Direct Translation of Chiral Recognition into Mechanical Motion

Abstract

Chiral recognition is of fundamental importance in nature, and also for a manifold of chemical applications. Enantioselectivity is the landmark of many new synthetic strategies and the basis of separation techniques, for example, for drugs. In all these processes the recognition manifests itself in the predominant formation of one enantiomer, be it after the synthesis or after the separation. The direct translation of chiral recognition into mechanical motions has to the best of our knowledge never been reported. The influence of chiral effector compounds or constituents on gelation, on the structure of gel materials, [1] and on phase or sol-gel transitions [2] has received increasing attention. Until now, supramolecular reactions in chemomechanical polymers has been only known to be selective with respect to regioisomeric effector molecules.[3] We describe here how interaction of a chemomechanical hydrogel with chiral effector molecules in the aqueous surrounding can lead to a volume decrease of 95 %, whereas the optical isomer triggers a change of less than 20 %, depending on the effector concentration.We used chitosan as the hydrogel as it contains aminoglucose units as chiral centers, and has been used, for example, for the separation of enantiomers.[4] The gel contains about 47 % water after preswelling with water; the gels exhibit additional volume expansion if exposed to anionic effector molecules which can bind to the protonated chitosan amino groups. Preliminary tests showed with O,O'-dibenzoyltartaric acids (DBTA, 1 c) that significant differences between the optical isomers already exist, and become smaller with increasing pH value (see the Supporting Information). Dramatically increased effects could be observed with gels containing acetic acid as cofactors. Investigations with other chemomechanical polymers have already shown that the cooperative action of different effector compounds in a gel can lead to significant alterations and enlargements of the volume changes. [5] Treatment of chitosan first with 50 mm acetic acid yields a gel with higher water content (96 %). Exposure of this gel to a number of different anionic effectors then results in significant volume contraction instead expansion.[6] The volume differences of the resulting gels formed with l-or d-DBTA can amount to be almost a factor of ten, depending on the effector concentration (Figure 1). Noticeably, the d isomer triggers large motions at quite low concentrations, at which the l enantiomer is barely active. As expected, a racemic mixture ([d + l-DBTA] = 5 mm) leads to a slightly smaller effect (À93 %) than that of the d isomer (À95 %), as the l isomer binds to a lesser degree.At higher concentrations, in contrast, the l isomer has larger effects than the d enantiomer. The concentration profiles can be ascribed to a higher affinity of the d isomer, so that the l enantiomer is fully bound only at higher concentrations. The contraction is believed to be the result of ionic cross-linking by the two carboxylate groups of the DBTA, which...

Supramolecular Interactions in Chitosan Gels

“…Only with His was there a moderate enantioselectivity in the presence of acetic acid as cofactor (Table S3). As already described in our preliminary communication, [31] the most dramatic enantioselectivity is reached with tartaric acid derivatives in which phenyl moieties provide for significant cation-π interactions with the protonated nitrogen centres of chitosan. The corresponding mode of interaction was corroborated by MAS-NMR spectra, with quite selective ring-current effects of the effector phenyl groups on the aminoglucose protons of the polymer.…”

“…[30] This compartmentalization effect allows a ten-fold sensitivity increase of the chitosan gel. [31] For example, the same contraction occurs at a 1 m DBTA effector concentration at a particle volume of about 7 mm 3 as at 0.1 m with a gel piece of about 0.2 mm 3 ( Table S4). The chemomechanical response velocity can also be increased significantly if, with smaller gel particles, the surface to volume ratio (S/V) is increased.…”

“…The presence of chiral centres in chitosan renders these gels promising candidates for enantioselective motions, which indeed have been found for the first time with these intelligent materials by using a variety of optical active compounds as possible effectors [31] (see Table S1 of the Supporting Information). The difference between tartaric acid derivatives such as O,OЈ-dibenzoyltartaric acid (DBTA) or O,OЈ-di-p-toluoyltartaric acid (DTTA) as effectors at pH 3.5 seems to be large (Table S1), but results essentially from very abrupt changes in the concentration profiles (see Figure S1).…”

Simple physical model for chemomechanics

“…[1][2][3][4] Polymers are also excellent platforms for molecular recognition of guest molecules as they often respond differently to closely related species, such as structural isomers and even enantiomers. 5,6 In most cases the molecular recognition process taking place between the polymer and its guest molecules involves diffusion of the guest molecule inside the polymer and formation of specific interactions between binding sites of the polymer and complimentary binding sites of the guest molecules. This normally involves some degree of structural reorganization followed by changes in macroscopic properties of the medium, such as the volume and the Young's modulus.…”

Chirality‐Triggered Wettability Switching on a Smart Polymer Surface

“…[ 4 , 5 ] For different chiral molecular species, many synthetic chiral recognition systems have been developed, [6][7][8] with applications such as chiral detection and separation, and asymmetric catalysis, etc. However, recognition is usually only the fi rst step, and a more appealing task is to translate the chiral signals into other processes or macroscopic properties and functions of materials, [ 9 ] which will bring great advantages to applications, but is quite challenging for chemists. Here we report the chirality-triggered wettability switching on smart copolymer fi lms containing dipeptide units.…”