Exploitation of noncovalent/dynamic covalent interactions in sensing, self-assembly and membrane transport

Abstract

This thesis reports the development of several novel chemical sensing systems, self-assembled aggregates, and membrane transporters in which noncovalent and/or dynamic covalent interactions operate.

Perylenebisimide dyes functionalized with boronic acid groups were designed as effective chirality sensors for [alpha]-hydroxy carboxylates. Binding of chiral ?-hydroxy carboxylate guests via boronate ester linkage leads to formation of optically active helical stacks of perylenebisimide dyes in water, giving diagnostic induced circular dichroism signals in the perylene absorption region.

A boronic acid-functionalized pyrene fluorophore forms excimer-emissive stacks upon cooperative binding of fluoride ion and catechol to the boron centre, allowing sensitive sensing of fluoride at ppm levels in aqueous solution which was unprecedented for boronic acids. The stabilization of boron-fluoride adduct in the aggregate and increase of Lewis acidity via catechol binding were proposed responsible for the unprecedented affinity, as supported by control experiments.

A dynamic covalent amphiphile comprised of 4-formylphenylboronic acid and octylamine forms vesicular aggregates selectively with glucose which can bind two boronic acids thus forming “Gemini-type” amphiphiles. The aggregates feature stabilization of imine bond and boronate ester linkage, with the two dynamic covalent bonds working in synergy promoting the formation of each other despite the spatial separation. The system allows selective sensing of glucose against the interference of fructose, for the first time without resorting to any synthesis.

A dynamic covalent approach was employed to transmembrane transport of amino acids by the formation of a three-component assembly. A mixture of a squaramide and a lipophilic and electrophilic aldehyde is shown to synergistically transport glycine across phospholipid vesicle membranes. The transport is proposed to occur via a hydrogen-bonded anionic glycine hemiaminal/imine, with control experiments supporting the role of hemiaminal/imine in the observed facilitated glycine transport

Finally, the issue of electrogenic/electroneutral transport mechanisms and potential proton or hydroxide transport for synthetic anionophores were examined. It is shown that depending on acidity, many synthetic anionophores can facilitate electrogenic proton or hydroxide transport. However, two newly-developed small molecules are shown to promote chloride transport without significant proton/hydroxide transport (pH gradient dissipation) at low concentrations, essentially mimicking the electrogenic cationophore valinomycin. The chloride > proton/hydroxide selective anionophores feature encapsulation of chloride ion via weak hydrogen or halogen bonds.

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ORCID for Phil Gale: orcid.org/0000-0001-9751-4910

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