A 1H-NMR study of bilirubin IX alpha solubilization by cholate micelles: application of nuclear Overhauser effects

Effect of the guest size and shape on its binding dynamics with sodium cholate aggregates

The binding dynamics of the guests acenaphthene, phenanthrene, fluorene, and acenaphthenol with sodium cholate aggregates were studied using laser flash photolysis and fluorescence. The location of the guests in the bile salt aggregate is determined by the guest's hydrophobicity, where acenaphthene, phenanthrene, and fluorene bind to the primary aggregates, while acenaphthenol binds to the secondary bile salt aggregates. The residence time of the guests in the primary aggregates and the access of ionic species from the aqueous phase to the guest in the aggregate depend on the size and the shape of the guest. These results show that bile salt aggregates are adaptable supramolecular host systems.

Spectroscopic probes of the individual and combined effects of Triton X-100 and chloroform on serum albumins and serum-albumin.bilirubin complexes

The effects of the non-ionic surfactant Triton X-100 on the biphasic induced CD spectra of bilirubin complexes of human and bovine serum albumins (HSA and BSA) are divergent. While Triton X-100 inverts the induced CD spectrum of HSA.bilirubin, this surfactant enhances the ellipticity values of induced CD of BSA.bilirubin without inversion. The effect of Triton X-100 on the characteristic ultraviolet-CD spectra of the albumins are similar; both the albumins are denatured from their native globular structures. The anionic surfactant SDS, unlike non-ionic Triton X-100, dislodges the ligand from its protein complexes, indicating that both electrostatic and hydrophobic forces are involved in binding of bilirubin to the albumins. The aprotic solvent chloroform inverts the biphasic induced CD spectra of HSA.bilirubin and BSA.bilirubin, whereas CHCl3 has relatively little effect on the ultraviolet CD spectra of the albumins. The combined effect of Triton X-100 and CHCl3 shows that the effect of CHCl3 predominates over that of Triton X-100. The perturbing effects of Triton X-100 and CHCl3 on the CD or induced CD spectra of the proteins or their bilirubin complexes are reversible, and independent of the order in which components were added. The observations suggest that the denaturation of the albumins by Triton X-100 or solvation of CHCl3 within albumins markedly alter the internal topography or dynamics of the receptor sites, triggering alterations of the chirality of the bound pigment in sign and/or magnitude.

On the interaction of polypeptides with bile salts or bilirubin-IX alpha

Aqueous solutions formed by polypeptides, simple models of proteins, and bile salts (sodium cholate and deoxycholate, NaC and NaDC, respectively) or bilirubin-IX alpha (BR) have been studied by CD measurements. They could mimic more complicated biliary systems, thus supplying a possible interpretation of the behavior of some amino acid residues in the biliary proteins. The aggregation of NaDC and NaC in water can be monitored by CD measurements. Bile salts, in submicellar and micellar form, stabilize poly(L-Lys) (PLL) in alpha-helical conformation. The alpha-helix content increases with increasing bile salt concentration and ionic strength. NaDC seems to be a slightly better stabilizing agent of the alpha-helix conformation than NaC. Models characterized by hydrogen bonds between bile salts and PLL are proposed, also resorting to previous data available on the systems formed by NaDC and poly(L-Leu-L-Leu-L-Lys) (PLLL) or poly(L-Leu-L-Leu-L-Asp) (PLLA). Binding of BR to PLL, poly(D-Lys), poly(L-Glu), PLLL, and PLLA in water has been investigated by CD spectra in order to clarify the nature of the association complexes and the mechanism of the BR enantioselective complexation. Potential energy calculations provide binding models capable of explaining the enantioselective ability of the PLL and PLLL alpha-helices toward the left- and right-handed enantiomer of BR, respectively. BR is bound to -NH2 groups of PLL and PLLL lying on a right- and left-handed spiral, respectively. These results, together with those formerly obtained for some bile salts-BR systems, indicate that the selectivity originates from a binding that involves large regions of the BR molecule and gives rise, very probably, to moderate conformational changes from the "ridge tile" structure observed in the crystals. In some cases van der Waals forces can play a crucial role in the chiral recognition of bilirubin. Moreover, possible interaction models of BR with human serum albumin are proposed on the basis of a recent x-ray crystal structure of the protein.