An Unprecedented Memory of Macromolecular Helicity Induced in an Achiral Polyisocyanide in Water

Redox-Triggered Reversible Switching between Dynamic and Quasi-static α-Helical Peptides

We report the reversible transformation between a singly stapled dynamic α-helical peptide and a doubly stapled quasi-static one through redox-triggered dithiol/disulfide conversions of a stapling moiety. This process allows the rate of interconversion between the right-handed (P) and left-handed (M) α-helices to be altered by a factor of approximately 103 before and after the transformation. An as-obtained doubly stapled α-helical peptide, which is composed of an achiral peptide having an l-valine carboxylic acid residue at the C-terminus, a disulfide-based reversible staple, and a biphenyl-based fixed staple, adopts an (M)-rich form as a kinetically trapped state. The (M)-rich helix was subsequently transformed into the thermodynamically stable (P)-rich form in 1,1,2,2-tetrachloroethane with the half-life time (t1/2) of approximately 44 days at 25 °C. Reduction of the doubly stapled peptide with tri-n-butylphosphine in tetrahydrofuran/water (10/1, v/v) produced the corresponding singly stapled dynamic α-helical peptide bearing two thiol groups at the side chains, which underwent solvent-induced reversible helicity inversion. The resulting dithiol of the singly stapled peptide could be reoxidized to form the original doubly stapled form using 4,4'-dithiodipyridine. Furthermore, the P/M interconversion of a doubly stapled peptide with two flexible hydrocarbon-based staples is considerably more rapid than that with more rigid staples.

Homochiral π-Rich Covalent Organic Frameworks Enabled Chirality Imprinting in Conjugated Polymers: Confined Polymerization and Chiral Memory from Scratch

Optically active π-conjugated polymers (OACPs) have garnered increasing research interest for their resemblance to biological helices and intriguing chirality-related functions. Traditional methods for synthesizing involve decorating achiral conjugated polymer architectures with enantiopure side substituents through complex organic synthesis. Here, we report a new approach: the templated synthesis of unsubstituted OACPs via supramolecularly confined polymerizations of achiral monomers within nanopores of 2D or 3D chiral covalent organic frameworks (CCOFs). We show that the chiral π-rich nanospaces facilitate the in situ enantiospecific polymerization and self-propagation, akin to nonenzymatic polymerase chain reaction (PCR) system, resulting in chiral imprinting. The stacked polymer chains are kinetically inert enough to memorize the chiral information after liberating from CCOFs, and even after treatment at temperature up to 200 °C. The isolated OACPs demonstrate robust enantiodiscrimination, achieving up to 85 % ee in separating racemic amino acids. This underscores the potential of utilizing CCOFs as templates for supramolecularly imprinting optical activity into CPs, paving the way for synthetic evolution and advanced functional exploration of OACPs.

Synthesis and applications of helical polymers with dynamic and static memories of helicity

This review mainly highlights our studies on the synthesis of one-handed helical polymers with a static memory of helicity based on the noncovalent helicity induction with a helical-sense bias and subsequent memory of the helicity approach that we developed during the past decade. Apart from the previous approaches, an excess one-handed helical conformation, once induced by nonracemic molecules, is immediately retained ("memorized") after the complete removal of the nonracemic molecules, accompanied by a significant amplification of the asymmetry, providing novel switchable chiral materials for chromatographic enantioseparation and asymmetric catalysis as well as a highly sensitive colorimetric and fluorescence chiral sensor. A conceptually new one-handed helix formation in a racemic helical polymer composed of racemic repeating units through the deracemization of the pendants is described.

Synthesis and Light-Mediated Structural Disruption of an Azobenzene-Containing Helical Poly(isocyanide)

Helical poly(isocyanide)s are an important class of synthetic polymers possessing a static helical structure. Since their initial discovery, numerous examples of these helices have been fabricated. In this contribution, the synthesis of a chiral, azobenzene (azo)-containing isocyanide monomer is reported. Upon polymerization with nickel(II) catalysts, a well-defined circular dichroism (CD) trace is obtained, corresponding to the formation of a right-handed polymeric helix. The helical polymer, dissolved in chloroform and irradiated with UV light (365 nm), undergoes a cis to trans isomerization of the azobenzene side-chains. After the isomerization, a change in conformation of the helix occurs, as evidenced by CD spectroscopy. When the solution is irradiated with LED light, the polymer returns to a right-handed helical conformation. To open up the possibility for chain-end post-polymerization modification of this light-responsive system, an alkyne-functionalized nickel(II) catalyst is also used in the polymerization of the azobenzene monomer, resulting in a stimuli-responsive, terminal-alkyne-containing helical poly(isocyanide).

Сhiral and Racemic Fields Concept for Understanding of the Homochirality Origin, Asymmetric Catalysis, Chiral Superstructure Formation from Achiral Molecules, and B-Z DNA Conformational Transition

The four most important and well-studied phenomena of mirror symmetry breaking of molecules were analyzed for the first time in terms of available common features and regularities. Mirror symmetry breaking of the primary origin of biological homochirality requires the involvement of an external chiral inductor (environmental chirality). All reviewed mirror symmetry breaking phenomena were considered from that standpoint. A concept of chiral and racemic fields was highly helpful in this analysis. A chiral gravitational field in combination with a static magnetic field (Earth’s environmental conditions) may be regarded as a hypothetical long-term chiral inductor. Experimental evidences suggest a possible effect of the environmental chiral inductor as a chiral trigger on the mirror symmetry breaking effect. Also, this effect explains a conformational transition of the right-handed double DNA helix to the left-handed double DNA helix (B-Z DNA transition) as possible DNA damage.

Multivalent Dendrimers and their Differential Recognition of Short Single-Stranded DNAs of Various Length and Sequence

Polycationic dendrimers were generated through simple and versatile reversible amine/aldehyde-imine chemistry. The inherent CD spectroscopic signal arising from the helical structures of single-stranded DNA (ssDNA) undergoes a dramatic amplification in the presence of the synthesised polycationic dendrimers. Compared to the first-generation core molecule, the second-generation dendrimer shows high spectroscopic responses upon chiral recognition of short ssDNA, owing to the combination of self-assembly and multivalency effects. The maximum signal variation is reached at the molar ratio at which the ratio between the negative charges in ssDNA balance the positive charges of the dendrimers, thus the approach enables differential recognition of ssDNAs of different lengths. Altogether, these results accelerate the simple and systematic discovery of efficient adaptive molecules for biomimetic recognition of ssDNA with high accuracy.

Vapour-Induced Liquid Crystallinity and Self-Recovery Mechanochromism of Helical Block Copolymer

New molecular design of conjugated polymer that possess high sensitivity to vapour and self-recovering property against pressure is proposed. We synthesised a rod-rod diblock copolymer, poly(3-((3S)-3,7-dimethyl-octyl)-thiophene)-block-poly(4-octyl phenylisocyanide) (PTh- b -PPI), composed of a π-conjugated polymer and a rod-type helical coiled polymer. Introduction of PPI block in the block copolymer architecture enabled PTh- b -PPI film to exhibit solid-to-liquid crystal phase transition by exposure to chloroform vapour, accompanied with colour change (purple-to-yellow), which is the first report on a new phenomenon of "vapour-induced liquid crystallinity". In addition, PTh- b -PPI film showed colour change (purple-to-vermillion) during mechanical shearing, and spontaneously recovered under ambient conditions. We concluded that rod-type helical coiled polymer PPI block performs crucial roles as intrinsically vapour-induced liquid crystallinity and self-reassembling property in the architecture of PTh- b -PPI.

Helix-sense-selective co-precipitation for preparing optically active helical polymer nanoparticles/graphene oxide hybrid nanocomposites

Constructing optically active helical polymer based nanomaterials without using expensive and limited chirally helical polymers has become an extremely attractive research topic in both chemical and materials science. In this study, we prepared a series of optically active helical polymer nanoparticles/graphene oxide (OAHPNs/GO) hybrid nanocomposites through an unprecedented strategy-the co-precipitation of optically inactive helical polymers and chirally modified GO. This approach is named helix-sense-selective co-precipitation (HSSCP), in which the chirally modified GO acted as a chiral source for inducing and further stabilizing the predominantly one-handed helicity in the optically inactive helical polymers. SEM and TEM images show quite similar morphologies of all the obtained OAHPNs/GO nanocomposites; specifically, the chirally modified GO sheets were uniformly decorated with spherical polymer nanoparticles. Circular dichroism (CD) and UV-vis absorption spectra confirmed the preferentially induced helicity in the helical polymers and the optical activity of the nanocomposites. The established HSSCP strategy is thus proven to be widely applicable and is expected to produce numerous functional OAHPNs/GO nanocomposites and even the analogues.

Supramolecular Helical Systems: Helical Assemblies of Small Molecules, Foldamers, and Polymers with Chiral Amplification and Their Functions

In this review, we describe the recent advances in supramolecular helical assemblies formed from chiral and achiral small molecules, oligomers (foldamers), and helical and nonhelical polymers from the viewpoints of their formations with unique chiral phenomena, such as amplification of chirality during the dynamic helically assembled processes, properties, and specific functionalities, some of which have not been observed in or achieved by biological systems. In addition, a brief historical overview of the helical assemblies of small molecules and remarkable progress in the synthesis of single-stranded and multistranded helical foldamers and polymers, their properties, structures, and functions, mainly since 2009, will also be described.

Dynamic foldamer chemistry

Foldamers can be made more than pieces of static, conformationally uniform molecular architecture by designing into their structure the conformational dynamism characteristic of functional molecular machines. We show that these dynamic foldamers display biomimetic properties reminiscent of allosteric proteins and receptor molecules. They can translate chemical signals into conformational changes, and hence into chemical outputs such as control of reactivity and selectivity. Future developments could see dynamic foldamers operating in the membrane phase providing artificial mechanisms for communication and control that integrate synthetic chemistry into synthetic biology.

Supramolecular rulers enabling selective detection of pure short ssDNA via chiral self-assembly

TPA propellers appear to be compatible with the general requirements for amplified chiral supramolecular rulers used to determine the number of base pairs of short ssDNAs.

Flaws in foldamers: conformational uniformity and signal decay in achiral helical peptide oligomers

Although foldamers, by definition, are extended molecular structures with a well-defined conformation, minor conformers must be populated at least to some extent in solution. We present a quantitative analysis of these minor conformers for a series of helical oligomers built from achiral but helicogenic α-amino acids. By measuring the chain length dependence or chain position dependence of NMR or CD quantities that measure screw-sense preference in a helical oligomer, we quantify values for the decay constant of a conformational signal as it passes through the molecular structure. This conformational signal is a perturbation of the racemic mixture of M and P helices that such oligomers typically adopt by the inclusion of an N or C terminal chiral inducer. We show that decay constants may be very low (<1% signal loss per residue) in non-polar solvents, and we evaluate the increase in decay constant that results in polar solvents, at higher temperatures, and with more conformationally flexible residues such as Gly. Decay constants are independent of whether the signal originates from the N or the C terminus. By interpreting the decay constant in terms of the probability with which conformations containing a screw-sense reversal are populated, we quantify the populations of these alternative minor conformers within the overall ensemble of secondary structures adopted by the foldamer. We deduce helical persistence lengths for Aib polymers that allow us to show that in a non-polar solvent a peptide helix, even in the absence of chiral residues, may continue with the same screw sense for approximately 200 residues.

Drastic change in racemization barrier upon redox reactions: novel chiral-memory units based on dynamic redox systems

The helical configuration of dication dyes 2(2+) with a dihydrodibenzoxepin unit remained unchanged even at high temperature, whereas the corresponding neutral electron donors 1 with a tetrahydrophenanthroxepin skeleton easily underwent racemization. Due to their electrochemical bistability, electron exchange between 1 and 2(2+) is prohibited. Thus, the above electrochromic pairs can serve as novel chiral-memory units where redox reactions trigger switching between an "erasable/writable"-state (1) and a "memorizing"-state (2(2+)).

Ion-pair induced self-assembly in aqueous solvents

The intriguing advantages of supramolecular chemistry and particularly the application of self-assembly for the construction of defined nanostructures from small, preferably synthetically easily accessible molecules has become a promising area of modern chemistry in the last years. However, the main focus of early work was based on H-bond induced self-assembly which is limited to nonpolar organic solvents. In the past years the field started to shift more and more towards obtaining self-assembling architectures in polar solvents and even water. This tutorial review will discuss some representative examples for self-assembling systems in polar solvents in order to illustrate the different concepts and strategies that can be used. We will also briefly discuss the special properties of water as the ultimate protic solvent from the perspective of a supramolecular chemist to elucidate the challenges that this solvent still poses even today to obtain specific self-assembled nanostructures.