Complexation of Sodium Cholate and Sodium Deoxycholate by -Cyclodextrin and Derivatives † P. Ramos Cabrer, ‡ E. Alvarez-Parrilla, ‡ F. Meijide, ‡ J. A. Seijas, § E. Rodrı ´guez Nu ´n ˜ ez, | and J. Va ´ zquez Tato* ,‡ Facultad de Ciencias, Departamentos de Quı ´mica Fı ´sica, Quı ´mica Orga ´ nica, and Fı ´sica Aplicada, Universidad de Santiago de Compostela, Lugo, Spain Received December 16, 1998. In Final Form: April 29, 1999 The complexation behavior of two bile saltsssodium cholate (NaC) and sodium deoxycholate (NaDC)s with -cyclodextrin (-CD), 6-deoxy-6-amino--cyclodextrin (-CDNH2), and dimer I (N,N′-bis(6-deoxy- -cyclodextrin)pyromellic acid diamide) was studied by NMR techniques. Complexes formed between -CD and -CDNH2 with NaC and NaDC have 1:1 and 2:1 (host:guest) stoichiometries, respectively. Complexes with -CDNH2 show higher equilibrium constants than those with -CD because of the electrostatic effect of the protonated amine group. Dimer I showed 1:2 and n:n stoichiometries with NaC and NaDC, respectively. ROESY spectra stated that bile salts enter first with their 5-C ring forward the inner cavity by the side of the secondary hydroxyl groups of cyclodextrins. In the complexes formed with -CDNH2, the steroid body of the bile salt enters deeper in the cavity, while the carboxylated side chain is extended toward the protonated amine group at C-6, allowing an electrostatic interaction between both groups. In the case of the 2:1 stoichiometry, the second cyclodextrin complexes ring A of the steroid body. Introduction Bile salts are involved in one of the most important pathways for the metabolism and excretion of cholesterol in mammals and represent an example of the liver capacity to convert lipid-soluble material into excretable water- soluble products. 1 Bile salts have a characteristic steroid structure, with a side chain at C-17, methyl groups at C-10, C-13, and C-20, different numbers of hydroxyl groups at C-3, C-7, and C-12, and a carboxylic acid at C-23, which can or cannot be conjugated with an amino acid. 1 Because of their amphipatic nature, they behave as biosurfactants and are used as drugs in gallstone disease treatments. Among the bile salts, the most often studied are sodium cholate (NaC) and sodium deoxycholate (NaDC) 2,3 (Figure 1a). Cyclodextrins are cyclic oligomers built up from 6, 7, or 8 glucopyranose units, linked by R-(1-4)-glycosidic link- ages, named R, , and γ-cyclodextrins, respectively (Figure 1b). They form inclusion complexes in water with a variety of organic molecules, a property used to increase the bioavailability of poorly soluble drugs. 4 Several cyclodex- trin dimers have been synthesized, and their effect in the inclusion complex formation has been studied. In general, dimers show higher binding constants in comparison with cyclodextrins. 5-10 Dimers can be divided into three groups depending on the side of the cyclodextrin where the linking is carried out: head to head, tail to tail, and head to tail, where head and tail are the primary and secondary hydroxyl sites of cyclodextrin. 9 Several linking bridges have been described: diamine, diether, diester, disulfide, dithioether, imidazolium, and diamide are the most common ones. 11 The complexation of surfactants by cyclodextrins pro- duces a change in their physicochemical properties, because of the insertion of the hydrophobic chain into the cyclodextrin cavity. A large number of studies have been carried out to study their complexation behavior by a variety of experimental techniques. 12 However, the num- ber of studies on the complexation of biosurfactants, such as bile acids, is really small, and particularly the structure of complexes is still unknown. Yang and Breslow 13 reported that they could not determine the stability constant between -cyclodextrin (-CD) and cholic acid by titration calorimetry. Mucci et al. 14,15 reported that cholic acid showed lower stability constants with hydroxypropyl-- cyclodextrin (HPCD) compared with those for chenode- oxycholic and ursodeoxycholic acids, because of the hydroxyl group at C-12, which is close to the complexation site. They could not confirm the inclusion of bile acids in HPCD by differential scanning calorimetry and X-ray † Part of this paper was presented at the 9th International Symposium on Cyclodextrins, Santiago de Compostela, Spain, May 1998. ‡ Departamento de Quı ´mica Fı ´sica. § Departamento de Quı ´mica Orga ´ nica. | Departamento de Fı ´sica Aplicada. (1) Danielsson, H.; Sjo ¨vall, J. Sterols and Bile Acids; Elsevier Sci. Ltd.: Amsterdam, The Netherlands, 1985; Chapter 13. (2) Campredon, M.; Quiroa, V.; Thevand, A.; Allouche, A.; Pouzard, G. Magn. Reson. Chem. 1986, 24, 624. (3) Coello, A.; Meijide, F.; Rodrı ´guez Nu ´n ˜ ez, E.; Va ´ zquez Tato, J. J. Pharm. Sci. 1996, 85, 9. (4) Szejtli, J.; Osa, T. Comprehensive Supramolecular Chemistry Vol. 3: Cyclodextrins; Elsevier Sci. Ltd.: Oxford, U.K., 1996. (5) Harada, A.; Furue, M.; Nozakura, S. I. Polym. J. 1980, 12, 29. (6) Coates, J. H.; Easton, C. J.; van Eyk, S. J.; Lincoln, S. F. M.; May, B. L.; Whalland, C. B.; Williams, M. L. J. 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Inclusion Phenom. Mol. Recognit. Chem. 1996, 26, 233. 5489 Langmuir 1999, 15, 5489-5495 10.1021/la9817359 CCC: $18.00 © 1999 American Chemical Society Published on Web 07/13/1999