Design and synthesis of multivalent receptors towards the detection of biologically relevant carbohydrates

Di Pasquale, Alice (2022). Design and synthesis of multivalent receptors towards the detection of biologically relevant carbohydrates. University of Birmingham. Ph.D.

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Abstract

Changes in the glycan phenotypes of cells is one of the main hallmarks of cancer. Glycans are carbohydrate chains linked to protein carriers, forming glycoproteins. Under or overexpression of glycans has been observed in cancer cells, alongside with expression of entirely new glycans. Many biomarkers currently employed in cancer diagnostics are proteins which are also expressed in benign conditions, resulting in high rates of false positive diagnoses and the associated negative consequences on patients. Conversely to currently employed protein biomarkers, there are particular glycoforms, expressed solely by cancer cells, which are tumour specific and therefore present great potential in cancer diagnostics. In particular, a higher degree of sialylation has been closely associated to cancer. This results in a higher expression of sialosides, glycans terminating with a sialic acid unit. The sialic acid unit is linked to the carbohydrate chain in α2-3 or α2-6 fashion with these glycoforms being expressed by altered and healthy cells, respectively. Thus, the selective detection of α2-3 sialosides, over the α2-6 glycoforms, can be exploited in cancer diagnostics. Furthermore, sialic acid, in its unbound free form, is also a relevant biomarker for certain neurodegenerative diseases and alcoholism. As a result, sialic acid is an important saccharide, for which detection is key in a variety of pathological conditions.

The detection of carbohydrates for diagnostics purposes can be achieved with natural or synthetic lectins. Among these, boron-based receptors, such as phenylboronic acids and benzoboroxoles, present the key advantage of forming covalent complexes with carbohydrates. Boron-based receptors have been known for decades for their sugar-binding properties and have been extensively employed as receptors for neutral monosaccharides. Nonetheless, there is no consensus in the literature with regards to the way these receptors bind to sialic acid. Therefore, it was pivotal to accurately identify the site via which boron-based receptors bind to sialic acid.

Different analogues of sialic acid were employed in calorimetric and spectroscopic studies allowing the unequivocal identification of the α-hydroxyacid group as the only site of binding. This key finding enabled the design and synthesis of small functionalised benzoboroxole receptors to selectively target this monosaccharide. In particular, the functionalisation with charged groups afforded benzoboroxole receptors which display up to a 4.5-fold activity compared to the non-functionalised benzoboroxole. This study resulted in a binding model being postulated highlighting the value of multiple cooperative interactions in sugar recognition.

Following on from this, the focus was moved on the detection of small sialosides representing biologically relevant sialylated glycans. Sialylated trisaccharide epitopes, SAα2-3LacNAc and SAα2-6LacNAc, were enzymatically synthesised to be employed as target molecules. Furthermore, dynamic combinatorial chemistry (DCC) was reviewed as a technique for the generation of carbohydrate receptors. In order to target the aforementioned oligosaccharides, a scaffold-based approach was adopted to provide interactions with a high degree of multivalency. A peptidomimetic scaffold was designed and synthesised and a series of sugar-binding small molecules were selected creating the foundations for the future application of the DCC approach in glycosensing.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):