)·H2O (R-,S-3F) in 20-40 vol % methanol/H2O or ethanol/H2O; and superhelices of R-,S-Ni(Brpemp)(H2O)·H2O (R-,S-4H or 5H) in 40 vol % propanol/H2O. Interestingly, the helicity of the superhelix can be controlled by using a propanol isomer in water. For the Ni/R-BrpempH2 system, a left-handed superhelix of R-4H(M) was obtained in 40 vol % NPA/H2O, while a right-handed superhelix of R-5H(P) was isolated in 40 vol % IPA/H2O. These results were rationalized by theoretical calculations. Adsorption studies revealed the chiral recognition behavior of these compounds. This work may contribute to the development of chiral CPs with a macroscopic helical morphology and interesting functionalities.

Twisting short dsDNA with applied tension

The twisting deformation of mechanically stretched DNA molecules is studied by a coarse grained Hamiltonian model incorporating the fundamental interactions that stabilize the double helix and accounting for the radial and angular base pair fluctuations. The latter are all the more important at short length scales in which DNA fragments maintain an intrinsic flexibility. The presented computational method simulates a broad ensemble of possible molecule conformations characterized by a specific average twist and determines the energetically most convenient helical twist by free energy minimization. As this is done for any external load, the method yields the characteristic twist-stretch profile of the molecule and also computes the changes in the macroscopic helix parameters i.e. average diameter and rise distance. It is predicted that short molecules under stretching should first over-twist and then untwist by increasing the external load. Moreover, applying a constant load and simulating a torsional strain which over-twists the helix, it is found that the average helix diameter shrinks while the molecule elongates, in agreement with the experimental trend observed in kilo-base long sequences. The quantitative relation between percent relative elongation and superhelical density at fixed load is derived. The proposed theoretical model and computational method offer a general approach to characterize specific DNA fragments and predict their macroscopic elastic response as a function of the effective potential parameters of the mesoscopic Hamiltonian.

Heat-Driven Rotation in Cholesteric Droplets with a Double Twisted Structure

In an isotropic–cholesteric coexistence system, a single-helix structure is formed in the cholesteric droplets, and when a temperature gradient is applied, unidirectional rotations are induced in these droplets. However, in a previous work, we showed that a double twisted structure was also formed in the droplets by changing droplet size or chirality. In this paper, we find that unidirectional rotations are also induced by applying a temperature gradient to droplets with a double twisted structure. Here, however, the rotational behavior is strongly dependent on the relationship between the direction of the helical axis and the temperature gradient. Unidirectional rotation is induced when one of the helical axes is parallel to the gradient, whereas no rotation is found when all of the axes are perpendicular to the temperature gradient. These results suggest that the macroscopic helix plays a significant role in the heat-driven rotational dynamics of cholesteric droplets.

Lehmann effect in a compensated cholesteric liquid crystal: Experimental evidence with fixed and gliding boundary conditions

In a recent letter (Europhys. Lett. 80, 26001 (2007)), we have shown that a compensated cholesteric liquid crystal (in which the macroscopic helix completely unwinds) may be subjected to a thermomechanical torque (the so-called Lehmann effect), in agreement with previous findings of Eber and Jánossy (Mol. Cryst. Liq. Cryst. Lett. 72, 233 (1982)). These results prove that one must take into account the chirality of the molecules and the absence of inversion symmetry at the macroscopic scale when deriving the constitutive equations of the phase at the compensation temperature. In this paper, we present the details of our experimental work and a new experiment performed in a sample treated for planar gliding anchoring. The latter experiment, coupled with a numerical simulation, supports the existence of a thermomechanical coupling in a compensated cholesteric.

Macroscopic DNA Helices

Presently there is great interest in the construction of nanostructures from DNA fragments. Here we report the preparation of much larger helical textures with different shapes from pure DNA fragments. We have observed them while preparing crystals of the dodecamer d(AAAAAATTTTTT), which only contains adenine and thymine. Noncoding regions of the genome are always rich in these two bases. We have found a strong influence of ions either monovalent or divalent, with which different crystalline structures are found. All of them contain long helical stacks of duplexes in the unit cell. The most remarkable structures are macroscopic helices with diameters and pitch in the range of 20-40 microm. Thus, pure DNA oligonucleotides may form a hierarchy of helical structures, going from the B-form double helix (pitch, p = 33 A) to helical stacks of duplexes (p approximately 900 A), and to macroscopic helices (p approximately 300,000 A). These different levels of organization are reminiscent of the different levels of organization of DNA in eukaryotic chromosomes.

Stereospecific Morphogenesis of Aspartic Acid Helical Crystals through Molecular Recognition

Helical morphologies were generated from aspartic acid (Asp) crystals in agar gel matrix. The morphogenesis stereospecifically proceeded in the helical crystal growth: D- and L-Asp provided left- and right-handed structures, respectively. The backbone of the helical morphology was twisted twins of tilted unit crystals, as was the case with inorganic helical crystals. The molecular recognition between the Asp crystals and agar matrix molecules resulted in the stereospecific morphogenesis. The chirality in Asp and agar molecules, the enantiomorph of unit crystals, and the resultant macroscopic helix were exquisitely associated with each other.

Electron transfer across α-helical peptides: Potential influence of molecular dynamics

Three hydrophobic leucine-rich peptides Fc18L, Ac18L and 18LAc were prepared. These peptides are equipped with a cystein sulfhydryl group which enables the formation of thin films on gold surfaces. Using these peptides, two types of films of α-helical peptides have been prepared, in which the redox-active peptide Fc18L is diluted by Ac18L ( SAM1) or by a mixture of Ac18L and 18LAc ( SAM2). In SAM1, the dipole moments of the peptides are aligned in the same direction, whereas in SAM2, they are opposite. Reflection absorption infrared spectroscopy (RAIRS) revealed that the peptides are more vertically oriented in SAM2 compared to those in SAM1. The interaction among the macroscopic helix dipoles gives tighter packing of the peptides in SAM2. Importantly, the electron transfer properties in the two films are significantly different, which is rationalized by differences in the molecular dynamics of the two films.

Amplification of chirality from molecules into morphology of crystals through molecular recognition.

We have found a novel type of morphological chiral tuning on inorganic helical crystals through stereochemical recognition of organic molecules. Helical forms consisting of twisted twins emerged from triclinic crystals under diffusion-limited conditions. The proportion of the right- and left-handed helices was precisely tuned with the addition of a specified amount of chiral molecules, such as d- and l-glutamic acids. The chiral molecules recognized the enantiomeric surface of the triclinic crystal and then changed the growth behavior of the helical morphology. As a result, the microscopic chiral information, at a molecular level, was amplified into the macroscopic helices consisting of inorganic achiral components.

Colloids, helices, and patterned films made from heme proteins and manganese oxide.

New colloidal materials with enzyme-like peroxidase activity were made from octahedral layered manganese oxide and myoglobin and hemoglobin, and were converted to macroscopic helices and patterned films.

A Macroscopic Helix Formation Induced by the Shrinking of a Cylindrical Polymeric Hydrogel

Since the work of Tanaka et al.,1, 2 volume phase transitions of polymeric hydrogels has been a topic intensively studied. Pattern formation in a hydrogel associated with this transition is also of interest,3–8 since studies on the underlying pattern formation may elucidate mechanical properties of gels and pattern evolutions in some biological systems. Quasi-hexagonal patterns were found in swelling of an ionized polyacrylamide gel slab by Tanaka et al.3 Afterwards, bubble, bamboo, and tube like patterns were observed in the shrinking of a gel cylinder suddenly immersed into an acetone-rich solution.4 Similar patterns were observed in poly(N-isopropylacrylamide) (PNIPAM) gel induced by temperature,9 in polyacrylate gel induced by a small molecular agent, dodecylpyridinium chloride (DPC)10 and in many other cases. Some complicated phenomena as intermediate states in a sudden volume change have also been reported such as whitening of PNIPAM gel at the early stage of gel shinking.11 In our study, a novel pattern tentatively named “gel helix” was observed during a sudden pH-induced shrinkage of a poly(sodium acrylate/acrylamide) gel cylinder. This process is described qualitatively in this report.

The influence of the chiral environment in the photosynthesis of enantiomerically enriched hexahelicene. A sudden change in the chirality of hexahelicene by a change in the temperature of a chiral crystalline medium

AbstractThe enrichment of one of the enantiomers (P or M) of hexahelicene (3) synthesized by the photochemical cyclodehydrogenation of 2‐styryl‐benzo[c]phenanthrene (1) in chiral media has been (re)investigated. Four different chiral solvents, two cholesteric liquid crystals, two chiral crystals, all at several temperatures, and two chiral polymers were used as media. To avoid erroneous values from circular‐dichroism (CD) measurements due to contributions of chiral side‐products, the enantiomeric excess (ee) was determined by HPLC analysis.In general the observed ee is small (< 7%). In chiral solvents the ee increases with decreasing temperatures. In the liquid crystals the macroscopic helix of the cholesteric phase has a small but distinct effect on the ee, probably due to the preferred fitting of one enantiomer of Z‐1 to the helix of the ‘solvent’. In chiral crystals the ee is relatively larger. A peculiar effect is observed when 1 is irradiated in ethyl (S)‐(+)‐O‐(4‐phenylbenzoyl)lactate. Below −19°C the M enantiomer and above −14°C the P enantiomer of 3 is the preferred photoproduct. In the polymer triacetylcellulose, M‐hexahelicene is formed in 4.5% excess; in β‐cyclodextrine only a very small amount of 3 is formed, due to the preferred photo‐isomerization into E‐1.