Laser Light Scattering Evidence for a Common Wormlike Growth Structure of Mixed Micelles in Bile Salt− and Straight-Chain Detergent−Phosphatidylcholine Aqueous Systems:  Relevance to the Micellar Structure of Bile

Abstract

We employed quasielastic and static light scattering to measure apparent values of the mean hydrodynamic radii (R h ) app , molecular weights (M app ), and radii of gyration (R g ) app in solutions containing mixed micelles composed of bile salts (cholate and taurochenodeoxycholate, both cholanoyl derivatives) and the glycoacyl chain detergent, octyl glucoside, with egg yolk phosphatidylcholine (EYPC) as functions of total lipid concentration (0.1-10 g/dL), EYPC/detergent molar ratio (0-1.2), and ionic strength (0.15-0.4 M NaCl) at 20°C and 1 atm. As the mixed micellar phase boundaries were approached by dilution, (R h ) app , M app , and (R g ) app values increased markedly by up to 20-fold. For each micellar system, the scaling ratios (R h ) app /M app 1/2 and (R g ) app /(R h ) app remained essentially constant at 0.018 nm/(g/mol) 1/2 and 1.5 (dimensionless), respectively, despite large variations in total lipid concentration, detergent molecular species, and ionic strength. Refined data analysis is inconsistent with a flat "mixed-disc" model for bile salt-EYPC micelles [Mazer, N. A., Benedek, G. B., and Carey, M. C. (1980) Biochemistry 19, 601] and octyl glucoside-EYPC micelles principally because the numerical value of (R h ) app /M app 1/2 corresponds to a hypothetical disk thickness of ∼1 nm, which is 4-fold smaller than the bimolecular width of EYPC molecules, and for a disk, (R g ) app /(R h ) app ratios should be close to 1 at low total lipid concentrations. Assuming disc-shaped micelles, we show that intermicellar excluded volume interactions would have only a minor effect on M app and cannot account for the unrealistic disk thickness. Instead, locally cylindrical, semiflexible wormlike micelles of diameter d ) 4 nm and persistence length p ) 17 nm in solution are compatible with the observed (R h ) app /M app 1/2 and (R g ) app /(R h ) app values when intermicellar excluded-volume interactions are considered. With EYPC/taurochenodeoxycholate ) 0.6 and EYPC/cholate ) 1.0 in 0.15 M NaCl, independent micelles grow upon dilution and use of the second virial coefficient [Egelhaaf, S. U., and Schurtenberger, P. (1994) J. Phys. Chem. 98, 8560] is adequate for estimating micellar weights. The systems EYPC/cholate ) 1.0 in 0.4 M NaCl, EYPC/cholate ) 1.2 in 0.15 M NaCl, and EYPC/octyl glucoside ) 0.13 in 0.15 M NaCl all form highly overlapping, semidilute polymer solutions, which mimic the observed scaling ratios. In such semidilute systems, use of the second virial coefficient alone to account for intermicellar interactions is inadequate for estimating micellar weights. The results of the present study, in combination with locations of known phase boundaries of the ternary bile salt-EYPCwater phase diagram at high dilution, suggest that elongation, as well as entanglement of wormlike mixed micelles may occur at concentrations approaching the micellar phase limit.Aqueous mixed micellar solutions of steroidal or straightchain detergents with long-chain phosphatidylcholine (PC) 1 are of considerable s...

Study of a Standardized Taurocholate– Lecithin Powder for Preparing the Biorelevant Media FeSSIF and FaSSIF

“…After equilibration, the Z av is between 30 and 70 nm for FaSSIF. This finding agrees with the structures found in natural bile (18,22). Visually, FaSSIF is clear to slightly opalescent.…”

“…The phase diagram of lecithin/water/taurocholate is detailed in the literature (17,18). However, the exact phase diagrams of FeSSIF and FaSSIF are not described, because these media have different pH and salt compositions.…”

“…The transition process is induced by the dilution of a more concentrated mixed micellar mixture towards the CMC region of the detergents. It is also common practice for physicists to equilibrate mixed micelles before studying their properties (18,24).…”

Incorporation of cholesterol in sphingomyelin- phosphatidylcholine vesicles has profound effects on detergent-induced phase transitions

“…Previous studies have examined in detail phase transitions induced by the addition of detergent to egg yolk PCcontaining vesicles or by dilution of micellar solutions, with formation of long cylindrical micelles as intermediate structures (10,11,30,31). In the present study, we have focused on phase transitions of SM-PC vesicles, with or without cholesterol incorporated in the bilayer, after addition of various amounts of taurocholate.…”

Molecular Dynamics Simulations of Mixed Micelles Modeling Human Bile

“…At lower PC/BS ratio's a phase region is entered with simple bile salt micelles coexisting with mixed micelles. Increasing the ratio of PC/ BS results in an elongation of the micelles into rodlike micelles and eventually to the system forming hexagonal and lamellar phases (6). The same effect is achieved upon dilution as the micelles become more enriched in phospholipids due to the migration of the highly soluble bile salts into the aqueous phase.…”