Chiral Aggregates Research Articles

Concentration Dependent Specific Rotations of Chiral Surfactants: Experimental and Computational Studies.

Recent experimental studies have shown unexpected chiroptical response from some chiral surfactant molecules, where the specific rotations changed significantly as a function of concentration. To establish a theoretical understanding of this experimentally observed phenomena, a novel methodology for studying chiral surfactants via combined molecular dynamics (MD) and quantum mechanical (QM) calculations is presented. MD simulations on the +10 000 atom surfactant systems have been performed using MD and QM/molecular mechanics (MM) approaches. QM calculations performed on MD snapshots coupled with extensive analysis on lauryl ester of phenylalanine (LEP) surfactant system indicate that the experimentally observed variation of specific rotation with concentration may be due to the conformational differences of the surfactant monomers in the aggregates. Though traditional MM simulations did not show significant differences in the conformer populations, QM/MM simulations using the forces derived from the PM6 method did predict conformational differences between aggregated and nonaggregated LEP molecules, which is consistent with experimental data. Additionally the electrostatic environment of charged surfactants may also be important, since dramatic changes in the Boltzmann populations of surfactant monomers can be noted in the presence of an electric field generated by the chiral ionic aggregates.

Hydrophobically driven chiral column aggregates of hydrazide macrocycles and side chains-controlled chirality conversion

Four hydrogen bonded rigid aromatic hydrazide macrocycles, which bear different number of chiral and/or achiral oligo(ethylene glycol) chains, have been synthesized. 1H NMR, fluorescence, UV–vis absorption, and circular dichroism experiments showed that the three macrocycles bearing chiral side chains gave rise to one-dimensional chiral column aggregates in aqueous solution. Remarkably, the chiral side chains at two different positions induced the helicity chirality in opposite directions.

Electrochemical Preparation of Electro-active Polymers having Nitrogen Atom

New functions of polymer film can be developed by using template. We synthesized polypyrrole (PPy) as a conducting polymer in liquid crystal electrolyte solution. The liquid crystal electrolyte solution for the electrochemical polymerization contains monomer, electrolyte, and matrix liquid crystal. The electrochemical polymerization in liquid crystal was simply performed by using sandwich cell electrochemical polymerization method, which we have originally developed. Application of constant voltage between the two electrodes produces a solid polymer film at anode side through electrochemical oxidation polymerization process. Polarizing optical microscopy observation revealed that the polymer film thus obtained show beautiful optical texture like liquid crystal. The surface texture was derived from molecular structure transcription from the liquid crystal electrolyte solution. The transcription is occurred from molecular level to macroscopic level. Infrared absorption spectroscopy measurements, UV-Vis absorption measurements confirm that the polymer film is exactly pi-conjugated film, although the optical structure is very similar to that of liquid crystal. Furthermore, the polymer film can be tuned its color via electrochemical redox reaction (doping-dedoping). Therefore, the polymer has both liquid crystal order and electro-active property. Lyotropic liquid crystals have good affinity with water, and form crystal like aggregation structure via a self-assembly. Lyotropic liquid crystal shows variety in liquid crystal type, such as columnar phase, smectic phase, nematic phase, and cholesteric phase. In this research, we employed lyotropic liquid crystals for electrolyte solution. Columnar liquid crystals have paid attention as an electrical conduction materials having good carrier mobility. Therefore, the polymer thus prepared can be expected to show structure based unique electrical conduction function through columnar form produced by the transcription from the liquid crystal electrolyte solution in the electrochemical polymerization. Cholesteric liquid crystals show chiral aggregation form. The polymer prepared in cholesteric liquid crystal shows fingerprint texture under the observation of polarizing optical microscope, and optical rotation and circular dichroism. The optical rotation angle can be controlled with electrochemical redox reaction. In this presentation, we report, 1) Preparation of liquid crystal electrolyte solution and characterization 2) Electrochemical polymerization 3) Spectroelectrochemistry. This research can be involved molecularly imprinting (MIP) [3,4]. We discuss the method of electrochemical polymerization, functions, and future prospective of the polymer films prepared in liquid crystals based on electrochemistry.

Formation of complex self-assembled aggregates in non-ionic chromonics: dimer and trimer columns, layer structures and spontaneous chirality.

Dissipative particle dynamics (DPD) simulations are used to model the aqueous self-assembly of three variants of the non-ionic triphenylene-based chromonic mesogen, TP6EO2M. In the variants studied, one to three of the six methoxy poly(ethylene glycol) chains of TP6EO2M are replaced by short hydrophobic-lipophobic chains, causing a remarkable change in the structure of the mesophases formed. In the 100 wt% limit, corresponding to pure thermotropic phases, complex columnar phases arise, in which the underlying hexagonal packing is supplemented by additional order resulting from microphase separation of hydrophobic-lipophobic regions. With addition of water an array of novel chromonic phases are seen. In these phases supramolecular aggregates form in which hydrophobic-lipophobic chains are excluded from water by the joining together of single molecule chromonic stacks into dimers or trimers. These aggregates form chromonic N and M phases and, in the case of a "Janus mesogen" (with three hydrophobic-lipophobic chains on one side of the molecule), form a novel smectic chromonic phase. Spontaneous symmetry breaking is seen in columns composed of trimer stacks with defects. Here achiral molecular aggregates develop a spontaneous twist, inducing the formation of either left-handed or right-handed chiral aggregates. On the long time-scales accessible to DPD simulations, chiral aggregates are seen to be dynamic structures in which chirality inversion can take place over long periods of time.

T-shaped monopyridazinotetrathiafulvalene-amino acid diad based chiral organogels with aggregation-induced fluorescence emission.

A series of pyridazine coupled tetrathiafulvalene T-shaped derivatives with varying amino acid moieties have been synthesized and their gelation properties were studied in various organic solvents. Among these derivatives, two gelators bearing glycine or phenylalanine units display efficient gelation in aromatic and polar solvents. Interestingly, these gelators, except for the gelator containing two tryptophan units, are able to gel DMF via a solution-to-gel transformation when triggered with sonication for less than 20 s or cooled below zero. A number of experiments revealed that these gelator molecules self-assembled into elastically interpenetrating three-dimensional chiral fibrillar aggregates. Importantly, all of the resulting gels result in a dramatic enhancement of the fluorescence intensity compared with their hot solution in spite of the absence of a conventional fluorophore unit and the fluorescence was effectively quenched by the introduction of C60. Moreover, the gelators can be utilized for the removal of different types of toxic molecules, such as aromatic solvents and cationic dyes, from wastewater.

Effect of chiral 2-ethylhexyl side chains on chiroptical properties of the narrow bandgap conjugated polymers PCPDTBT and PCDTPT.

Two narrow bandgap conjugated polymers containing chiral 2-ethylhexyl side chains were synthesized: poly[(4,4-bis(2-ethylhexyl)cyclopenta-[2,1-b:3,4-b']dithiophene)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl] (PCPDTBT*) and poly[(4,4-bis(2-ethylhexyl)cyclopenta[2,1-b:3,4-b0]dithiophene)-2,6-diyl-alt-[1,2,5]-thiadiazolo[3,4-c]pyridine] (PCDTPT*). The presence of a chiral substituent provides a handle to study the geometry of interchain aggregates and/or the secondary structure of these conjugated polymers in solution and in thin films via circular dichroism (CD) spectroscopy, provided that the asymmetry in the side-chain is translated to the optically active conjugated backbone. CD signals are observed for PCPDTBT* and PCDTPT* in poor solvent systems, which indicate the presence of chiral ordering in the aggregates. PCPDTBT* shows greater chiral order than PCDTPT* based on their relative anisotropy factors. Additionally, GIWAXS analysis reveals that PCPDTBT* films are more ordered than what is observed for the same polymer containing racemic 2-ethylhexyl chains. Upon solution deposition, the chiral ordering is found to translate to the solid-state microstructure for PCPDTBT* but not PCDTPT*. The presence of a pyridyl nitrogen on the thiadiazolo[3,4-c]pyridine ring of PCDTPT* favors a planar conformation for the backbone such that it has a higher rotational barrier compared to PCPDTBT*. This larger rotational barrier appears to limit the ability of PCDTPT* to adopt a helical structure or relevant chain distortions for achieving chiral aggregates.

Synthesis of optically active conjugated polymers containing platinum in the main chain: Control of the higher‐order structures by substituents and solvents

ABSTRACTThe Sonogashira–Hagihara coupling polymerization of d‐hydroxyphenylglycine‐derived diiodo monomers 1–4 and platinum‐containing diethynyl monomer 5 gave the corresponding polymers [poly(1–5)–(2–5)] with number‐average molecular weights of 19,000–25,000 quantitatively. The polymers were soluble in CHCl3, CH2Cl2, THF, and DMF. CD and UV–vis spectroscopic analysis revealed that amide‐substituted polymers [poly(1–5) and poly(2–5)] formed chiral higher‐order structures in solution, while ester‐substituted polymers [poly(3–5) and poly(4–5)] did not. Poly(1–5) formed one‐handed helices in THF/toluene mixtures, while it formed chiral aggregates in THF/MeOH mixtures. Poly(1–5) emitted fluorescence with quantum yields ranging from 0.8 to 1.3%. The polymers usually aggregated in the solid state. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015, 53, 2452–2461

Chiroptical Studies on Supramolecular Chirality of Molecular Aggregates

The attempts of applying chiroptical spectroscopy to supramolecular chirality are reviewed with a focus on vibrational circular dichroism (VCD). Examples were taken from gels, solids, and monolayers formed by low-molecular mass weight chiral gelators. Particular attention was paid to a group of gelators with perfluoroalkyl chains. The effects of the helical conformation of the perfluoroalkyl chains on the formation of chiral architectures are reported. It is described how the conformation of a chiral gelator was determined by comparing the experimental and theoretical VCD spectra together with a model proposed for the molecular aggregation in fibrils. The results demonstrate the potential utility of the chiroptical method in analyzing organized chiral aggregates.

Synthesis of radial silicas: A dynamic templating approach

Understanding how radial growth of silica spines on synthetic silica spheres relates to the biosilification mechanism on their analogous biological diatoms will help to better understand the natural process. However, the number of reports of the growth of silicas with radial spines is few. In this work we report the synthesis and use of a pair of low-molecular-weight amphiphiles derived from 1,2-diphenylethylenediamine to form chiral aggregates in deionized water at a concentration of 25gL−1 at 25°C. Silica nanotubes and hollow nanospheres with radial spines are prepared using one of the amphiphiles as the template, 3-(aminopropyl)trimethoxysilane as a co-structure-directing agent and tetraethyl orthosilicate as the silica precursor. The pH value of the reaction mixture plays an important role in the formation of radial spines, which are found to grow within the pH range of 7.43–10.52. It is proposed that the hierarchical nanostructures form through a dynamic templating process.

Oligo(p-phenylene ethynylene) Electrolytes: A Novel Molecular Scaffold for Optical Tracking of Amyloids.

Finding new optical probes to detect and track amyloid protein aggregates is key to understanding and defeating the myriad of neurodegenerative and other diseases associated with these misfolded proteins. Herein we report that a series of fluorescent, soluble oligo(p-phenylene ethynylene)s (OPEs) are able to detect amyloids in vitro by massive binding-activated superluminescence, with low micromolar affinity and high selectivity for the amyloid conformer. The OPEs track the kinetics of amyloid fibril formation from native hen egg white lysozyme (HEWL) similarly to thioflavin T (ThT), and the dependence of binding affinity on OPE length supports the theory of a linear binding groove. We hypothesize, based on spectral properties, induced circular dichroism, and previous work in analogous systems, that the fluorescence turn-on mechanism is a combination of the reduction of static solvent-mediated quenching at the ethyl ester end groups of the phenylene ethynylene fluorophore and the formation of chiral J-type aggregates templated on the amyloid fibril surface.

A Remarkably Complex Supramolecular Hydrogen-Bonded Decameric Capsule Formed from an Enantiopure C₂-Symmetric Monomer by Solvent-Responsive Aggregation.

The formation of an unprecedented decameric capsule in carbon disulfide, held together by the combination of double and triple hydrogen bonds between isocytosine units embedded in an enantiomerically pure bicyclic framework is reported. The aggregation occurs via symmetry breaking of the enantiopure intrinsically C2-symmetric monomer brought about by solvent, induced tautomerization of the hydrogen-bonding unit. We show that the topology of the aggregate is responsive to the solvent in which the assembly takes place. In this study we demonstrate that in carbon disulfide the chiral decameric cavity aggregate consisting of three forms of the same monomer, differing in their hydrogen bonding to each other is selectively formed, representing a tube-like structure capped with two C2-symmetric monomers. The large cylindrical cavity produced selectively accommodates one partially solvated C60 molecule, and molecular dynamic simulations revealed the special role of the solvent in the inclusion mechanism. The strategy described herein represents the first step toward the creation of a new class of hydrogen-bonded tubular objects from only one small symmetric building block by solvent-responsive aggregation.

Application of Octaethylporphyrin Structural Motif for Supramolecular Chirogenesis

Supramolecular chirogenesis is a smart combination of supramolecular chemistry and chirality associated with the processes of asymmetry induction, amplification, modulation in non-covalently linked multicomponent assemblies.1Porphyrin stuctures are of particular interest to study this phenomen owing to their unique spectral, physico-chemical, and synthetic properties. Octaethylporphyrin (OEP) is one of the simplest porphyrins, yet being easily accessible via the standard synthetic procedure or commercially available from many reagent suppliers. Therefore, monomeric ZnOEP was firstly tested in the process of supramolecular chirality induction. Hence, upon interation with chiral amines (L) the induced optical activity in inherently circular dichroism (CD) silent ZnOEP was almost neglidgible that indicated inefficiency of the chirality transfer process in these supramolecular systems.2 However, in the case of solid state (a glassy KBr matrix) there was the time-dependent formation of (ZnOEP.L)n chiral aggregates posessing a high degree of optical activity with the anisotropy factor of 0.015.3 The chirality transfer mechanism was based upon the unidirectional arrnagement of ZnOEP molecules. In contrast to monomeric ZnOEP, dimeric (ZnOEP)2 exhibited considerable optical activity upon interaction with chiral guests.2,3 Thus, monodentate ligands resulted in formation of the stable 1:2 supramolecular chiral complexes and induction of the noticeable (moderate-to-strong) exciton couplet CD signals in the absorption region of porphyrin. It was established that the mechanism of asymmetry transfer from chiral guests to achiral (ZnOEP)2 consists of two major steps. At the first step there is formation of the host-guest complex owing to abilities of chiral guests to interact with (ZnOEP)2 resulting in the initial syn-to-anti conformational switching. The second step is another type of structural deformation in (ZnOEP)2 induced by the size difference of ligand’s substituents at the chirogenic center resulting in asymmetric screw formation in the resulting complexes. For example, when the bulkiness order of the guest substituents around the chiral center coincides with the substituents’ priority rule, the (R)-enantiomer yields a negative first Cotton effect and a positive second Cotton effect, while the guest with the (S) absolute configuration produces CD signal of the opposite signs, thus making this sensing protocol to be effectively used for determining the absolute configuration of different classes of organic molecules. In the solid state the situation is different; (R)-ligands upon interaction with (ZnOEP)2 induce a positive first Cotton effect, whilst the corresponding (S)-ligands produce a mirror image CD spectra.3 Remarkably, the induced chirality in the solid state exhibits enhanced intensity, with an inverted sign in comparison to that in the solution phase owing to formation of chiral aggregates of regular structure, in which the intermolecular screw sense is opposite to the intramolecular screw sense of the single molecule. The generated CD sign unambiguously correlates with the induced helicity allowing straightforward determination of the absolute configuration of various chiral guests and thus making it possible to apply (ZnOEP)2 as effective and universal chirality sensors for different types of optically active compounds as in solution and in the solid state. For investigation of the weak host-guest interaction mode a special tetrameric (NiOEP)4 host has been designed.4 This host satisfies the major following requirements: (1) total inertness towards most of the conventional host-guest binding modes, (2) possession of an open cavity between the two bis-porphyrin subunits owing to the syn-orientation in (NiOEP)4, which allows chiral guest molecules to be incorporated into it and thus to interact with the host in a stereospecific manner, and (3) presence of a spectrally separated B|| electronic transitions aligned along the diacetylene bridges and located at the low energy region of the porphyrin B band due to the enhanced interporphyrin interactions through the unsaturated linkage. The chirogenic process in (NiOEP)4 was monitored upon interaction with various chiral solvents and resulted in induction of optical activity in the region of B|| electronic transitions due to the corresponding host-guest interactions. In particular, (R)-solvents yield a positive first and negative second Cotton effects, while the (S)-enantiomers gives the opposite signs. The apparent mechanism of chirality induction in (NiOEP)4 includes penetration of the solvent molecules inside the interporphyrin cavity to enhance the solvent-solute interactions and thus transfer the chiral information to an achiral host by inducing a unidirectional screw in (NiOEP)4 and following the guest’s absolute configuration. V. V. Borovkov, Y. Inoue, Top. Curr. Chem. 2006, 265, 89.V. V. Borovkov, J. M. Lintuluoto, Y. Inoue, J. Am. Chem. Soc. 2001, 123, 2979V. V. Borovkov, G. A. Hembury, Y. Inoue, Acc. Chem. Res. 2004, 37, 449.V. V. Borovkov, T. Yamamoto, H. Higuchi, Y. Inoue, Org. Lett. 2008, 10, 1283.

New Generation of Hybrid Materials Based on Hierarchical Assembly of Porphyrin on Supramolecular Scaffolds or Nanostructures

Porphyrins multifunctional properties make these macrocycles intringuing building blocks to build up complex supramolecular assemblies via covalent or non-covalent approaches, either in solution or on inorganic surfaces. In the perspective of development of hybrids organic-inorganic materials, mastering of non-covalent strategies to transfer water soluble porphyrin based assemblies on nanostructured surfaces paves the way to a cost effective and environmentally friendly research that combine the unique properties of nanomaterials with the synthetic versatility of organic chemistry. Noteworthy, the control of spontaneous self-assembly is not trivial, as it is a hierarchically driven process that can proceed through thermodynamic or kinetic pathways. However, in water, complex porphyrin based supramolecular assemblies having a pre-determined and controlled stechiometry, chirality and composition have been successfully synthetized. Transfer of these assemblies (i.e. chiral porphyrin chiral aggregates, porphyrin-calixarenes, porphyrin-DNA) on inorganic surfaces opens the way to development of new generation of chiral surfaces and sensors, where porphyrins played an important role as spectroscopic probes, capable to overcome sensitivity limitations. Chirality induction and memory is, accordingly, another important requirement that need to be kept during this migration from solution to the solid state. The robustness of non-covalent approaches to achieve a straightforward, controllable and environmentally friendly functionalization of inorganic nanostructures strongly hinges on the competition between retention at the solid state rather than in solution and depends on several factors. Among them, polyelectrolytes secondary structure strongly influences the outcome of spontaneous deposition of porphyrin assemblies on solid surfaces often overriding the mere system electrostatics.

Synthesis and Properties of Novel Optically Active Poly(thiophenyleneethynylene-phenyleneethynylene)s

Abstract The Sonogashira–Hagihara coupling polymerization of l-alanine-derived 2,5-dibromothiophene monomers 1a–1d with p- and m-diethynylbenzenes 2p and 2m gave novel optically active poly(thiophenyleneethynylene-phenyleneethynylene)s poly(1a-2p)–poly(1d-2m). p-Phenylene-linked poly(1b-2p)–poly(1d-2p) bearing amide-amide side chains exhibited CD signals based on chiral aggregates, while the m-phenylene-linked counterparts poly(1b-2m)–poly(1d-2m) exhibited no evidence of the formation of secondary structures. Poly(1a-2p) and poly(1a-2m) bearing amide-ester side chains exhibited no CD signal as well.

Optical rotation by plasmonic circular dichroism of isolated gold nanorod aggregates

We show that plasmonic chirality in single gold nanorod (GNR) aggregates leads to the rotation of polarization of the scattered light. 3D glasses in conjunction with linearly polarized dark field scattering microspectroscopy were used to study the chirality of single GNR aggregates. Using this hetero-polarizer setup, we not only detect but also quantify their chirality. A polar mapping strategy was used for providing direct evidence for the emergence of light of different polarization angles when chiral GNR aggregates were excited with circularly polarized light of different handedness. Further, we have developed a methodology to eliminate fluctuations in the scattering intensity by averaging and normalizing the data. This allows calculation of plasmonic circular dichroism scattering spectra with high accuracy.

Reversibility of the supramolecular chirality of bridged binaphtol derivatives at the air-water interface

Second Harmonic Generation (SHG) was used to observe the reversible formation of chiral molecular aggregates at the air-water interface. The aggregates were formed by the chiral bridged binaphthol derivative 1( + ) under lateral compression in a Langmuir trough. The S polarized output SHG intensity was measured as a function of the input polarization angle of the fundamental beam during several compression cycles of the molecular film. The chirality of the supramolecular aggregates was established from the non vanishing SHG intensities collected for the p and s input polarization angles. It is demonstrated that the formation of the chiral domains with compression is a reversible process.

ChemInform Abstract: Application of Electronic Circular Dichroism in the Study of Supramolecular Systems

AbstractReview: 50 refs.

Circularly polarized luminescence from individual microstructures of conjugated polymer aggregates with solvent-induced chirality.

We report a microscopic study of circularly polarized luminescence (CPL) from R and S isomers of aggregates of the conjugated polymer F8BT, a copolymer containing fluorene and thiophene monomer units. Chirality in the aggregates is induced by addition of either (R)- or (S)-limonene during aggregation in solution. Solid-state samples are prepared either by drop-casting or spin-coating of the aggregate solution. CPL dissymmetry parameter g measured for individual aggregate microstructures shows a broad distribution of values ranging from positive to negative even for the same isomer. The difference in the centers of the g distribution between the R and S isomers reflects the intrinsic CPL from the chiral aggregates, and is found to depend on the size of the aggregate structures. On the other hand, the large widths of the distributions are ascribed to an optical effect arising from phase retardation along the optical path of the CPL inside the sample.

2D Chiral Structures in Quinoline Mixed Langmuir Monolayers

Chirality at interfaces is a relevant topic in nanoscience, as well as a key point for prebiotic chemistry. Mixed Langmuir monolayers, composed of the anionic phospholipid dimyristoyl phosphatidic acid (DMPA) and the cationic amphiphilic quinoline derivative 2-methyl-1-octadecylquinoline (MQ) have been built at the air–water interface. Both DMPA and MQ molecules are miscible, thus the equimolar mixture yields homogeneous monolayers completely. Chiral domains have been formed by this monolayer and observed in situ by Brewster angle microscopy (BAM). These chiral domains display a large array of shapes and sizes. The chirality of the monolayers has been confirmed by circular dichroism spectroscopy. The ordered aggregation of the quinoline group into large chiral supramolecular structures is proposed as the molecular origin of the observed chirality. Theoretical simulations using molecular mechanics confirm the strong trend of the quinoline group to form chiral aggregates. The great diversity in the size and shape of the chiral domains has been found to be strongly influenced by the competition between two nuclei growth mechanisms. An experimental procedure allowing a minimized growth through one of these mechanism is proposed, achieving a homogeneous distribution of ring-shaped domains. An overshoot in the π–A isotherms of this mixed monolayer appears at an intermediate surface pressure. This overshoot is interpreted as being due to the large difference between the surface pressure which starts the nuclei formation, πcrit, and the superficial pressure in which the nuclei can grow, πe. The rather small πe value compared to πcrit observed for this system must be attributed to the molecular interactions involved in the mixed monolayer, which facilitate the incorporation of molecules in preformed nuclei.

Copper Coordination Study in a Metal-Induced Chiral Polythiophene Aggregate

The chiral regioregular polythiophene, poly[(R)-3-(4-(4-ethyl-2-oxazolin-2-yl)phenyl)-thiophene] (PEOPT), forms chiral aggregates in chloroform under copper salt presence, with chirality disappearing upon the addition of acetonitrile. We report here a comprehensive quantum mechanical study of intermolecular assembly adducts with a copper ion that may be involved in this switching process. Results indicate that copper adducts made of undissociated copper salt in chloroform prefer a syn-(EOPT)2 disposition of ligands belonging to different polythiophene chains, favoring a double-stranded helical conformation. In contrast, doped polymer with copper ions in acetonitrile prefers an anti-(EOPT)2 arrangement for the different polythiophene side chains, favoring a nonchiral π-stacked packing. Furthermore, another cross-linked interaction besides the most stable NN-type has been characterized. Thus, NO-type interactions on the [Cu(EDOT)2]Cl2 adducts in chloroform have been shown to be energetically stable. However, hybrid quantum mechanical/molecular mechanical molecular dynamics (QM/MM MD) simulations indicate that only those adducts that involve helical chains keep structural stability under kinetic considerations.