Handedness inversion of chiral amphiphilic molecular assemblies evidenced by supramolecular chiral imprinting in mesoporous silica assemblies

Antipodal twisted helical ribbons with lamellar bilayer structure were obtained by self-assembly of chiral amphiphilic molecules in water and water/ethanol. The handedness inversion of the molecular arrangement in these antipodal helical ribbons was investigated by using chiroptical spectroscopy and molecular probes in their antipodal mesoporous silica assemblies synthesized through pairing interaction between the head group of the chiral amphiphilic molecules and a co-structure-directing agent. The supramolecular chirality is imprinted in the pore surface through the organic group of the co-structure-directing agent. The mirror-image diffuse-reflectance circular dichroism spectra of the conjugated discotic probing molecule introduced into their supramolecular chiral imprinted mesoporous silica demonstrated the origin of inverse chirality from the antipodal helical stacking of the molecules.     

Inversion of the Supramolecular Chirality of Nanofibrous Structures through Co‐Assembly with Achiral Molecules

To understand the behavior of chiral nanostructures, it is of critical importance to study how achiral molecules regulate the chirality of such nanostructures and what the main driving forces for the regulation processes are. In this work, the supramolecular chirality of helical nanofibers consisting of phenylalanine‐based enantiomers is inverted by achiral bis(pyridinyl) derivatives through co‐assembly. This inversion is mainly mediated by intermolecular hydrogen bonding interactions between the achiral additives and the chiral molecules, which may induce stereoselective interactions and different reorientations for the assembled molecules, as confirmed by calculations. This work not only exemplifies a feasible method to invert the helicity of chiral nanostructures by the addition of achiral molecules, but also provides a method to explore their functions in environments where chiral and achiral molecules are in close proximity.     

Nonracemic dopant-mediated hierarchical amplification of macromolecular helicity in a charged polyacetylene leading to a cholesteric liquid crystal in water

Here we show the first example of a helical polyacetylene that forms a lyotropic liquid crystal (LC) through a hierarchical amplification of a macromolecular helicity process in water. The macromolecular helicity with an excess of one helical sense was first induced in the positively charged polyacetylene upon complexation with an extremely small oppositely charged nonracemic dopant through electrostatic interaction in water. Subsequently, the helicity was significantly amplified in the polymer backbone as an almost perfect single-handed helix through self-assembly into supramolecular helical arrays in a lyotropic cholesteric state. The present results will allow the detection of a tiny imbalance in chiral molecules and also provide new approaches for the design of novel water-soluble helical architectures and the construction of new chiral materials in areas such as biotechnology and materials science.     

Molecular structure of self-assembled chiral nanoribbons and nanotubules revealed in the hydrated state

A detailed molecular organization of racemic 16-2-16 tartrate self-assembled multi-bilayer ribbons in the hydrated state is proposed where 16-2-16 amphiphiles, tartrate ions, and water molecules are all accurately positioned by comparing experimental X-ray powder diffraction and diffraction patterns derived from modeling studies. X-ray diffuse scattering studies show that molecular organization is not fundamentally altered when comparing the flat ribbons of the racemate to chirally twisted or helical ribbons of the pure tartrate enantiomer. Essential features of the three-dimensional molecular organizations of these structures include interdigitation of alkyl chains within each bilayer and well-defined networks of ionic and hydrogen bonds between cations, anions, and water molecules between bilayers. The detailed study of diffraction patterns also indicated that the gemini headgroups are oriented parallel to the long edge of the ribbons. The structure thus possesses a high cohesion and good crystallinity, and for the first time, we could relate the packing of the chiral molecules to the expression of the chirality at a mesoscopic scale. The organization of the ribbons at the molecular level sheds light on a number of their macroscopic features. Among these are the reason why enantiomerically pure 16-2-16 tartrate forms ribbons that consist of exactly two bilayers, and a plausible mechanism by which a chirally twisted or helical shape may emerge from the packing of chiral tartrate ions. Importantly, the distinction between commonly observed helical and twisted morphologies could be related to a subtle symmetry breaking. These results demonstrate that accurately solving the molecular structure of self-assembled soft materials--a process rarely achieved--is within reach, that it is a valid approach to correlate molecular parameters to macroscopic properties, and thus that it offers opportunities to modulate properties through molecular design.     

Anion-dependent self-assembly of silver(I) and diaminotriazines to coordination polymers: non-covalent bonds and role interchange between silver and hydrogen bonds

New coordination polymers have been obtained by the self-assembly of silver salts AgX (X = BF 4, PF 6, CF 3SO 3) and 2,4-diamino-6-R-1,3,5-triazines L (R = phenyl and p-tolyl) of formulas AgLX ( 1- 6). A complex of different stoichiometry, [Ag 3L 2(H 2O)(acetone) 2](BF 4) 3, 7 (R = phenyl), has also been synthesized. The three-dimensional structures of five compounds have been determined by X-ray diffraction studies. For the AgLX complexes, when X = BF 4 and R = phenyl or p-tolyl, chiral chains with alternating Ag and L are formed. The chains are cross-linked by the counteranions in a three-dimensional fashion through hydrogen bonds and weak Ag...F interactions giving rise to a structure with solvent-filled channels. Different and more compact structures have been found when the counteranion is CF 3SO 3 (OTf). When R = phenyl, sheets are formed which consist of [Ag 2(OTf) 2L 2] units with double triflate bridges and which contain columns of pi-pi stacked arenes. Hydrogen bonds connect the sheets. When AgOTf is used and R is p-tolyl, a different and unusual ladderlike structure is obtained in which the rungs are double asymmetric bridges consisting of the triflate groups bonded to Ag in kappa (2) O,mu 2- O and kappa (1) O,mu 2- O fashion. The ladders are parallel to each other and are mutually linked by N-H...N hydrogen bonds to give a 3D architecture. A very similar ladderlike structure has been found for 7 but with a water molecule and a BF 4 (-) group acting as bridges. The role played by the hydrogen bonds in complex 6 to form the 3-D structure is played in 7 by [Ag(acetone) 2] fragments. The noncovalent interactions play an important role in the different solid-state 3D structures. The behavior of the new derivatives in solution has also been analyzed. A new species has been detected at low temperatures, and this exhibits restricted rotation of the phenyl ring.     

Supramolecular structure of racemic 5‐hydroxy‐4‐(4‐methoxyphenyl)‐10‐oxa‐4‐azatricyclo[5.2.1.02,6]dec‐8‐en‐3‐one

The structure of the title compound, C₁₅H₁₅NO₄, comprises a racemic mixture of chiral molecules containing five stereogenic centres. The cyclohexane ring tends towards a boat conformation and the two tetrahydrofuran rings adopt envelope conformations. Molecules are linked into sheets parallel to (100) by a combination of O—H...O, C—H...O and C—H...π hydrogen bonds, leading to a two‐dimensional supramolecular structure.     

Autoamplification of Molecular Chirality through the Induction of Supramolecular Chirality

The novel concept for the autoamplification of molecular chirality, wherein the amplification proceeds through the induction of supramolecular chirality, is presented. A solution of prochiral, ring‐open diarylethenes is doped with a small amount of their chiral, ring‐closed counterpart. The molecules co‐assemble into helical fibers through hydrogen bonding and the handedness of the fibers is biased by the chiral, ring‐closed diarylethene. Photochemical ring closure of the open diarylethene yields the ring‐closed product, which is enriched in the template enantiomer.     

β-Sheet to Helical-Sheet Evolution Induced by Topochemical Polymerization: Cross-α-Amyloid-like Packing in a Pseudoprotein with Gly-Phe-Gly Repeats

Protein-mimics are of great interest for their structure, stability, and properties. We are interested in the synthesis of protein-mimics containing triazole linkages as peptide-bond surrogate by topochemical azide-alkyne cycloaddition (TAAC) polymerization of azide- and alkyne-modified peptides. The rationally designed dipeptide N₃-CH₂CO-Phe-NHCH₂CCH ( 1 ) crystallized in a parallel β-sheet arrangement and are head-to-tail aligned in a direction perpendicular to the β-sheet-direction. Upon heating, crystals of 1 underwent single-crystal-to-single-crystal polymerization forming a triazole-linked pseudoprotein with Gly-Phe-Gly repeats. During TAAC polymerization, the pseudoprotein evolved as helical chains. These helical chains are laterally assembled by backbone hydrogen bonding in a direction perpendicular to the helical axis to form helical sheets. This interesting helical-sheet orientation in the crystal resembles the cross-α-amyloids, where α-helices are arranged laterally as sheets.     

Hierarchical superstructures with control of helicity from the self-assembly of chiral bent-core molecules

Herein, two asymmetric chiral bent-core molecules, 3-[(4-{[4-(heptyloxy)benzoyl]oxy}benzoyl)oxy]-phenyl-4-[(4-{[(1R)-1-methylheptyl]oxy}benzoyl)oxy] benzoate (BC7R) and 3-[(4-{[4-(heptyloxy)benzoyl]oxy}benzoyl)oxy]-phenyl-4-[(4-{[(1S)-1-methylheptyl]oxy}benzoyl)oxy] benzoate (BC7S), were synthesized to demonstrate control of the helicity of their self-assembled hierarchical superstructures. Mirror-imaged CD spectra showed a split-type Cotton effect after the formation of self-assembled aggregates of BC7R and BC7S, thereby suggesting the formation of intermolecular exciton couplets with opposite optical activities. Both twisted and helical ribbons with preferential helicity that corresponded to the twisting character of the intermolecular exciton couplet were found in the aggregates. The formation of helical ribbons was attributed to the merging of twisted ribbons through an increase in width to improve morphological stability. As a result, control of the helicity of hierarchical superstructures from the self-assembly of bent-core molecules could be achieved by taking advantage of the transfer of chiral information from the molecular level onto the hierarchical scale.     

Supramolecular helical stacking of metallomesogens derived from enantiopure and racemic polycatenar oxazolines

The present report undertakes a challenge of general interest in supramolecular chemistry: the achievement of helical organizations with controlled structure. To achieve this target we considered the possibility of inducing supramolecular chirality using molecules that were designed to organize into columnar mesophases. The use of oxazoline-derived ligands and metal coordination served as tools to prepare molecules with a phasmidic-like structure, which show columnar organization in the liquid crystalline state. To ensure the formation of chiral mesophases, these complexes bear stereogenic centers in the rigid coordination environment of the metal. X-ray and circular dichroism experiments have revealed that chirality transfer does indeed take place from the chiral molecule to the columnar liquid crystal organization. This chiral columnar organization appears as a helix consisting of stacks of molecules that rotate with respect to one another along the column while maintaining their mean planes parallel to each other. In fact, it has been concluded that packing of these polycatenar molecules must be more efficient upon rotation of a molecule with respect to the adjacent one along the column. Furthermore, the same type of helical supraorganization has been found to be present in the mesophase of the racemic mixture and the mixture of diastereomers prepared from the racemic ligand. In this case, segregation of the optical isomers is proposed to occur to give rise to both types of helix (right-handed and left-handed).     

Self‐Assembly of α‐Helical Polypeptides Driven by Complex Coacervation

Reported is the ability of α‐helical polypeptides to self‐assemble with oppositely‐charged polypeptides to form liquid complexes while maintaining their α‐helical secondary structure. Coupling the α‐helical polypeptide to a neutral, hydrophilic polymer and subsequent complexation enables the formation of nanoscale coacervate‐core micelles. While previous reports on polypeptide complexation demonstrated a critical dependence of the nature of the complex (liquid versus solid) on chirality, the α‐helical structure of the positively charged polypeptide prevents the formation of β‐sheets, which would otherwise drive the assembly into a solid state, thereby, enabling coacervate formation between two chiral components. The higher charge density of the assembly, a result of the folding of the α‐helical polypeptide, provides enhanced resistance to salts known to inhibit polypeptide complexation. The unique combination of properties of these materials can enhance the known potential of fluid polypeptide complexes for delivery of biologically relevant molecules.     

The hydrogen‐bonding patterns of 3‐phenylpropylammonium benzoate and 3‐phenylpropylammonium 3‐iodobenzoate: generation of chiral crystals from achiral molecules

The crystal structures and hydrogen‐bonding patterns of 3‐phenylpropylammonium benzoate, C₉H₁₄N⁺·C₇H₅O₂⁻, (I), and 3‐phenylpropylammonium 3‐iodobenzoate, C₉H₁₄N⁺·C₇H₄IO₂⁻, (II), are reported and compared. The addition of the I atom on the anion in (II) produces a different hydrogen‐bonding pattern to that of (I). In addition, the supramolecular heterosynthon of (II) produces a chiral crystal packing not observed in (I). Compound (I) packs in a centrosymmetric fashion and forms achiral one‐dimensional hydrogen‐bonded columns through charge‐assisted N—H...O hydrogen bonds. Compound (II) packs in a chiral space group and forms helical one‐dimensional hydrogen‐bonded columns with 2₁ symmetry, consisting of repeating R₄³(10) hydrogen‐bonded rings that are commonly observed in ammonium carboxylate salts containing chiral molecules. This hydrogen‐bond pattern, which has been observed repeatedly in ammonium carboxylate salts, thus provides a means of producing chiral crystal structures from achiral molecules.     

β‐Sheet to Helical‐Sheet Evolution Induced by Topochemical Polymerization: Cross‐α‐Amyloid‐like Packing in a Pseudoprotein with Gly‐Phe‐Gly Repeats

Protein‐mimics are of great interest for their structure, stability, and properties. We are interested in the synthesis of protein‐mimics containing triazole linkages as peptide‐bond surrogate by topochemical azide‐alkyne cycloaddition (TAAC) polymerization of azide‐ and alkyne‐modified peptides. The rationally designed dipeptide N₃‐CH₂CO‐Phe‐NHCH₂CCH ( 1 ) crystallized in a parallel β‐sheet arrangement and are head‐to‐tail aligned in a direction perpendicular to the β‐sheet‐direction. Upon heating, crystals of 1 underwent single‐crystal‐to‐single‐crystal polymerization forming a triazole‐linked pseudoprotein with Gly‐Phe‐Gly repeats. During TAAC polymerization, the pseudoprotein evolved as helical chains. These helical chains are laterally assembled by backbone hydrogen bonding in a direction perpendicular to the helical axis to form helical sheets. This interesting helical‐sheet orientation in the crystal resembles the cross‐α‐amyloids, where α‐helices are arranged laterally as sheets.     

Tunable helical assemblies of L-alanine methyl ester-containing polyphenylacetylene

The self-assemblying behaviors of L-alanine methyl ester-containing polyphenylacetylene (PPA-Ala, in Chart 1 ) were investigated upon the evaporation of its solvent on mica and on air/water interfaces. The introduction of chiral amino acid attachments to the polyphenylacetylene backbone induced a helical conformation of the backbone, which was stabilized by various noncovalent interactions, especially hydrophobic effect and hydrogen bonds. The helicity of the polymer was further amplified in its higher-order self-assemblies as the formation of helical fibers on the surface of mica upon natural evaporation of its THF solution. By LB technique, the polymer chains were guided to form ordered parallel ridges and highly aligned, with their helical conformation still remaining. The reorganization of the chiral polymer chains on air/water interface was associated with the additional hydrophobic effect of PPA-Ala on an air/water interface. The polymer backbones had to adopt different arrangements to minimize their contact with water, and this adjustment led to the formation of aligned polymer ridges under proper surface pressure.     

Crystal structure and hydrogen-bonding system in cellulose Ibeta from synchrotron X-ray and neutron fiber diffraction

The crystal and molecular structure together with the hydrogen-bonding system in cellulose Ibeta has been determined using synchrotron and neutron diffraction data recorded from oriented fibrous samples prepared by aligning cellulose microcrystals from tunicin. These samples diffracted both synchrotron X-rays and neutrons to better than 1A resolution (>300 unique reflections; P2(1)). The X-ray data were used to determine the C and O atom positions. The resulting structure consisted of two parallel chains having slightly different conformations and organized in sheets packed in a "parallel-up" fashion, with all hydroxymethyl groups adopting the tg conformation. The positions of hydrogen atoms involved in hydrogen-bonding were determined from a Fourier-difference analysis using neutron diffraction data collected from hydrogenated and deuterated samples. The hydrogen atoms involved in the intramolecular O3...O5 hydrogen bonds have well-defined positions, whereas those corresponding to O2 and O6 covered a wider volume, indicative of multiple geometry with partial occupation. The observation of this disorder substantiates a recent infrared analysis and indicates that, despite their high crystallinity, crystals of cellulose Ibeta have an inherent disorganization of the intermolecular H-bond network that maintains the cellulose chains in sheets.     

Structural transitions in model beta-sheet tapes

We present a molecular-scale simulation study of the structural transitions between helicoidal, helical, and tubular geometries in supramolecular beta-sheet tapes. Such geometries have been observed in different self-assembled amyloid systems (based on either natural or synthetic peptides) for which the beta-sheet tapes represent the simplest fibrillar aggregates. A coarse-grained model for the beta-sheet tapes is proposed, with chiral degrees of freedom and asymmetrical chemical properties, which provides a quantitative characterization of the structural transitions. A quantitative connection is established between the molecular properties and the elastic parameters of the supramolecular tapes.     

Construction of multi-component supramolecular architectures of bile acids and cinchona alkaloids through helical-pitch-synchronized crystallization

Molecular assemblies based on helical motifs are of substantial interest from the view point of fundamental science as well as application. In this study, we propose a new class of organic crystal, that is, heteroH-MOC (multi-component organic crystal containing different kinds of helical motifs consisted of different components), and describe successful construction of heteroH-MOCs with P2(1) and P2(1)2(1)2(1) space groups by using steroidal bile acids and cinchona alkaloids. In the P2(1) crystals, two kinds of helices composed of the steroid and alkaloid are arranged in a parallel fashion, while, in the P2(1)2(1)2(1) crystals, those are in a perpendicular fashion. It is remarkable that, in such systems, particularly in the latter crystals, components ingeniously achieved highly-ordered synchronization of periodicity (helical pitches r and periodic distances in the array of helices p), which is first demonstrated in this study through hierarchical interpretation of the crystal structures.     

Chiral translation and cooperative self-assembly of discrete helical structures using molecular recognition dyads

Complementary diaminopyridine (DAP) and flavin derivatives self-assemble into discrete helically stacked tetrads in hydrocarbon solvents. The self-assembled structure was demonstrated through induced circular dichroism using DAPs with chiral side-chains and flavin with achiral side-chains. Flavin derivatives with chiral side-chains were synthesized; cooperativity in the self-assembly was established through circular dichroism (CD) profiles and melting curves. It was found that placing stereocenters in both recognition units resulted in a strong bisignated profile and enhancement of complex stability, indicative of cooperative self-assembly.     

N‐(tert‐Butoxycarbonyl)‐L‐valyl‐L‐valine methyl ester: a twisted parallel β‐sheet in the crystal structure of a protected dipeptide

The title compound, C₁₆H₃₀N₂O₅, crystallizes with three molecules in the asymmetric unit, each adopting a β‐strand/polyproline II backbone conformation. The main‐chain functional groups are hydrogen bonded into tapes having the characteristics of parallel β‐sheets. Each tape has a left‐handed twist and thus forms a helix, with six peptide molecules needed to complete a full 360° rotation. A comparison of hydrogen‐bond lengths and twisting modes is made with other related structures of protected dipeptides and with a hexapeptide derived from amyloid‐β containing the Val–Val segment. Additionally, a comparison of the backbone conformation is made with that of the Val141–Val142 segment of the water channel aquaporin‐4 (AQP4).     

Tubular hydrogen-bonded networks sustained by water molecules.

The design concept of functional solids relies on controlling the topology of crystal packing through exploitation of weak intermolecular forces. In the context of cyclic aggregates, the ability to anticipate the consequences of ring constituents and their stereochemistries on ring conformation is vitally important since even an apparently slight structural change effected on molecules can dramatically alter the crystal structure. We have found that solid-state structures formed by hydroxy acids with a general structure (+/-)-1 depend on steric interactions. Thus, with the exception of molecules 1b and 1e, compounds (+/-)-1a-(+/-)-1m, which possess bulky and conformationally rigid substituents, aggregate by forming tapes and sheets by alternating (+) and (-) subunits held together through carboxylic acid-to-alcohol hydrogen bonds. Homologue (+/-)-1n, with conformationally flexible substituents which allow conformational deformation, gives, by incorporation of molecules of water, an efficient hexagonal assembly which extends to the third dimension to form tubular H-bonding networks. Each puckered channel can be described as interconnected closely packed hexagons in chairlike conformations. The ethyl groups presented in (+/-)-1n gave the volume required to lock the inner hexagonal wall into a rigid structure. Attempts to obtain cyclic aggregates using small substituents, compounds (+/-)-1o-(+/-)-1q, failed. The observed supramolecular assemblies of the anhydrous compounds can be classified into one-dimensional strands and two-dimensional sheets, while three-dimensional networks are present only in the hydrated molecules (1b, 1e, and 1n). The crystal structure of the anhydrous (+/-)-1n compound confirms the important role played by water molecules in the formation of tubular structures.