Biosensors and Bioelectronics 20 (2004) 1233–1241 Bile acid amidoalcohols: simple organogelators Arto Valkonen ∗ , Manu Lahtinen, Elina Virtanen, Suvi Kaikkonen, Erkki Kolehmainen Department of Chemistry, University of Jyv¨ askyl¨ a, P.O. Box 35, FIN-40014 Jyv¨ askyl¨ a, Finland Received 12 November 2003; received in revised form 22 April 2004; accepted 10 June 2004 Available online 13 September 2004 Abstract Simple bile acid amide synthesis of lithocholic and deoxycholic acids with 2-aminoethanol and 3-aminopropanol are reported. The structural properties of these amides were examined by NMR spectroscopic, ESI-TOF mass spectral, and X-ray crystallographic methods. The gelation properties of these amides in common organic solvents and in three different water solutions were also investigated using Tyndall effect, SEM, TEM, and optical microscopy. 2-Hydroxyethylamides were found to be effective gelators in chlorinated organic solvents and 3- hydroxypropylamides in aromatic solvents. Both derivatives thicken neutral and acidic water solutions. © 2004 Elsevier B.V. All rights reserved. Keywords: Bile acid; Amino alcohol; Structural analysis; Low molecular mass organogelator; Gel 1. Introduction Thirty years ago Paul Flory defined a substance to be a gel if it has a macroscopic, continuous structure, and it is rheologically solidlike (Flory, 1974). Although cova- lently crosslinked polymeric and biomacromolecular gels have been known and applied for a long time many aspects of gels are still poorly understood (Maitra et al., 2001; Prost and Rondelez, 1991). In many cases in preparation of gel, it is dif- ficult to specify the conditions and the structural requirements for the gelator. The differences or the relationships between the packing arrangements of gelator molecules in crystalline states and in aggregates in the gels as well as the influence of it on the properties of the gels are more or less unanswered (Terech and Weiss, 1997; Abdallah and Weiss, 2000). Driving forces of aggregation of gelators in aqueous and in nonaque- ous systems are very different. In water hydrophobic attrac- tions are the most important (Nemethy and Scheraga, 1962), but in nonaqueous media dipolar interactions and possibly intermolecular hydrogen or metal-coordination bonding are necessary (Ruckenstein and Nagarajan, 1980). ∗ Corresponding author. Tel.: +358 14 2602670; fax: +358 14 2602501. E-mail address: amvalkon@cc.jyu.fi (A. Valkonen). Low molecular weight gelators (M w < 1000) have attracted interest only upon the last 20 years. Many potential applica- tions of these rather new gelators have been predicted and they have also been the stimulus of rapidly growing, detailed research. Most mono-component low molecular weight gela- tors published in literature are organic fatty acid, steroid, an- thryl, amino acid, or organometallic derivatives (Terech and Weiss, 1997), and are called as organogelators. Organogel systems are good examples of supramolecular chemistry. In solution, low molecular weight organogelators self-assemble (i.e., self-aggregate) by non-covalent interactions, such as hy- drogen bonding, – stacking, electrostatic, van der Waals, or dipole–dipole. Most of such organogelators have hydrogen bond donors and acceptors that promote self-aggregation and subsequent, fiberlike, three-dimensional network formation (Clavier et al., 1998; Estroff and Hamilton, 2000). Due to lim- ited solubilities of gelators at room temperature organogels are usually prepared by warming a mixture of a gelator and a liquid until the solid dissolves and then cooling the solu- tion. During warming the gelator molecules self-aggregate into linear fibers, which form the three-dimensional network by entangling during cooling. The network formed immo- bilises the liquid component to a variable extent by trapping the solvent molecules into interstitial spaces. The network is 0956-5663/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.bios.2004.06.029