Synthesis, Characterisation and DFT Calculations of Saccharide-BODIPY conjugates

Abstract

Saccharides, comprising one of the largest groups within the carbohydrate family, are found throughout nature and are an essential component to all life. For example, on the surface of cells they function as receptors in cell-to-cell recognition and the expression of certain types of glycoproteins are associated with cancer. An in depth understanding of their chemical behaviour is necessary in order to advance the accurate identification of biologically relevant saccharides. The research presented here is therefore to further this understanding by synthesizing a fluorescent marker for the selective binding to monosaccharides using the well-known F-BODIPY (4,4-difluoro-4-boro-3a,4a-s-indacene) dye as the fluorophore. Chapter 1 gives an introduction to saccharide chemistry and information on their molecular characteristics necessary for this study as well as the current methods used for their sensing, which typically involve the boronic acid moiety due to its favourable binding with adjacent diol groups. Next, the F-BODIPY is introduced as one such saccharide reporter with an overview on their synthesis and properties. Chapter 2 reports on the process of establishing a new synthetic method for conjugation of the BODIPY and monosaccharide. The synthesis and characterisation of two different BODIPY dyes is then detailed which are used as starting material in the subsequent saccharide conjugation. The first is a simple F-BODIPY with four methyl groups strategically placed around the core to increase the fluorescence and second with additional of styryl groups (F-styrylBODIPY) at the conjugation site for the purpose of introducing steric hindrance to limit possible adducts during conjugation. Both dyes are then functionalised with methoxy groups (MeO-BODIPY, MeO-styrylBODIPY) affording the necessary scaffold for monosaccharide conjugation. Crystal structures were obtained for both these dyes and their photophysical properties analysed together with DFT calculations. Chapter 3 describes conjugation between the aldopentoses D-ribose and D-xylose and MeO-BODIPY using the new methodology. These monosaccharides were tested first due to their higher solubility compared to hexoses. The results were reproducible and indicated some favourability to specific saccharide conformations. A detailed analysis of both 1D and 2D NMR techniques applied to the conjugates as well as DFT calculations is then presented iv assigning the geometry of the structure. Their spectral properties are measured and compared with the starting MeO-BODIPY. Chapter 4 extends this study to include the reactions between MeO-BODIPY and representatives of the hexose D-series including fructose, glucose, mannose and galactose. A number of different conjugates formed with each in reasonable to good yields and were fully characterised by both 1D and 2D NMR and DFT techniques. The spectral properties of these conjugates were also measured. Two crystal structures were obtained from the conjugation with D-fructose and D-glucose confirming the observed optimised geometries. Chapter 5 looks at the second BODIPY type with styryl attachments and its reaction with D-fructose. The resulting conjugates were fully characterised as before and show a similar isomer preference when binding compared to reactions between MeO-BODIPY and that same monosaccharide. Chapter 6 is an overall summary of the main research outcomes.