Supra-molecular association and polymorphic behaviour in systems containing bile acid salts

Abstract

A wide number of supra-molecular association modes are observed in mixtures containing water and bile salts, BS, (with, eventually, other components). Molecular or micellar solutions transform into hydrated solids, fibres, lyotropic liquid crystals and/or gels by raising the concentration, the temperature, adding electrolytes, surfactants, lipids and proteins. Amorphous or ordered phases may be formed accordingly. The forces responsible for this very rich polymorphism presumably arise from the unusual combination of electrostatic, hydrophobic and hydrogen-bond contributions to the system stability, with subsequent control of the supra-molecular organisation modes. The stabilising effect due to hydrogen bonds does not occur in almost all surfactants or lipids and is peculiar to bile acids and salts. Some supra-molecular organisation modes, supposed to be related to malfunctions and dis-metabolic diseases in vivo, are briefly reported and discussed.

Scheme 1

The most common chemical substances interacting with bile salts in aqueous solution or in vivo and significantly affecting their supra-molecular association modes.

Figure 1

Comparison of the chemical structures of fatty acids salts, left, di-glycerides, in the centre, and BS, respectively. Oxygen atoms are in black, the glycerol carbons of di-glycerides in dark grey. Molecular sizes are not in scale.

Figure 2

Molecular structure of cholanic acids. OH groups are indicated by dotted lines, methyl ones by full lines. The arrow indicates a counter-clockwise numbering of carbon atoms.

Figure 3

Dependence of some physical-chemical properties on surfactant concentration. The arrow indicates the cmc value, above which micelles dominate. Concentration scales are normal or logarithmic (in surface tension plots).

Figure 4

Surfactant self-diffusion, D, in 10^-10 m² s^-1, vs. surfactant wt%, for NaTC, circles, and NaTDC, squares, at 25.0 °C. Data are in semi-logarithmic scale. They refer to the region of existence of the solution phase. Note the significant differences between NaTC and NaTDC.

Figure 5

Phase evolution with time, in hours, in a solution containing 41.5 wt% sodium taurodeoxycholate, NaTDC, viewed between crossed polarisers. The solution was formerly heated and equilibrated at 25.0 °C. Note the presence of an upper anisotropic layer, changing its thickness and consistency with time (lanes 2-4), the formation of small crystals in the bottom (lanes 2 and 3), creaming (lane 4), and formation of a birefringent, liquid crystal (lane 5).

Figure 6

Optical anisotropic textures observed by polarising microscopy, at 20 °C, in water-sodium deoxycholate (NaDC) mixtures. From the left, fibres (26.6 wt %), random and ordered anisotropic domains (47.1 wt %), at equilibrium, or by shearing the sample between glass slides. The last image suggests rod-like, or fibre-like, arrangement. Magnification is 200 x.

Figures 7a-c

Partial phase diagrams of the water-NaDC, upper left, of the water-NaTDC, upper right, and of the water-NaGDC binary systems. Single phase and multi-phase areas are indicated. More concentrated mixtures consist in hydrated crystals and are not reported.

Figure 8

SAXS spectrum of a sample (48.4 wt%) in the water-NaTDC, at 25.0 °C. The presence of peaks, in the order 1:1.73:1.89, is representative of a hexagonal structure.

Figure 9

Proposed cylindrical arrangement of BS into reverse hexagonal liquid crystalline phases. Each unit represents a BS, with polar regions, in black, pointing toward the inner part of the cylinder. The structure has a six-fold symmetry axis. The above cylinders are arranged into a hexagonal pattern.

Figure 10

²H-NMR spectra of different liquid crystalline samples, made up of heavy water and different BS, at room temperature. The quadrupole splitting, i.e. the distance between the two upper peaks, can be detected. In the water-NaTDC system the quadrupolar splitting overlaps with an isotropic signal, due to the coexistence of liquid crystalline and solution phases. The 500 Hz wide bar indicates the amplitude of quadrupolar splitting. Spectra relative to NaDC suggest that many chemically different binding sites for heavy water exist. The overall spectrum is originated by the overlapping of many contributions.

Figure 11

Equivalent conductance plot, Λ (in S cm² eq^-1) vs. the BS/BSA mole ratio, at 25.0 °C. The minimum in the curve indicates the protein titration threshold by NaTDC, and corresponds to a number of BS molecules per protein between 18 and 20. Its value increases when pH is lower than 5.0.

Figure 12

Partial phase diagram of the ternary system water-NaTDC-BSA, at 25.0 °C and pH 6.4. The gel, in dark grey, is essentially elastic at temperatures below 40.0 °C and becomes viscous above that limit. The two-phase region gel + solution is in light grey colour. The dotted lines in the low left side of the figure indicate precipitation when the pH is 5.0, light grey, or 3.0, black colour, respectively.

Figure 13

Simplified phase diagram of the system water, NaTDC, C₁₆ soybean lecithin (free from lower and unsaturated homologues) and BSA, at 25.0 °C. For practical purposes, the figure was drawn as a trigonal structure, with the water axis normal to the ternary phase diagrams. Solutions (in dark grey), two-phase regions (middle grey), solutions + gel (pale), solutions plus liquid crystalline dispersions (light grey), solutions + precipitate (middle dark grey) are indicated. The gel phase extending along the BSA rich side is moderate in size. It shrinks with lecithin content and has been omitted.