Fluorescent chiral selective receptor system for applications in diabetes management

Diabetes mellitus, simply called diabetes, is a metabolic disorder characterized by the presence of abnormally high concentrations of glucose in the blood. Existing methods for diagnosing diabetes rely on traditional techniques for detecting glucose in blood serum samples – a process that is usually tedious and expensive.

Molecular recognition is the science of precisely detecting specific compounds by exploiting their binding properties. Here, a receptor molecule – a kind of sensor – binds selectively to a target molecule. This process causes some reaction, say, a change in fluorescence. As a result, the target is located. Chemical sensors, specialized polymers and some catalysis techniques work on this principle.

Despite advances in molecular recognition over decades, the development of receptors to detect chiral (or asymmetric) molecules remains a challenge. Chirality results in pairs of enantiomers, which are non-superimposed “mirror” images of the same molecule. They have identical physical and chemical properties but different biological functions. Their similar structures make them difficult to distinguish from each other. Therefore, researchers must use complex and expensive techniques, such as high-performance liquid chromatography, to tell them apart.

In this light, a team of researchers, including Professor Takashi Hayashita and Dr. Yota Suzuki of the Department of Materials and Life Sciences at Sofia University, have designed a brand new fluorescence recognition method for the detection of D-glucose, a chiral monosaccharide, water. Their work was made available online on December 20, 2022 and was published at ACS sensors on January 27, 2023.

Dr. Hayashita describes the motivation behind the research: “Most approaches to design D-glucose chemical sensors require complex formulations, often have poor water solubility and sometimes low selectivity. Therefore, a new detection mechanism has been developed. ”

The researchers developed a complex composed of γ-cyclodextrin (γ-CyD), which has a cavity that provides a hydrophobic microenvironment for the spontaneous encapsulation of hydrophobic compounds in an aqueous environment. Then, they easily synthesized two types of simple hydrophobic fluorescent receptors based on monoboronic acid: a receptor based on 3-fluorophenylboronic acid (1F) and a receptor based on pyridylboronic acid (2N). They attached two molecules of either receptor to γ-CyD.

The resulting inclusion complexes (1F/γ-CyD or 2N/γ-CyD) formed a pseudo-diboronic acid moiety that selectively recognized D-glucose in water at its two positions. This strongly enhanced the fluorescence of the solution. In contrast, only weak fluorescence was observed for nine other saccharides tested, including D-fructose, D-galactose, and D-mannose, which were typical saccharides found in blood. 1F/γ-CyD and 2N/γ-CyD increased fluorescence by 2.0- and 6.3-fold, respectively, for D-glucose, relative to the enantiomer L-glucose.

“To our knowledge, 2N/γ-CyD has the highest D/L selectivity among other reported receptors based on a fluorescent diboronic acid molecule,” says Dr. Suzuki.

The researchers further investigated this phenomenon through spectral and nuclear magnetic resonance induced circular dichroism studies. They found that one molecule of D-glucose bridges the two molecules of monoboronic acid. Hardens the composite structure and enhances fluorescence. In the case of non-glucose saccharides, two different molecules are attached to the two positions of the pseudodiboronic acid moiety. As a result, the fluorescence remains weak.

In addition to high selectivity, the developed complexes also show remarkable sensitivity. 1F/γ-CyD and 2N/γ-CyD could detect concentrations of D-glucose with low limits of detection (LODs) of 1.1 μM and 1.8 μM, respectively. Therefore, both complexes can serve as simple D-glucose chemosensors. They have excellent selectivity, sensitivity and chiral selectivity.

“The developed fluorescent sensors are useful for the selective detection of D-glucose and the discrimination of glucose enantiomers. They can also serve as next-generation diagnostic systems for diabetes that can be used with small amounts of blood sample, which is a necessary feature when drawing blood Since their chemical structures are quite simple, these sensors will help develop affordable and reproducible kits for its early diagnosis,” concludes Dr. Hayashita.