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Sodium Taurodeoxycholate Structure from Solid to Liquid Phase Luciano Galantini,* Edoardo Giglio, Camillo La Mesa, Nicolae Viorel Pavel, and Francesco Punzo Dipartimento di Chimica, Universita` di Roma “La Sapienza”, P.le Aldo Moro 5, 00185 Roma, Italy Received October 8, 2001. In Final Form: January 14, 2002 A 7/1 helix was previously identified by X-ray diffraction analysis as the structural unit of a sodium taurodeoxycholate (NaTDC) fiber drawn from an aqueous micellar solution and proposed as a model of NaTDC micellar aggregates. The repetitive unit of the 7/1 helix is a trimer formed by three NaTDC molecules related by a 3-fold rotation axis. This model was supported by a study of NaTDC aqueous micellar solutions performed with different experimental techniques. Moreover, the phase behavior of the NaTDC-water system was recently studied as a function of concentration and temperature with emphasis on concentrated regions beyond the isotropic solution phase. In the present paper, X-ray measurements carried out on NaTDC concentrated aqueous solutions and fibrils that grow from these solutions are presented. Experimental results strongly support the following. The NaTDC aggregates are helices formed by trimers both in concentrated aqueous solutions and in fibrils. Fibrils are composed of the same 7/1 helices of the fiber, whereas concentrated aqueous solutions contain helices with a larger cross section and a shorter identity period along the helical axis than the 7/1 helices. The trimer repeat along the helical axis and the radius are, respectively, about 6.4 and 10.2 Å in the fibrils and 3.6 and 15.85 Å in the concentrated aqueous solutions. Both of the helices are packed into rectangular unit cells which can be derived from trigonal or hexagonal unit cells. Because the trimer seems to flatten by decreasing the concentration, the radius of the structural unit in the isotropic solution phase should be greater than about 15.85 Å. These results permit a reasonable guess about the structural evolution of NaTDC aggregates from the solid to the liquid phase. Some suggestions about the sodium deoxycholate and taurocholate behavior are also provided.
Introduction Some bile salts give rise to transitions [aqueous micellar solution] f [gel] f [fiber] and, sometimes, [fiber] f [crystal]. Among others, there are the sodium salts of 3R,12R-dihydroxy-5β-cholan-24-oic acid (sodium deoxycholate, NaDC),1,2 3R,12R-dihydroxy-5β-cholanoyltaurine (sodium taurodeoxycholate, NaTDC),3,4 3R,12R-dihydroxy5β-cholanoylglycine (sodium glycodeoxycholate, NaGDC),4 3R,7R,12R-trihydroxy-5β-cholanoyltaurine (sodium taurocholate, NaTC),5 and 3R,7R,12R-trihydroxy-5β-cholanoylglycine (sodium glycocholate),6 together with lithium, potassium, and rubidium deoxycholate,2,7 rubidium4 and calcium8 taurodeoxycholate, and rubidium glycodeoxycholate.4 Glassy and birefringent fibers can be drawn from gels or aqueous micellar solutions near the gelation point, and sometimes crystals can be obtained from fibers by aging. Models with well-established geometry were obtained by solving several crystal3,9-16 and fiber2-8,17 structures * Corresponding author. Tel: (+int.)-6-49913687. Fax: (+int.)6-490631. E-mail:
[email protected]. (1) Conte, G.; Di Blasi, R.; Giglio, E.; Parretta, A.; Pavel, N. V. J. Phys. Chem. 1984, 88, 5720. (2) D’Archivio, A. A.; Galantini, L.; Giglio, E.; Jover, A. Langmuir 1998, 14, 4776. (3) D’Alagni, M.; D’Archivio, A. A.; Giglio, E.; Scaramuzza, L. J. Phys. Chem. 1994, 98, 343. (4) Briganti, G.; D’Archivio, A. A.; Galantini, L.; Giglio, E. Langmuir 1996, 12, 1180. (5) Bottari, E.; D’Archivio, A. A.; Festa, M. R.; Galantini, L.; Giglio, E. Langmuir 1999, 15, 2996. (6) Bonincontro, A.; D’Archivio, A. A.; Galantini, L.; Giglio, E.; Punzo, F. Langmuir 2000, 16, 10436. (7) Bonincontro, A.; D’Archivio, A. A.; Galantini, L.; Giglio, E.; Punzo, F. J. Phys. Chem. B 1999, 103, 4986. (8) D’Archivio, A. A.; Galantini, L.; Giglio, E. Langmuir 1997, 13, 4197.
and were used as micellar aggregate models that were verified in the study of aqueous micellar solutions. Obviously, fiber models should be more reliable than crystal ones, the fiber being more similar than the crystal to the aqueous micellar solution. As a matter of fact, [aqueous micellar solution] f [gel] f [fiber] transitions could occur without drastic structural changes, because the gel is obtained by concentrating the aqueous micellar solution and the fiber is drawn from the gel by means of the weak force of gravity. Fibers of NaTDC, NaGDC, rubidium taurodeoxycholate and glycodeoxycholate,4 and calcium taurodeoxycholate8 are formed by 7/1 clockwise helices stabilized by strong polar interactions involving in particular the cations as inferred mainly from electrolytic conductance and dielectric measurements.6,7 A trimer (Figure 1a) is the basic building block of the 7/1 helix. In agreement with the above model, on one hand NaTDC micellar aggregation numbers, (9) Campanelli, A. R.; Candeloro De Sanctis, S.; Giglio, E.; Petriconi, S. Acta Crystallogr., Sect. C 1984, 40, 631. (10) Campanelli, A. R.; Candeloro De Sanctis, S.; Giglio, E.; Scaramuzza, L. J. Lipid Res. 1987, 28, 483. (11) Campanelli, A. R.; Candeloro De Sanctis, S.; Chiessi, E.; D’Alagni, M.; Giglio, E.; Scaramuzza, L. J. Phys. Chem. 1989, 93, 1536. (12) Campanelli, A. R.; Candeloro De Sanctis, S.; Galantini, L.; Giglio, E.; Scaramuzza, L. J. Inclusion Phenom. Mol. Recognit. Chem. 1991, 10, 367. (13) Campanelli, A. R.; Candeloro De Sanctis, S.; D’Archivio, A. A.; Giglio, E.; Scaramuzza, L. J. Inclusion Phenom. Mol. Recognit. Chem. 1991, 11, 247. (14) D’Alagni, M.; Galantini, L.; Giglio, E.; Gavuzzo, E.; Scaramuzza, L. Trans. Faraday Soc. 1994, 90, 1523. (15) D’Archivio, A. A.; Galantini, L.; Gavuzzo, E.; Giglio, E.; Scaramuzza, L. Langmuir 1996, 12, 4660. (16) D’Archivio, A. A.; Galantini, L.; Gavuzzo, E.; Giglio, E.; Mazza, F. Langmuir 1997, 13, 3090. (17) D’Alagni, M.; D’Archivio, A. A.; Galantini, L.; Giglio, E. Langmuir 1997, 13, 5811.
10.1021/la011519r CCC: $22.00 © 2002 American Chemical Society Published on Web 03/09/2002
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Figure 1. Projection of NaTDC trimer anions along an axis perpendicular to the 3-fold rotation axis. Possible trimers with different openings and radii are shown in (a) the fiber, (b) the 40.0 wt % sample, and (c) the isotropic solution phase.
obtained from electromotive force measurements as a function of ionic strength, pH, and bile salt concentration, are generally multiples of three;18 on the other, NaGDC shows always the presence of trimers, even at the highest ionic strengths.19 Moreover, the 7/1 clockwise helix agrees with the enantioselective complexation of the left-handed conformer of bilirubin-IXR to NaTDC micellar aggregates.3 The two wings of this butterfly-shaped molecule have opposite screw sense in the left- and right-handed enantiomers. Because the left-handed screw sense coincides with that of the 7/1 clockwise helix, the left-handed conformer interacts with the 7/1 helix better than the right-handed one. Recently, Edlund, Khan, and La Mesa (EKL) and Marques, Edlund, La Mesa, and Khan (MELK) have (18) Bottari, E.; Festa, M. R. Langmuir 1996, 12, 1777. (19) Bottari, E.; Festa, M. R. Mh. Chem. 1993, 124, 1119.
investigated the phase behavior of NaTDC-, NaDC-, NaGDC-, NaTC-, and sodium taurochenodeoxycholatewater systems20,21 as a function of concentration and temperature by means of phase diagrams, polarizing microscopy, 2H NMR, pulsed-field gradient NMR selfdiffusion, and small-angle X-ray scattering (SAXS) measurements. The formation of anisotropic liquid crystals, possibly of the reverse type, beyond the micellar solution for all the bile salts was invoked. Phase diagrams with liquid crystalline regions of hexagonal-like structure were reported on the basis of optical textures as well as of 2H NMR and SAXS data.21 The aim of this paper is to connect the NaTDC fiber structure with that of the aqueous solution, thus clarifying (20) Edlund, H.; Khan, A.; La Mesa, C. Langmuir 1998, 14, 3691. (21) Marques, E. F.; Edlund, H.; La Mesa, C.; Khan, A. Langmuir 2000, 16, 5186.
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the NaTDC structural path from the solid to the liquid phase. Some possible NaDC and NaTC structural paths are also discussed on the basis of MELK and our previous results. Experimental Section Materials. NaTDC (Sigma) was twice crystallized from a mixture of water and acetone. Crystals contained water10 that was removed by heating at 50 °C under a pressure of 10-2 Torr for 1 day before preparing the aqueous solutions. Analytical grade acetone (Carlo Erba) was used as received. NaCl was from Merck (suprapur). Sample Preparation. NaTDC fibers, drawn from concentrated aqueous solutions or from aqueous solutions containing NaCl, gave identical X-ray diffraction patterns. NaTDC aqueous solutions were prepared by weight (40.0 wt %) in tightly sealed glass tubes. The contents of the samples were mixed by repeated centrifugation at low speed (3000-5000 rpm), heated in an air oven at 50 °C for 30 min, and centrifuged again to avoid bubble formation. After thorough mixing, samples were allowed to equilibrate at 25 °C for several days.20 Formation of fibrils was observed with time on the sample surface. X-ray Measurements. X-ray diffraction patterns of NaTDC fibers and fibrils were recorded on flat films by means of a Buerger precession camera. The most intense NaCl interplanar spacings, which are known with great precision, were used to determine those of NaTDC samples. SAXS experiments were performed by a Kiessig camera with pinhole collimation and sample-to-film distances of 100 and 200 mm. Samples were held in Lindemann capillaries of 0.5 mm internal diameter at a temperature of about 20 °C. The space containing the sample and the film was kept under vacuum to minimize air scattering. Ni-filtered Cu KR radiation (λ ) 1.5418 Å) was used.
Results X-ray Data of NaTDC Fibers. X-ray photographs of NaTDC fibers were formerly interpreted by means of a packing of 7/1 helices formed by trimers.4 A trimer is constituted by three NaTDC and some water molecules related by a 3-fold rotation axis (Figure 1a) and is the repetitive unit of the 7/1 helix. This helix is generated by a trimer clockwise rotation of 34.29° around and a trimer translation of 6.4 Å along the 3-fold rotation axis. The 7/1 helices pack in a unit cell with a ) 70.8 Å, b ) 40.8 Å, c ) 45.0 Å, and R ) β ) γ ) 90° (unit cell A), the helical axis being parallel to c and b/a ) tan 30° (hexagonal packing). Intense equatorial reflections can be all indexed by halving a (35.4 Å) and b (20.4 Å) (unit cell B). Each unit cell A contains eight helices with an identity period along the helical axis of 45.0 Å and with a radius of 10.2 Å, assuming that the helices are closely packed cylinders. Unit cells A and B are characterized by a two-dimensional packing of two types of nonequivalent helices. Nonequivalent helices can be obtained by small rotations around and translations along the helical axis, as previously reported.4 Furthermore, nonequivalence could be generated by up-pointing and down-pointing identical helices, which are related by a 180° rotation around an axis perpendicular to the helical one. X-ray Data of NaTDC Fibrils and Concentrated Aqueous Solutions. Slit-smeared SAXS patterns of NaTDC samples at 22 °C and concentrations 40.0, 41.8, 43.1, and 44.7 wt % were published by EKL. Three Bragg peaks were observed for all the samples with relative positions in the ratio 1:x3:1.89 instead of the ratio 1:x3:2, typical for hexagonal structures (i.e., hexagonal packing of cylinders or helices), corresponding to the ratio between the 100 spacing and that of 100, 110, and 200 (or 020). The 100 spacing varies from 50.3 to 52.3 Å. The proposed hexagonal two-dimensional lattice for the 40.0 wt % sample is a ) b ) 59.5 Å and γ ) 120°. However,
Galantini et al. Table 1. Observed (do) and Calculated (dc) Spacings (Å) and Possible Miller Indices in Parentheses of NaTDC Fibrils (a ) 70.8 Å, b ) 40.8 Å, c ) 45.0 Å, and r ) β ) γ ) 90°) do
dc
dc
35.6 20.4 18.6 13.6 10.2 7.3 6.43 6.04 5.45 5.05 4.41 3.45
35.4 (2 0 0) 20.4 (0 2 0) 18.6 (0 2 1) 13.6 (0 3 0) 10.2 (0 4 0) 7.3 (2 0 6) 6.43 (0 0 7) 6.04 (0 4 6) 5.44 (0 4 7) 5.04 (0 6 6) 4.42 (8 8 0) 3.46 (0 0 13)
35.4 (1 1 0) 20.4 (3 1 0) 18.6 (3 1 1) 10.2 (6 2 0) 7.3 (1 1 6) 6.40 (1 0 7) 6.04 (2 2 7) 5.44 (6 2 7) 5.04 (8 2 7) 4.40 (8 8 1)
dc
dc
6.04 (4 0 7) 5.42 (0 2 8) 5.07 (0 8 1) 4.38 (8 6 6)
6.05 (6 2 6) 5.08 (6 0 8)
the 200 observed spacing is too far from that expected (i.e., 27.2 against 25.8 Å). Moreover, the two greatest observed spacings of 51.5 and 29.9 Å (40.0 wt % sample) give rise to a constant ratio of 0.73 with a and b of the NaTDC fiber unit cell A (1.46 if unit cell B is considered). These results seem to suggest that the NaTDC packing in solution is related to that in the fiber. To throw light on this point, a NaTDC 40.0 wt % sample (anisotropic liquid crystal phase21) has been prepared and studied by X-ray diffraction analysis. A Kiessig camera with pinhole collimation has been used in order to overcome the distortion of the theoretical distribution of scattered intensity that is peculiar of slit-smeared SAXS patterns. An assemblage of rodlike fibrils is formed with time on the sample surface. X-ray diffraction patterns show that fibrils present axial orientation. Sharp arclike reflections centered on the equator and the meridian are observed. Their spacings are in very good agreement with those observed for the fiber, and their sharpness permits establishment of the possible Miller indices of strong reflections belonging to layers with l > 0, not determined in the previous work.4 The observed and calculated spacings and their possible Miller indices are given in Table 1. As formerly stated,4 the structural unit is the 7/1 helix and the most intense layer lines have l ) 0, 1, 6, 7, 8, and 13. Not all the reflections corresponding to the observed spacings are reported. The reflections with h ) 2n and k ) 2n, that can be indexed also in the unit cell B, have been taken mainly into account because they are the most intense. Other possible Miller indices with proper spacings are omitted in Table 1, because they are not observed in the fiber X-ray photographs. X-ray photographs of the NaTDC 40.0 wt % sample show the presence of almost sharp rings that are more intense around the equatorial or meridional region. The observed and calculated reflection spacings together with their possible Miller indices are given in Table 2 in agreement with unit cell constants a ) 54.8 Å, b ) 31.7 Å, c ) 7.2 Å, and R ) β ) γ ) 90° (unit cell C). Both 54.8 and 31.7 Å values give rise to a constant ratio of 1.06 with those of 51.5 and 29.9 Å reported by EKL and of 1.55 with those of 35.4 and 20.4 Å of the fiber unit cell B.4 Of course, this last ratio is halved if the fiber unit cell A is considered. Because b/a ) tan 30°, the packing is of hexagonal type. Therefore, our data seem to be related to those both of EKL and of the fiber. Discussion Inspection of Tables 1 and 2 points out that, very likely, the 40.0 wt % sample has a structural unit cross section greater than that of the fibril 7/1 helix, their radii being
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Table 2. Observed (do) and Calculated (dc) Spacings (Å) and Possible Miller Indices in Parentheses of the NaTDC 40.0 wt % Sample (a ) 54.8 Å, b ) 31.7 Å, c ) 7.2 Å, and r ) β ) γ ) 90°) do
dc
dc
dc
dc
54.8 31.7 12.6 9.9 9.12 7.18 6.53 6.28 6.03 5.45 4.98
54.8 (1 0 0) 31.7 (0 1 0) 12.6 (4 1 0) 9.9 (2 3 0) 9.13 (6 0 0) 7.14 (1 0 1) 6.55 (3 1 1) 6.25 (4 1 1) 6.02 (5 0 1) 5.46 (4 3 1) 4.97 (4 4 1)
9.15 (3 3 0) 7.20 (0 0 1) 6.51 (1 2 1) 6.29 (7 3 0) 5.99 (3 5 0) 5.48 (10 0 0) 4.99 (6 3 1)
6.56 (0 2 1) 6.29 (8 2 0) 5.99 (6 4 0)
6.30 (1 5 0)
4.96 (8 0 1)
15.85 and 10.2 Å, respectively (see parts b and a of Figure 1). An identity period decrease along the helical axis (7.2 against 45.0 Å) corresponds to a cross section enlargement. The first arclike reflection centered on the meridian has a spacing of 7.2 Å, which should correspond to that of the first layer line. As a matter of fact, the reflections with l ) 1 are the most intense and, hence, can represent the pitch of a helix with two turns in the identity period. If the trimer is rotated by 60, 180, or 300° (these rotations are equivalent, owing to the presence of a 3-fold rotation axis in the trimer), a 21 helix is obtained. Thus, the system is constituted by a packing of 21 helices or, if the rotation angle is not 60°, by a packing of helices with two rotation angles, the sum of which is 120°. Of course, the spacings of the 40.0 wt % sample can be obtained also in the hexagonal unit cell a ) b ) 63.3 Å, c ) 7.2 Å, R ) β ) 90°, and γ ) 120° (unit cell D), which has a and b a little longer than those of EKL (59.5 Å, ratio of 1.06) and has a volume twice that of the unit cell C. Unit cells C and D contain two and four helices, respectively, assuming that the distance between the axes of two adjacent helices is 31.7 Å, namely, the b edge of the unit cell C. Thus, the cylindrical aggregate radius is no longer than 15.85 Å, a value that is much smaller than 20.3 Å (calculated by EKL) and is much greater than 10.2 Å observed in the fiber. The two weak reflections of comparable intensities corresponding to 29.9 and 27.2 Å spacings of EKL give rise to 31.7 and 28.8 Å spacings if multiplied by 1.06. Surprisingly, only the first one has been observed in our X-ray photographs of the NaTDC 40.0 wt % sample, and therefore no violation of the hexagonal packing occurs, because the 28.8 Å spacing is not allowed in the unit cells C and D. Unit cell C is the simplest one that agrees with the observed data. The same elongation ratio of 1.55 between its a and b edges and those of the fiber unit cell B strongly suggests that the fiber and the 40.0 wt % sample are both characterized by the same hexagonal packing but by structural units with different cross sections. A cross section increase can be obtained by filling the structural unit interior with water molecules, in accordance with sedimentation and diffusion experiments which indicated that NaTDC micellar aggregates, even though in salt solutions, are highly hydrated.22 Moreover, the swelling could be accompanied by an elongation of the NaTDC anion side chain. Very probably, the side chain conformation in the fibrils is not fully extended in order to decrease the empty space and to better satisfy the close-packing principle.23 This is supported by interatomic distance (22) Laurent, T. C.; Persson, H. Biochim. Biophys. Acta 1965, 106, 616.
calculations performed for the very similar NaGDC fiber structure (7/1 helix, radius of 10.1 Å and identity period along the helical axis of 45.1 Å), which show that the NaGDC anion side chain is bent.4 Therefore, a radius of 15.85 Å for a trimer with a 3-fold rotation axis can be obtained by shifting the molecules of the fiber trimer (Figure 1a) away from this axis, owing to the water incorporation, and by increasing the angle between the symmetry and anion long axis (Figure 1b). This trimer is more open than that of the fiber. Its greater opening justifies both the trimer repeat decreasing from 6.4 to 3.6 Å and the 34.29° rotation angle increasing toward 60°. The rotation of 60° or about 60° gives rise to good intermolecular contacts between two adjacent trimers and to the formation of a 21 or pseudo 21 helix. The results obtained for fibrils and 40.0 wt % sample (≈0.84 M) permit speculation about the micellar aggregate structure in the more dilute isotropic solution phase. SAXS measurements of 0.1 M NaTDC aqueous solutions, containing NaCl at concentrations ranging from 0.2 to 1.0 M, indicated that the micellar aggregate shape is rodlike, in agreement with a helical structure.24 Later on, the 7/1 helix structure was proposed for NaTDC micellar aggregates.4,18,25 However, on the basis of the present results it is reasonable to suppose that solutions with or without NaCl, less concentrated than the 40.0 wt % sample, contain 21 or two rotation angle helices with a longer radius than 15.85 Å and a trimer repeat very close to 3.6 Å. The greater volume available to the micellar aggregates in less concentrated solutions makes unnecessary structural changes, such as the side chain bend caused by packing reasons, and favors the side chain elongation giving rise to a longer radius (Figure 1c). In this context, it must be noticed that a value of 18 Å was inferred from SAXS measurements.24 Therefore, the trimer path from the fiber to the isotropic solution phase resembles the opening of an umbrella, and the trimer flattening occurs by decreasing the concentration. This is also supported by the trend to slightly increase a (from 58.1 to 60.4 Å) in a narrow range of NaTDC concentration (from 44.7 to 40.0 wt %) as observed by EKL.20 On the other hand, a possible side chain elongation should not vary much the distances among polar heads, sodium ions, and coordinated water molecules that are mainly responsible for the trimer repeat. Slit-smeared SAXS data of MELK on NaDC (39.1 wt %, isotropic solution and anisotropic liquid crystal two-phase region) and NaTC (59.9 wt %, anisotropic liquid crystal phase)21 deserve attention. MELK proposed hexagonal two-dimensional unit cells with a ) b ) 54.2 Å (NaDC), a ) b ) 42.7 Å (NaTC), and γ ) 120°. The agreement between observed and calculated spacings is shown in Table 3. The 8/1 helices formed by trimers similar to those of the NaTDC fiber were identified by X-ray diffraction analysis in lithium, sodium, potassium, and rubidium deoxycholate fibers.2,7 The NaDC fiber has unit cell constants a ) 32.3 Å, b ) 18.7 Å, c ) 52.0 Å, and R ) β ) γ ) 90°, with b/a ) tan 30° (hexagonal packing). The 8/1 helix has a trimer with a 3-fold rotation axis as the repetitive unit, and is generated by a clockwise rotation of 45° around and a translation of 6.5 Å along the 3-fold rotation axis (helical axis) that is parallel to c. As in the case of NaTDC, the ratio between the two observed (23) Kitaigorodsky, A. I. Organic Chemical Crystallography; Consultants Bureau: New York, 1961. (24) Matsuoka, H.; Kratohvil, J. P.; Ise, N. J. Colloid Interface Sci. 1987, 118, 387. (25) Bonincontro, A.; Briganti, G.; D’Archivio, A. A.; Galantini, L.; Giglio, E. J. Phys. Chem. B 1997, 101, 10303.
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Table 3. Observed (do) and Calculated (dc) Spacings (Å) and Miller Indices hk in Parentheses of NaDC (39.1 wt % Sample) and NaTC (59.9 wt % Sample) NaDC
NaTC
do
dc (MELK)
dca
do
dc (MELK)
dca
46.9 27.4 23.3 17.4 12.8
46.9 (1 0) 27.1 (1 1) 23.5 (2 0) 17.7 (2 1) 13.0 (3 1)
46.9 (1 0) 27.4 (0 1) 23.3 (1 1) 17.5 (2 1) 13.0 (1 2)
37.0 21.2 18.5 13.3 11.9
37.0 (1 0) 21.3 (1 1) 18.5 (2 0) 14.0 (2 1) 12.3 (3 0)
37.0 (1 0) 21.2 (0 1) 18.5 (2 0) 13.1 (3 -1) 11.8 (2 1)
a Spacings calculated with unit cell constants a ) 46.9 Å, b ) 27.4 Å, and γ ) 92° (NaDC) and a ) 40.2 Å, c ) 23.0 Å, and β ) 113° (NaTC).
greatest spacings (46.9 and 27.4 Å, see Table 3) and a and b of the NaDC fiber is 1.45 and 1.47 (1.46 for NaTDC). This nearly constant ratio seems to indicate that the NaDC packing in solution is related to that in the fiber. Unit cell constants a ) 46.9 Å, b ) 27.4 Å, and γ ) 92° produce spacings listed in the third column of Table 3, which better agree with the observed ones. Assuming that the distance between the axes of two adjacent closely packed cylindrical helices is 27.4 Å, namely, b, the unit cell contains two helices with a radius no longer than 13.7 Å, which must be compared with the value of 9.35 Å observed in the fiber.2 The increase of the fiber radius is supported by NaDC helical radii previously inferred from SAXS measurements of aqueous electrolyte (NaCl) solutions.26 The calculations were performed by using the anion helix found in the rubidium deoxycholate crystal structure9 as a basic framework. Although this and the 8/1 helices are different, the 8/1 trimer resembles three alternate anions of the rubidium deoxycholate helix.2 For this reason, we can be confident of the SAXS experimental data that were in accordance with a helix enlargment (radius of about 1213 Å). Thus, the NaDC trimer path from the fiber to the isotropic solution phase could be like the opening of an umbrella, as in the case of NaTDC. Very probably, the NaTC trihydroxy salt behaves in a different way than dihydroxy ones (NaTDC and NaDC). The crystal structure of a NaTC monoclinic crystal (space group C2) containing water and acetone was previously solved.14 Its unit cell constants are a ) 40.20 Å, b ) 7.66
Å, c ) 23.04 Å, R ) γ ) 90°, and β ) 92.8°. More recently, a fiber containing only water has been drawn from a gel obtained by concentrating a NaTC aqueous solution. Fiber unit cell constants a ) 19.65 Å, b ) 7.65 Å, c ) 25.70 Å, R ) γ ) 90°, and β ) 97° are strictly related to those of the monoclinic crystal (a is halved).5 Both crystal and fiber are formed by very stable dimers and octamers (assembly of four dimers)5,14 that are present in electrolyte aqueous solutions as confirmed by electromotive force measurements. Micellar aggregation numbers are always multiples of 2, and octamers and hexadecamers (very probably formed by two octamers) prevail in the sample with the highest concentration and ionic strength.5,27 A better agreement with the MELK NaTC data is obtained when the monoclinic crystal a and c values along with a greater β value (113° against 92.8°, see Table 3) are assumed. The β increase is not unexpected when acetone is lacking as in the 59.9 wt % sample. Trihydroxy salt crystals containing water and acetone, as the NaTC monoclinic crystal, are constituted by a packing of octamers and include acetone in apolar channels.12,14,15 Because of their shape, octamers suitably pack in tetragonal and monoclinic systems and not in the hexagonal one. When acetone is present, the channel cross section tends to assume a squared shape that is compatible with a rectangular or nearly rectangular unit cell with β ) 90° or very close to it (NaTC, 92.8°; tetragonal sodium glycocholate12 and rubidium taurocholate,15 90°). The lack of acetone could cause a β widening in order to decrease the empty space in the apolar channels and to get better interatomic contacts as in the NaTC fiber (β ) 97°). In conclusion, the NaTC path from the crystal and fiber to the isotropic solution phase should occur with negligible structural changes because of the dimer and octamer stiffness, at variance with the NaTDC and NaDC trimer behavior. Hence, the biggest NaTC aggregates could be formed by piling octamers as in the crystal and fiber structures. Acknowledgment. This work was sponsored by the Italian Ministero per l’Universita` e per la Ricerca Scientifica e Tecnologica. Thanks are due to Professor Alberto Ripamonti and Dr. Giuseppe Falini for their kind hospitality and for having made available the Kiessig camera. LA011519R
(26) Esposito, G.; Giglio, E.; Pavel, N. V.; Zanobi, A. J. Phys. Chem. 1987, 91, 356.
(27) Bottari, E.; Festa, M. R.; Franco, M. Analyst 1999, 124, 887.
