Synthesis and Photophysical Properties of Anthracene Conjugated Bile Acids

Abstract

Bile salts, having a unique molecular structure and unconventional amphiphilicity, belong to a special class of biosurfactants known as steroidal surfactants. These amphipathic molecules possess numerous biological functions, including eliminating cholesterol from the body, driving the flow of bile to eliminate catabolites, emulsifying fat-soluble vitamins to enable their absorption, aiding in motility and in reducing the bacteria flora found in the small intestine and biliary tract [1]. These diverse physiological functions of bile salts have opened an array of possibilities in their applications, making them important tools for active molecule delivery, metabolic disease treatments and emulsification processes in the food and drug industries [2]. The increasing demand for surfactants with unique features and enhanced functionality has led to the synthesis of a number of bile acids based biomimetic surfactants. Recently, fluorescent bile acid (FBA) derivatives are getting much attention owing to their intrinsic fluorescence in order to monitor the real-time activities in various biological processes [3]. With similar objectives, the present work aims at synthesis and characterization of fluorescent bile acids by conjugating anthracene as a fluorescent probe in the side chain of bile acids. Three anthracene conjugated bile acids based on cholic acid (CAA), deoxycholic acid (DCAA) and lithocholic acid (LCAA) have been synthesized for this purpose; and the structures were confirmed by 1H,13C NMR spectroscopy and HRMS. Prior to their applications, a detailed understanding of their photophysical properties becomes essential; thus absorption and fluorescence studies were carried out in different solvents. In all the solvents, the three derivatives showed structured absorption with maximum around 380 nm except in water. In the aqueous medium, the absorption spectrum was found to be very broad, devoid of the vibrational bands, and the maximum was found to be around 393 nm, which is relatively red shifted as compared to the other solvents. All of the FBA derivatives displayed weak emission with vibrational structures in the organic solvents, while a strong, broad and red-shifted aggregation-induced emission (AIE) in the aqueous medium (Fig.1), that increase with increase in the hydrophobicity from CAA to LCAA. The aggregation behaviour was further investigated by monitoring their absorption and fluorescence characteristics in Acetonitrile-H2O binary solvent mixtures and also using dynamic light scattering (DLS). Preliminary work suggests these FBAs as promising candidates as bio-probes.