Memory of macromolecular helicity assisted by interaction with achiral small molecules

Abstract

The helicity of biological macromolecules such as DNA and proteins is largely governed by the homochirality of their components (D-sugars and L-amino acids). In polymer and supramolecular chemistry, control of helicity is an attractive goal because of possible applications in materials science, chemical sensing and enantioselective catalysis 1-13 . We reported recently that macromolecular helicity can be induced in a polymer by an optically active amine 14 . Here we show that this helicity can be 'memorized' when the amine is replaced by various achiral amines. Although the maintenance of helicity in the polymer is not perfect, it can 'repair' itself over time. Small structural changes in the achiral amines influence the efficiency of helicity retention markedly.We recently reported the induction of helicity in a stereoregular, cis-transoidal poly((4-carboxyphenyl)acetylene) (poly-1; Fig. 1) 14 . The polymer possesses a large number of short helical units with many helix-reversal points, and is therefore achiral. However, in the presence of optically active amines such as (R)-2 and (S)-3 capable of interacting with the polymer's carboxy groups, a dynamic, onehanded macromolecular helicity is induced in the polymer, resulting in optical activity (Fig. 1). Analogous induction of macromolecular helicity occurs in the tobacco mosaic virus, which induces a right-handed helix in RNA on the inside of the protein shell 15 . The complexes of poly-1 with optically active amines show a characteristic, induced circular dichroism (ICD) in the ultravioletvisible region which depends on the absolute configuration of the chiral amines, and the signs corresponding to the helix-sense can be used as a probe for the chirality assignment of the amines. We now find that the macromolecular helicity of poly-1 induced by optically active amines remains even after removal and replacement of the optically active amines by achiral ones (Fig. 1). Figure 2, trace a shows the circular dichroism (CD) spectrum of poly-1 in the presence of (R)-2 (10 equiv. to monomer units of poly-1) in dimethyl sulphoxide (DMSO). The poly-1-(R)-2 complex shows a characteristic, exciton-type coupled ICD 16 . As expected, the optical activity of the poly-1-(R)-2 complex instantly disappeared when the complex was exposed to a stronger acid (such as trifluoroacetic acid), which frees the poly-1 so it reverts to the original, optically inactive (non-helical)polymer. On addition of 5 equiv. (S)-3 to the solution of poly-1-(R)-2, the ICD spontaneously changed and the signs inverted to give mirror images (Fig. 2, trace b). This clearly indicates that the induced helix of the poly-1 is dynamic in nature, and the helix-sense can be controlled with a rather small amount of the chiral amino alcohol (S)-3 ( Fig.

Chiral and Chirality Discrimination on Helical Polyacetylenes

“…The intensity of Cotton effects depends on the bulkiness of the chiral amines; the magnitude of the ICD increases with an increase in the bulkiness of the chiral amines, 24 < 23 < poly-16 was found to be able to be "memorized" when the chiral amine ((R)-20) was replaced by various achiral amines such as 2-aminoethanol and 1-butylamine in DMSO. 24 Although the maintenance of helicity in the poly-16 is not perfect just after the exchange reaction, it can "repair" itself over time. This helicity induction and memory process can be used to construct new supramolecular assemblies with controlled macromolecular helicity and desired molecules may be able to be arranged in a helical array on the poly-16.…”

Hierarchical formation of helical supramolecular polymers via stacking of hydrogen-bonded pairs in water

“…In synthetic systems, noncovalent interactions have been used to obtain well defined self-assembled architectures in organic solvents (2-7). Peripheral chiral centers in assemblies (8)(9)(10)(11) and chiral side chains attached to a polymer backbone (12)(13)(14)(15)(16)(17)(18)(19)(20) have been shown to bias chirality at the supramolecular level. Highly ordered multimolecular supramolecular structures stable in water, held together by hydrophobic interactions or with additional hydrogen bonding, are also known (21)(22)(23)(24)(25).…”

Supramolecular Structure of Extrinsically Chiral Porphyrin Hetero‐Assemblies and Achiral Analogues

“…But chirality is also receiving growing interest for the possible technological applications that span from chiroptical devices [1] to asymmetric catalysis, [2] memory systems, [3,4] and others.[5]The transfer of chiral information to achiral molecules through non-covalent interactions, expressed at a supramolecular level, has, in particular, gained great consideration. [6,7] In fact, non-covalent synthesis, especially when molecular recognition and self-assembly processes are involved, [8] presents many advantages: i) it is not time consuming, ii) it does not lead to side products, and iii) it dose not require an external energy source for reaction.…”

“…But chirality is also receiving growing interest for the possible technological applications that span from chiroptical devices [1] to asymmetric catalysis, [2] memory systems, [3,4] and others. [5] The transfer of chiral information to achiral molecules through non-covalent interactions, expressed at a supramolecular level, has, in particular, gained great consideration.…”