title: The apparent association constant of calcium and bilirubinate ions in Triton X-100 micelles.
abstract: Calcium bilirubinate [Ca(HB)2], a major component of pigment gallstones, rapidly precipitates from aqueous solutions, thereby precluding an accurate determination of the apparent association constant (K') of calcium (Ca2+ and monohydrogenated bilirubinate (HB) ions. In this study, we determine the K' in solutions of Triton X-100, a nonionic detergent with a critical micellar concentration of 0.01 g/dL, by analysis of spectral shifts, using a Hewlett-Packard diode array spectrophotometer. In the absence of Ca2+, 10 microM sodium bilirubinate (NaHB) solutions were incorporated into Triton X-100 micelles in 0.15 M NaCl, 0.10 M Tris Cl, pH 8, as evidenced by a progressive shift in the wavelength maximum (lambda max) of 10 microM HB from 438 to 458 nm in 0-0.1 g/dL (0-10 microM of micelles/L) Triton. Thus the lambda max plateau occurred with 1 mole of HB per micelle of Triton. As Ca2+, 0-90 microM, was added to 100 microM NaHB in 0.5 g/dL Triton X-100, there was a progressive downward shift in the lambda max of HB from 458 to 434 nm; an isosbestic point was present at 448 nm. A calibration curve using ratios of absorbance (Abs) 420/470 for known concentrations of Ca(HB)2 or NaHB was used to determine the mole fraction of Ca(HB)2. The spectral data were consistent with the equilibrium equation: Ca2+ + 2 HB = Ca(HB)2.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.forename: B W
authors.initials: BW
authors.lastname: Trotman
authors.forename: W L
authors.initials: WL
authors.lastname: Stone
mesh_terms: D001663:Bilirubin / Q000032:analysis / Q000737:chemistry*; D002118:Calcium / Q000032:analysis / Q000737:chemistry*; D003902:Detergents / Q000494:pharmacology*; D007477:Ions; D007700:Kinetics; D008823:Micelles; D017830:Octoxynol; D011092:Polyethylene Glycols / Q000737:chemistry*; D013057:Spectrum Analysis
chemical_list: D003902:Detergents; D007477:Ions; D008823:Micelles; D011092:Polyethylene Glycols; D017830:Octoxynol; D001663:Bilirubin; D002118:Calcium