Chapter 1. An Overview of Bile Acid Science This chapter deals with an overview of bile acid science (cholanology) compiling elevant literature review, covering bile acid chemistry, biosynthesis, bile salt evolution, physiology and medicinal values. Figure 1. (a) Digestive system; (b) enterohepatic circulation and (c) cholic acid Bile acids are the end products of cholesterol metabolism, secreted in the liver and stored in the gall bladder (Figure 1). They are normally conjugated with glycine (75%) or taurine (25%). Because of their facially amphiphilic nature, bile salts tend to form micellar aggregates in aqueous solution. They have remarkable ability to transform lamellar array of lipids into mixed micelles. All primary bile acids seem to have three features in common: (1) They are major products of cholesterol metabolism; (ii) they are secreted into the bile largely in a conjugated form and (iii) the conjugates are membrane impermeable, water soluble, amphiphilic molecules. Recent advances in molecular biology have greatly accelerated the knowledge relating to the significance of bile salts in a number of physiological functions. The new role of bile salts as pheromones and ligands for nuclear hormone receptors has been discussed. Chapter 2. Pythocholic Acid: A Major Constituent of Python’s Bile and 16α-Hydroxycholic Acid: A Minor Constituent of Avian’s Bile The first chemical synthesis of pythocholic acid (major constituent of python’s bile) and 16α-Hydroxycholic acid (a minor constituent of avian’s bile) were accomplished starting from cholic acid with overall yields of 5.0% and 5.5%, respectively. A biomimetic remote functionalization strategy was utilized as a key step to achieve the selective chlorination at C-17. Dehydrochlorination of 17-chlorosteroid resulted in the Δ16 olefin. Hydroboration-oxidation of the Δ16 olefin followed by the selective oxidation of the pentol under TEMPO mediated oxidation resulted in an ε-lactone. Hydrolysis of the lactone using 5% KOH in MeOH furnished the 16α-Hydroxycholic acid. On the other hand, selective oxidation of 7-OH of the lactone was achieved using N-bromosuccinimide in acetone/H2O to yield the 7-keto lactone. The ketolactone when subjected to the Huang-Minlon modification of the Wolf-Kishner reduction furnished pythocholic acid. Pythocholic acid showed unusual aggregation behavior and high cholesterol solubilization ability, compared to other trihydroxy bile acids. Chapter 3. 16-Epi-pythocholic acid: An Unnatural Analogue of Pythocholic Acid The synthesis of 16-epi-pythocholic acid, an unnatural analogue of pythocholic acid, was accomplished starting from cholic acid. Cholic acid was converted to Δ8-14) olefin using ZnCl2 in refluxing acetone. Methylation followed by isomerization in CHCl3 by passing dry. HCl at -78 oC resulted in the Δ14 olefin. Allylic oxidation using Na2Cr2O7.2H2O in the presence of N-hydroxysuccinimide in acetone furnished the enone. Selective reduction of the olefin using Pd/C-H2 resulted in 16-Epi-pythocholic acid the 16-keto steroid. NaBH4 reduction of this ketone in MeOH/THF (2:1 v/v) followed by hydrolysis produced the 16-OH bile acid. Analysis of spectral data confirmed that it is a 16β-epimer of pythocholic acid (3α,12α,16β-trihydroxy-5β-cholan-24-oic acid). Critical micellar concentration and cholesterol solubilization properties were studied. Chapter 4. Low Molecular Mass Organogelators Derived from Simple Esters of Cholic Acid This chapter begins with an introduction to low molecular mass organogelators and highlights their applications. Serendipitous gelation of a number of organic solvents by allyl cholate and the design of related simple esters of cholic acid are discussed. A series of simple and easily accessible esters of bile acids were prepared. Ethyl cholate and propyl cholate were found to immobilize a variety of organic solvents like benzene, toluene, xylene, mesitylene, 1,2-dichlorbenzene (DCB) and chlorobenzene (Figure 2). The morphology of the xerogels was well characterized using SEM, AFM and polarizing optical microscopy (POM) techniques, Which revealed the presence of highly entangled self-assembled 3D-fibrillar network (SAFINs). The fiber diameter was found to vary between 300-500 nm. Direct imaging of the collapse of this fibrillar network and direct observation of the evolution of nanofibers was achieved for the first time using variable temperature POM techniques. FT-IR studies, X-ray powder diffraction and variable temperature POM studies revealed the resemblance of SAFINs to the bulk solid. Formation of helical fibrillar network was observed in SEM images and the existence of chiral aggregates was confirmed by induced circular dichroism experiment using achiral Reichardt’s dye as the chromophore. Chapter 5. Ambidextrous Gelators Derived from Spacer Linked Bile Acid Derivatives Based on our observation of simple esters of cholic acid as organogelators a rational design of a series of spacer linked dimers and tripodal derivatives were carried out. Some of these molecules formed highly transparent gels in solvents like haloarenes, anisole, xylene and dibromoalkanes. These molecules also showed rapid gelation in DMF/H2O and DMSO/H2O mixtures in varying proportions of water and the co-solvent. These types of gelators are known as ambidextrous gelators. The xerogels were characterized using SEM, TEM and POM techniques and the presence of highly entangled 3D-fibrillar network (Figure 3) was observed. XRPD showed crystalline nature of bulk solid, whereas the xerogels were shown to lose their crystalline nature. (For figures and structural formula pl see the pdf file.)

Chapter 1: Functional and Responsive Supramolecular Gels In this chapter ‘supramolecular gels’ derived from small organic molecules with molecular mass of typically less than 2000 daltons are discussed. Representative examples of various low molecular weight gelators based on their natural availability and also divergent functionalities are mentioned (Scheme 1). Scheme 1 Advances in the recent years have been very rapid in the field of supramolecular chemistry of gels giving rise to ‘Tunable responsive gels’. Control of the gel property in a reversible fashion has been the highlight of responsive gels. A few of the gels which are responsive towards various stimuli such as pH, photoirradiation, cations, anions, neutral species have been discussed. Advances and scope of supramolecular gels in various applications have also been mentioned in detail with respective examples. Utilities of supramolecular gels in synthesis of nanostructures, in biology and medicine, enzyme recognition, catalysis etc are discussed. (Scheme 2). Chapter 2: Charge transfer triggered organogels of bis(bile acid)anthracene conjugates and 2,4.7-trinitrofluorenone. In this chapter the study involves the synthesis of a special class of anthracene based steroidal derivatives. The appending of two amphiphilic bile acid units imparts a unique hydrophobic/ hydrophilic balance on the chromophore. The 2,3-didecyloxyanthracene (DDOA) was reported to be a gelator of various organic solvents but none of the three bile acid derivatives of anthracene synthesized was a gelator on its own. It was also observed that dialkoxy (propyl, heptyl, decyl) derivatives of anthracene formed strong charge-transfer gels in the presence of 2,4,7-trinitrofluorenone (TNF). The addition of electron deficient TNF to the steroidal derivatives of anthracene resulted in the gelation of some specific organic solvents. The driving force behind the gel formation resulted from the charge-transfer (CT) interaction between the electron rich anthracene and electron deficient fluorenone. Figure 1. Chemical structures of 2,3-bis(bile acid)anthracenes and TNF (centre), a scanning electronic microscopy image of xerogels prepared from bis(deoxycholyl)anthracene and TNF (left) and a photograph of the gel of bis(deoxycholyl)anthracene and TNF in n-octanol. Thermochromic property (during sol to gel phase transition), absorption and variable temperature fluorescence measurements supported CT interaction. Thermal stability studies and dynamic rheology experiments confirmed that CT gels were thermally most stable and mechanically stronger with equi-molar amounts of the two components. Stiffness values obtained from rheological experiments also suggested that the gels were viscoelastic solids. Chapter 3(A): Tb(III) sensitization in an organogel matrix: Selective luminescence quenching by an aromatic nitro derivative In this chapter the discovery of metallo organogel formation by mixing methanolic solutions of Tb(OAc)3 and sodium deoxycholate (NaDCh) has been explored. Sensitization of Tb3+ was observed by doping micromolar quantities of 2,3-dihydroxynaphthalene (DHN). Mechanical properties of Tb3+-DCh gels were investigated by rheology at three different ratios of Tb3+ and DCh. It was observed that increasing in the Tb3+ to DCh ratio increased the mechanical property of the gels. Time delayed emission spectra were recorded with increasing concentration of DHN and luminescence increase was noticed in a linear fashion. Importance of gel matrix was demonstrated by measuring the Tb3+ luminescence at fixed concentration (5 mM) with/without DHN in the solution and gel media. Figure 2:: Schematic representation of Tb3+ sensitization by DHN. Photograph (right)) of the Tb3++-DCh (5/15 mM) gels with (a) 50 µM DHHN (b) No DHHN under UVV (365 nm). Sensitization by an electron rich chromophore created interest in us to dope relatively electron deficient compounds into the gel matrix for possible quenching off Tb3+-luminescence. Among the electron deficient analytes screened included 1,5-difluro-2,4-diinitrobenzenne (DFDNB)), 2,4 dinitrophenol (DNPPh), p-nitrobenzaldehydde (p-NB), 2,4,6-trinitrootoluene (TTNT) and 22,4,7¬trinitrofluuorenone (TTNF). Microscopy studies such as AFM, TEMM and SEMM revealed highly entangled fibrous network in the morphology of Tb3+--DCh xerogel. Solid state luminescence experiments suggested that sensiitization was observed in the xerogels and extent of sensitization was comparable to that of the gel state. Xerogel soaking studies inferred the strong adherence of the DHNN to the gel fibres. Chapter 3(B): Anion dependent structural, morphological and mechanical features of Ln(III)-Cholate gels In this chapter the counter anion influence on various aspects of hydrogels has been discussed. It has been reported from our laboratory that mixing of aqueous solutions of sodium cholate (15 mM) and various lanthanide acetates (5 mM) followed by sonication resulted in either transparent or transluscent gels. Unsurprisingly we found that aqueous solutions of lanthanide nitrates and lanthanide chlorides also formed hydrogels upon mixing with sodium cholate (Figure 33). Dried films of Tb3++-cholate and Eu3+-cholate gels prepared from their respective nitrate salts displayed birefringent structures under polarizing optical microscopy (POM). But no significant textures of any type were observed in the case of gels prepared from either chloride or acetate salts. Figure 3:: Photographs of the hydrogels prepared by mixing of aqueous solutions of various salts Tb33+ and Eu3+ with sodium cholate solutions. Scanning electron microscopic images exhibited fibrous structures for all the xerogels in the morphology. Atomic force microscopy and transmission electron microscopy measurements revealed helical morphology for xerogels prepared from nitrate salts where as flat tape-like cross linkage was observed for chloride or acetate based xerogels. Anion effect on mechanical properties was significant in the sense that gels prepared from acetate salts displayed highest mechanical strength followed by nitrate based gels which were stronger than that of chloride based gels. Titration of sodium cholate solution with various lanthanide salt solutions gave the direct evidence of thee pH variation as a function of the anions. Figure 4: TEM images of xerogels prepared from gels of nitrate salts of Tb3+ (left) and Eu3+ (right) Chapter 4: Design, synthesis a nd study of bile acid ‘click’ conjugates of perylene bisimides (PBIs) and naphthalene bisimides (NBIs) In this chapter the synthesis of novel bile acid derivatives of perylene and naphthalene bisimides is discussed. The ‘click’ chemistry procedure was used to link bile acid groups on to the chromophores. Azide derivatives of PBIs and NBIs were prepared inn 3 step methods which were coupled to propargyl esters of bile acids by following standard ‘click chemistry’ protocols to achieve the target molecules (Scheme 3). Scheme 3 The studies conducted mainly focused on Cholic acid (CA) conjugates of PBIs and NBIs. Steady state absorption and emission studies of CA conjugates were performed in 10% MeOH/DCM system. POM and fluorescence images showed red emissive aggregates in case of PBI films. TEM measurements revealed uniform aggregate sizes for both the films of PBI(CA)2 and NBI(CA)2. SEM and AFM (Fig 5) studies exhibited spherical aggregates of diameter around 100-200 nm for PBI(CA)2 films where as aggregates of diameter around 500-700 nm were observed for NBI(CA)2 films. Figure 5: AFM images and their corresponding height profiles of PBI films (left) and NBI films (right)