Elusive sulfate anions captured by luminous chains

The university’s Dr. Krzysztof Bąk and Dr. Michał Chmielewski, in cooperation with theoretician Professor Bartosz Trzaskowski, developed a new switchable compound - a catenane capable of selective binding and fluorescent detection of the sulfate anion. They published the results of their research in Chemical Science.

Sulfuric acid (VI) is called the blood of the chemical industry. It is used, for example, in the production of paper, medicines, artificial and explosive materials. The presence of sulfuric acid, its salts, esters or derivatives in a solution can be recognized by the presence of the sulfate anion.

'Sulfate is the fourth most abundant anion in human plasma (serum levels 0.3–0.5 mM) and has important functions in human physiology,’ the scientists write. The ion is present in the body when regulating functionality of proteins, hormones and signalling molecules. It is also needed in the process of blood clotting and the formation of connective tissues. 'Unusually low levels of sulfate have been found in the plasma of patients with rheumatoid arthritis and irritable bowel disease,’ the researchers point out. 

In industry, sulfate is known to interfere with the remediation of nuclear waste.

They add: ‘Sulfate-selective sensors could be used to elucidate the diverse roles of sulfate in living organisms, allow improved diagnostics, and facilitate monitoring of environmental samples.’

Although living organisms have developed proteins that capture the sulfate ion, there have been few ideas on how to recreate this process using receptors obtained in the laboratory in such a way that the anion capture is signalled, for example, by fluorescence (light emission), which is very convenient to observe.

'Sulfate remains one of the most challenging anionic targets for molecular recognition in water (...) Therefore, inspired by the sulfate binding protein, which strongly and selectively binds sulfate in water by encapsulating the anion using a network of hydrogen bonds,’ the researchers write. 

They figured out that incorporation of strong hydrogen bond donors inside a three-dimensional cavity of an electroneutral fluorescent catenane could mimic these characteristics. Sulfate anions will be drawn into such a structure, and once they attach there, fluorescence appears in the structure - light of a different wavelength will appear in the solution illuminated with appropriate light.

Catenanes (from Latin catena - chain) are structures that resemble chains. The links of these tiny chains are ring-shaped molecules. Since these rings are only woven around each other and not chemically bonded, they can rotate relative to each other. Such compounds are used, for example, to build molecular machines, i.e. molecules that work similarly to engines and switches known from the macroscopic world that perform their work at the atomic level.

Mechanically linked catenane rings form a three-dimensional gap that resembles a protein binding site, and - like proteins - they can interact strongly and selectively with their substrates. Thanks to these properties, catenanes are used as new molecular sensors and catalysts.

The scientists focused on constructing catenanes for selective binding of the sulfate anion. 'We were fascinated by the tetrahedral geometry of sulfate and its ability to force molecular self-assembly, i.e. spontaneous arrangement of molecules around this anion. It turned out that the relatively simple organic molecules obtained in our laboratory align perpendicularly to each other thanks to the interaction with this anion,’ says Dr. Chmielewski. He adds: 'In our work, we have shown that macrocyclisation of molecules arranged in this way allows us to obtain a catenane.

'We also used the unique properties of sulfate to construct a molecular switch. We have shown that we can control the relative position of the rings in our catenane by changing the pH. This means that in response to an external stimulus in the form of a change in pH, the catenane can adopt a compact structure, in which both rings bind the same anion, or an extended structure, in which each ring binds a different anion, and their binding sites try to move as far away from each other as possible,’ adds Dr. Krzysztof Bąk, the first author of the publication. 'This is the first example of using an anion in this role'.

The team led by Dr. Michał Chmielewski is currently working on the use of catenanes to transport anions across biological membranes. Thanks to their excellent ability to bind anions, in the future catenanes may replace damaged anion transport proteins, e.g. in people with cystic fibrosis.

PAP - Science in Poland, Ludwika Tomala

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