LASER-LIGHT SCATTERING EVIDENCE FOR A COMMON WORMLIKE GROWTH-STRUCTURE OF MIXED MICELLES IN BILE SALT-PHOSPHATIDYLCHOLINE AND STRAIGHT-CHAIN DETERGENT-PHOSPHATIDYLCHOLINE AQUEOUS SYSTEMS - RELEVANCE TO THE MICELLAR STRUCTURE OF BILE

We employed quasielastic and static light scattering to measure appare nt values of the mean hydrodynamic radii (R-h)(app), molecular weights (M-app), and radii of gyration (R-g)(app) in solutions containing mix ed 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 t otal lipid concentration (0.1-10 g/dL), EYPC/detergent molar ratio (0- 1.2), and ionic strength (0.15-0.3 M NaCl) at 20 degrees 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 var iations in total lipid concentration, detergent molecular species, and ionic strength. Refined data analysis is inconsistent with a flat ''m ixed-disc'' model for bile salt-EYPC micelles [Mazer, N. A,, Benedek, G. B., and Carey, M. C. (1980) Biochemistry 19, 601] and octyl glucosi de-EYPC micelles principally because the numerical value of (R-h)(app/ )M-app(1/2) corresponds to a hypothetical disk thickness of similar to 1 nm, which is 4-fold smaller than the bimolecular width of EYPC mole cules, 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, w e 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 micelle s of diameter d = 4 nm and persistence length xi(p) = 17 nm in solutio n 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 ar e 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 micella r 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 a ll form highly overlapping, semidilute polymer solutions, which mimic the observed scaling ratios. In such semidilute systems, use of the se cond virial coefficient alone to account for intermicellar interaction s is inadequate for estimating micellar weights. The results of the pr esent study, in combination with locations of known phase boundaries o f the ternary bile salt-EYPC-water phase diagram at high dilution, sug gest that elongation, as well as entanglement of wormlike mixed micell es may occur at concentrations approaching the micellar phase limit.