Quasi-elastic Light-scattering Measurements Research Articles

QELS and X-ray study of two dihydroxy bile salt aqueous solutions

Previously, aqueous solutions containing one dihydroxy and one trihydroxy bile salt together with 0.8 M NaCl were studied by means of small-angle X-ray scattering (SAXS) and quasi-elastic light-scattering (QELS) techniques. It was observed that even a small amount of the bile salt forming smaller size aggregates (trihydroxy salt) inhibits the growth of the bile salt forming bigger size aggregates (dihydroxy salt), giving rise to an apparent mean hydrodynamic radius ( R h) value lower than that of the dihydroxy salt alone. This phenomenon occurred when sodium glycocholate and taurocholate were added to sodium glycodeoxycholate (NaGDC) and taurodeoxycholate (NaTDC) cylindrical aggregates, formed by trimers. In this paper observed SAXS data of a NaGDC aqueous electrolyte solution are reported and compared with the calculated ones by using the cylindrical aggregate model. The satisfactory agreement strongly supports the validity of the model. Moreover, aqueous solutions containing two dihydroxy salts (NaGDC and NaTDC), together with 0.8 M NaCl to minimize charge interactions, are investigated by QELS measurements that give information on the growth processes of NaGDC and NaTDC aggregates as a function of their relative concentration. The R h-value of a fixed amount of bile salt increases by adding the other bile salt, in agreement with the presence of two-component aggregates formed by random or block association of NaGDC and NaTDC trimers and in disagreement with the hypotheses of the growth inhibition and the formation of two one-component aggregates. X-ray and atomic emission spectroscopy measurements of fibers drawn from NaGDC + NaTDC solutions show that the fiber composition reflects with good approximation that of the solution. Under the same bile salt and NaCl concentrations and at the same temperature, the R h-values at high ionic strength of one-bile-salt solution follow the order NaGDC > NaTDC. Our results show that the R h-value of a 100 mM two-bile-salt solution (NaGDC + NaTDC) is intermediate between those of 100 mM NaGDC and NaTDC pure solutions. Moreover, R h-values of solutions with constant NaGDC + NaTDC total concentration (100 mM) versus NaGDC or NaTDC concentration are fitted by straight lines, thus indicating that the aggregate size linearly depends on NaGDC and NaTDC molar fractions. All these results indicate that NaGDC and NaTDC trimers seem to be interchangeable to some extent in the two-component aggregates that very probably are formed by means of the block association process.

Study of thermal aggregation of oat globulin by laser light scattering.

The heat-induced aggregation of oat globulin was studied using size-exclusion chromatography (SEC) combined with on-line multiangle laser light scattering (MALLS) and quasi-elastic light scattering (QELS). The unheated oat globulin exists as a mixture of hexamer (>95%), trimer, and dimer forms of hexamer. The molecular weight of the hexamer was estimated by MALLS to be 330 000, close to that deduced from the genomic cloned data of the acidic and basic polypeptides of oat globulin. From QELS measurements, it can be predicted that the hexamer exists as two annular trimeric rings, with a diameter of 11.8 nm, placed on top of each other, forming an oblate cylinder with a height of about 8.5 nm. Upon heating at 100 degrees C, the oat globulin hexamers and trimers were dissociated into monomers. The heat-denatured monomers, probably assuming an extended structure, were associated to form small aggregates, which were further aggregated to high molecular weight complexes. Upon further heating (60 min), the soluble aggregates were associated to form insoluble aggregates. Aggregation of oat globulin occurred at a much faster rate at 110 degrees C. The results indicate that the SEC-MALLS-QELS system is suitable for studying thermal aggregation of food proteins.

Rpe65 Is a Retinyl Ester Binding Protein That Presents Insoluble Substrate to the Isomerase in Retinal Pigment Epithelial Cells

Photon capture by a rhodopsin pigment molecule induces 11-cis to all-trans isomerization of its retinaldehyde chromophore. To restore light sensitivity, the all-trans-retinaldehyde must be chemically re-isomerized by an enzyme pathway called the visual cycle. Rpe65, an abundant protein in retinal pigment epithelial (RPE) cells and a homolog of beta-carotene dioxygenase, appears to play a role in this pathway. Rpe65-/- knockout mice massively accumulate all-trans-retinyl esters but lack 11-cis-retinoids and rhodopsin visual pigment in their retinas. Mutations in the human RPE65 gene cause a severe recessive blinding disease called Leber's congenital amaurosis. The function of Rpe65, however, is unknown. Here we show that Rpe65 specifically binds all-trans-retinyl palmitate but not 11-cis-retinyl palmitate by a spectral-shift assay, by co-elution during gel filtration, and by co-immunoprecipitation. Using a novel fluorescent resonance energy transfer (FRET) binding assay in liposomes, we demonstrate that Rpe65 extracts all-trans-retinyl esters from phospholipid membranes. Assays of isomerase activity reveal that Rpe65 strongly stimulates the enzymatic conversion of all-trans-retinyl palmitate to 11-cis-retinol in microsomes from bovine RPE cells. Moreover, we show that addition of Rpe65 to membranes from rpe65-/- mice, which possess no detectable isomerase activity, restores isomerase activity to wild-type levels. Rpe65 by itself, however, has no intrinsic isomerase activity. These observations suggest that Rpe65 presents retinyl esters as substrate to the isomerase for synthesis of visual chromophore. This proposed function explains the phenotype in mice and humans lacking Rpe65.

Hydration Study of PEG/Water Mixtures by Quasi Elastic Light Scattering, Acoustic and Rheological Measurements

To study the hydration effect of poly(ethylene glycol) (PEG) and its dependence on the molecular weight, we report viscosity, compressibility, and quasi elastic light-scattering measurements on aqueous solutions of PEG with different mean molecular weight, Mw, at different concentration and temperature values. In particular, ultrasonic technique allows to evaluate the hydration number for PEG samples at different polymerization degrees. The values deduced by ultrasonic technique are then compared with those deduced from viscosity data following the Linow and Philipp's model. PCS technique allows to obtain information on the hydrodynamic radius and its dependence on the polymer Mw at different temperature values.

Stabilisation of kraft lignin solutions by surfactant additions

The colloidal stability of kraft lignin (KL) and some various KL–surfactant mixtures in aqueous alkaline solutions, pH 10.0–11.50, was tuned and investigated at high ionic strengths, 0.5–1.1 M, and elevated temperatures, 70 and 150 °C. The main techniques used were Quasi-Elastic Light-Scattering (QELS) and turbidity measurements, together with sample inspection by the naked eye. Samples without surfactant additions and samples containing various alkyl sulfonates showed a comparably low colloidal stability, whereas in mixtures containing sodium salts of bile acids, the stability was found greatly improved. The efficiency of different bile salts was investigated and it was found that the overall best solution stability is obtained in mixtures of KL and sodium taurodeoxycholate (STDC). STDC showed a relatively good stabilising effect also at very high temperatures (150 °C). Furthermore, in already aggregated KL solutions, additions of STDC were found to cause ‘de-aggregation’ and the number of formed aggregates was dramatically decreased. From the outcome of the investigation, it can be concluded that by introducing surfactants, and maybe other designed additives too, new possibilities in controlling the colloidal stability of KL at rough solution conditions are given.

On Tetramethylammonium Ion Role in Glycodeoxycholate Micellar Aggregate Formation

Previously, fiber and crystal structural models were proposed for bile acid salt micellar aggregates and verified in aqueous solutions. Electromotive force (emf) measurements on sodium salts versus ionic strength, pH, and bile salt concentration provided micellar aggregate compositions that supported the models. Ionic strength was varied by adding tetramethylammonium chloride (TMACl), although tetramethylammonium (TMA+) and Na+ ions could interfere in the aggregate formation and structure. In this case emf results cannot be used for sodium salts. Tetramethylammonium (TMAGDC) and sodium glycodeoxycholate (NaGDC), which forms helical aggregates constituted by trimers, are studied to clarify this point. TMAGDC crystal structure is solved, and circular dichroism (CD), quasi-elastic light-scattering (QELS), electrolytic conductance, and dielectric measurements on TMAGDC and NaGDC aqueous solutions are compared for determining similarities and dissimilarities in their behavior. TMA+ and Na+ coordinations in TMA...

Fluorescence and Light-Scattering Studies of the Aggregation of Cationic Surfactants in Aqueous Solution: Effects of Headgroup Structure

Aggregation of the following cationic surfactants has been studied in aqueous solution: cetyltrimethylammonium chloride, CMe3ACl; cetyldimethylphenylammonium chloride, CMe2PhACl; cetyldimethylbenzylammonium chloride, CMe2BzACl; cetyldimethyl-2-phenylethylammonium chloride, CMe2PhEtACl; and cetyldimethyl-3-phenylpropylammonium chloride, CMe2PhPrACl. Critical micelle concentrations, cmc's, were obtained from surface tension and conductance measurements; data of the latter were also employed to determine the degree of the surfactant counterion dissociation, α. Static and quasi-elastic light-scattering measurements were employed to obtain micellar weight-average molecular weights, aggregation numbers, Nagg, micellar hydrodynamic radii, Rh, and interfacial area/surfactant headgroup. The latter area was also obtained from surface tension measurements. Finally, time-resolved fluorescence decay measurements with pyrene as probe were employed to obtain Nagg. The structure of micelles of CMe2PhACl is different from that of the other phenyl-group-bearing surfactants because its aromatic ring cannot fold back on the micellar interface. Increasing the number of the methylene segments in the headgroup results in an increase in α and interfacial surface area/headgroup and a decrease in the cmc, Nagg, and Rh. There is good agreement between micellar properties obtained by the different techniques employed.

The Detection of Structural Transformations in Kaolin Suspensions by Ultrasound

Thefractal dimension of kaolin flocs suspended in water was increased by the addition of potassium chloride and indirectly monitored by low-power ultrasonic pulses in a test cell with a “pitch-catch” configuration. The results from these measurements were compared with fractal dimensions obtained from quasi-elastic light-scattering measurements in a kaolin suspension of a much lower solid volume fraction. Further comparison with a coupled phase theory of ultrasound propagation, utilizing measured slope viscosity and particle size distribution, revealed that the observed changes in the ultrasound signature were commensurate with changes in the fractal dimension observed by light scattering over a potassium chloride concentration range of approximately 0–1 mM.

Microscopic Parameters of Sodium Dodecyl Sulfate Micelles from Optical Probe Studies and Quasi-Elastic Light Scattering

Quasi-elastic light-scattering measurements of the diffusion of sodium dodecyl sulfate (SDS) micelles in aqueous solutions, and the diffusion of mesoscopic optical probes through the same solutions, were carried out at 35 °C and multiple solvent ionic strengths. From nonlinear least-squares fits to both probe and mutual diffusion data, assuming a spherical micelle, we deduced the micelle radius, aggregation number, charge, and hydration. For SDS micelles the charge that we find is lower than that in the literature (Hayter, J. B.; Penfold, J. Colloid Polym. Sci. 1983, 261, 1002. Triolo, R.; Caponetti, E.; Graziano, V. J. Phys. Chem. 1985, 89, 5743); the difference arises because we use an improved form for the intermicellar electrostatic potential. As further results of using the same potential, we find a smaller aggregation number and a larger micellar hydration than those reported in the literature.

Structural Study of the Micellar Aggregates of Sodium Taurodeoxycholate

Previously, fibers of sodium and rubidium salts of glycodeoxycholic and taurodeoxycholic acids have been drawn from aqueous micellar solutions. Their X-ray patterns have been interpreted by means of very similar unit cell parameters and helical structures, formed by trimers, having a 3-fold rotation axis, arranged in 7/1 helices. Micellar aggregates with an aggregation number N ≤ 12 and ≥ 15, having oblate and cylindrical (7/1 helix) shape, respectively, have been proposed for sodium taurodeoxycholate (NaTDC) and satisfactorily verified by means of electromotive force and quasi-elastic light-scattering (QELS) measurements. The aim of this paper is to further check this two-structure model by means of QELS, circular dichroism (CD), and dielectric measurements on NaTDC aqueous solutions within concentration and temperature ranges 10−100 mM and 5−45 °C. The average intensity scattered by the NaTDC samples does not depend on the temperature and can be fitted by two straight lines, which intersect at a concentration about 40 mM. Two structures seem to be present, one prevailing above and the other below this concentration. The change of structure, which occurs during the growth of the aggregates, has been monitored by means of CD spectra increasing the ionic strength. The CD data agree with the two-structure model assuming that the cylindrical aggregates have an enantioselective ability toward bilirubin-IXα lower than that of the oblate aggregates. The average electric dipole moment μ of a NaTDC monomer has been calculated from dielectric data. The μ values, plotted as a function of NaTDC concentration within the temperature range 5−45 °C, can be fitted by two straight lines and show a break at a concentration that is, once more, about 40 mM. The μ values vary from 33 to 68 D, depending on the temperature and NaTDC concentration. These high values can be justified by a remarkable hydration of the NaTDC aggregates. The moderate decrease of μ when the populations of the aggregates with N ≥ 15 increase can agree with the formation of 7/1 helices, but disagrees with the formation of disordered structures, especially when they have a pseudo center of symmetry. The temperature dependence of the relaxation time follows an Arrhenius law. The molar enthalpy of activation vs NaTDC concentration for the relaxation process can be fitted by two straight lines, which show a break point at a concentration about 40 mM and could be connected with two different structures.

X-Ray, QELS, and CD Studies of the Micellar Aggregates of Calcium Taurodeoxycholate

Calcium taurodeoxycholate (CaTDC) has been studied in order to throw light on the structure of its micellar aggregates and on the effect of the competition between calcium and sodium ions for the formation of aggregates in aqueous solution. A fiber has been drawn from a gel formed in an aqueous micellar solution of CaTDC. Its X-ray pattern shows a close resemblance with those of the fibers drawn from aqueous micellar solutions of the sodium and rubidium salts of glycodeoxycholic and taurodeoxycholic acids (NaGDC, RbGDC, NaTDC, and RbTDC). Therefore, it has been likewise interpreted by means of a structure composed of trimers arranged in 7/1 helices. The helical structures of the anions are similar for CaTDC, NaGDC, RbGDC, NaTDC, and RbTDC. Quasi-elastic light-scattering measurements, carried out on CaTDC aqueous solutions as a function of ionic strength, point out a micellar growth greater or smaller than that of NaTDC or calcium glycodeoxycholate, respectively. The NaTDC micellar size increases much more...

Calcium Ion Binding to Bile Salts

The nature of the interactions between calcium ions and bile salt anions and the effect of the competition between calcium and sodium ions for possible formation of micellar aggregates are investigated. The crystal structures of calcium deoxycholate (CaDC) and glycodeoxycholate (CaGDC) and quasi-elastic light-scattering measurements, carried out on sodium deoxycholate (NaDC) and glycodeoxycholate (NaGDC) aqueous solutions containing CaCl2, are discussed. The calcium ions are coordinated to oxygen atoms of carboxylate groups and water molecules by means of ion−ion and ion−dipole interactions. The hydroxyl groups of the bile salt anions are not involved. The Ca2+-bile salt complexes fulfill both a 1:1 (CaDC) and a 1:2 (CaGDC) stoichiometry. An assembly of wedge-shaped bilayers, cemented with CaDC anions, or an assembly of irregular trimeric units, each one containing three anions, can be recognized in the crystal packing of CaDC. Ca2+ and Cl- ions and water molecules are located in liquid-like regions. The crystal packing of CaGDC is characterized by 21 helices and units with a 2-fold rotation axis. The quasi-elastic light-scattering measurements show that NaDC and NaGDC aqueous solutions with low concentrations of Ca2+ ions have micellar aggregates with apparent hydrodynamic radii remarkably greater than those of the solutions containing only Na+ ions. The Ca2+ ions seem to have a greater affinity for the deoxycholate and glycodeoxycholate anions than the Na+ ions, and the micellar aggregates with Na+ and Ca2+ ions seem to be more stable than those containing only Na+ ions. It is proposed that the Ca2+ ions act as aggregation centers of the sodium micellar aggregates.

Growth of Mixed Nonionic Micelles

Static and quasielastic light-scattering measurements were utilized to investigate the shape, size, and polydispersity of mixed micelles composed of the nonionic surfactants dodecyl hexaoxyethylene...

Solution of telechelic ionomers in water/AOT/oil (w/o) microemulsions: a static and dynamic light scattering study

Static and quasielastic light-scattering measurements of endsulfonated polyisoprene in a water in oil (w/o) microemulsions were used to characterize the structure and diffusion properties of this complex system. The hydrophilic end groups of the polymer stick to the surfactant covered oil/water interface, thus bridging the water droplets. This structure formation decreases the mobility of the aqueous nanodroplets and polymer molecules. At interdroplet distances larger than the end-to-end distance of the ionomer chain a decrease of the osmotic modulus is observed. It can be explained by a depletion force of free ionomer chains acting on the nanodroplets. With increasing polymer concentration structure formation of the microemulsion is observed at nanodroplet concentrations where the ionomer chains just fit the average separation of two nanodroplets.

Structural Study of the Micellar Aggregates of Sodium and Rubidium Glyco- and Taurodeoxycholate

Some salts of bile acids increase the size of their micellar aggregates in aqueous solutions by varying parameters such as pH or ionic strength and give rise to transitions: aqueous micellar solution → gel → fiber → crystal. Helical fibers of sodium and rubidium salts of glycodeoxycholic and taurodeoxycholic acids (NaGDC, RbGDC, NaTDC, and RbTDC, respectively) were drawn from aqueous solutions and investigated by means of X-ray diffraction analysis to clarify the structure of their micellar aggregates and to verify whether the micellar structures of the sodium and rubidium salts were similar. In this case, the rubidium salts can be studied in place of the sodium ones by extended X-ray absorption fine structure spectroscopy. The X-ray patterns of the NaGDC, RbGDC, NaTDC, and RbTDC fibers showed a close resemblance and were interpreted by means of very similar unit cell parameters and helical structures, formed by trimers arranged in 7/1 helices. Calculations of interatomic distances provided a model of the 7/1 helix which satisfactorily packs into the unit cells of the fibers. Quasi-elastic light-scattering measurements carried out on aqueous micellar solutions up to a concentration of NaCl or RbCl of about 0.3 M supported the similarity among the structures of the NaGDC, RbGDC, NaTDC, and RbTDC micellar aggregates. Oligomers of small size were observed without NaCl or RbCl. The extent of micellar growth increased upon increasing NaCl or RbCl concentration. The micellar size, which seemed to be dependent on the peculiar cation−anion Coulombic interaction starting from a NaCl or RbCl concentration of about 0.2 M, was in the following order: NaGDC > NaTDC ≈ RbTDC > RbGDC. Electromotive force measurements accomplished on NaTDC solutions at constant ionic medium provided the distribution of micellar sizes. Most of the aggregates have aggregation numbers that are a multiple of 3. The fraction of bound Na+ ions is high in accordance with an ordered structure like that of the 7/1 helix. Further support for the 7/1 helix came from the calculation of mean hydrodynamic radii using the helical model. The agreement with quasi-elastic light-scattering measurements at low bile salt concentration and within a wide range of ionic strength was satisfactory. The helical structure of the anions seems to be similar in the micellar aggregates of NaGDC, RbGDC, NaTDC, RbTDC, and tetramethylammonium taurodeoxycholate.

Pyrene-labeled amphiphilic poly-(N-isopropylacrylamides) prepared by using a lipophilic radical initiator: synthesis, solution properties in water, and interactions with liposomes

Fluorescently labeled amphiphilic poly-(N-isopropylacrylamides) (PNIPAM) substituted with a N-[4-(1-pyrenyl)butyl]-N-n-octadecyl group at the chain end were prepared by free-radical polymerization in dioxane of N-isopropylacrylamide (NIPAM) using 4,4′-azobis{4-cyano-N,N-[4-(1-pyrenyl)butyl]-n-octadecyl}pentanamide as the initiator. The solution properties of the polymers in water were studied as a function of polymer concentration and temperature. Quasi-elastic light-scattering measurements and fluorescence experiments monitoring the pyrene excimer and pyrene monomer emissions revealed the presence of multimolecular polymeric micelles below the lower critical solution temperature (LCST) of PNIPAM. These underwent partial, reversible reorganization as they were heated above the LCST. The interactions of the pyrene-labeled amphiphilic PNIPAM with dimyristoylphosphatidylcholine (DMPC) liposomes have been examined in water at 25 °C. From fluorescence experiments it was established that the polymeric micelles are disrupted irreversibly upon contact with the liposomes. The anchoring of the polymer chains occurs by insertion of their hydrophobic tail within the phospholipidic bilayer, as evidenced from a large decrease of the pyrene excimer emission relative to pyrene monomer emission. The copolymers remained anchored within the bilayer as the temperature of the copolymer–liposome suspension was raised above the LCST of PNIPAM. Keywords: liposome, poly-(N-isopropylacrylamide), fluorescence, micelles.

Complex Formation between Sodium Dodecyl Sulfate and Poly(4-vinylpyridine N-oxide)

The interaction of poly(4-vinylpyridine N-oxide) (PVPNO) with sodium dodecyl sulfate in aqueous solutions was examined using surface tension, turbidimetry, and light scattering (quasielastic, electrophoretic, and total intensity light scattering). Surface tension measurements show that the polymer begins to interact with SDS at a concentration below the critical micelle concentration (cmc), in a manner similar to the behavior of other nonionic polymer-surfactant systems (e.g., PEO-SDS). Turbidimetric and quasielastic light scattering (QELS) studies at SDS concentrations much above the cmc clearly reveal the formation of a soluble polymer-micelle complex. On the other hand, phase separation, similar to that observed for polyelectrolyte-oppositely charged micellar systems, occurs at low pH, presumably as a consequence of protonation of the polymer to form a polycation. Total intensity data show saturation of the polymer with bound micelles at high SDS concentration, while QELS measurements clearly reveal two peaks : one corresponding to free micelles and the other to the complex. Quasielastic and electrophoretic light scattering measurements were carried out on mixtures of SDS and PVPNO, SDS and PEO, or SDS and both polymers. These indicate that the binding of SDS micelles to PVPNO in 0.10 M NaCl is not as strong as the binding to PEO.

Lamellar bodies coexist with vesicles and micelles in human gallbladder bile. Ursodeoxycholic acid prevents cholesterol crystal nucleation by increasing biliary lamellae

The aggregative forms of lipids in human gallbladder bile and their relation to cholesterol crystallization are controversial. Using combined chemical, gel-chromatographic, optical/electron microscopic and quasielastic light-scatttering methods, we investigated this issue in native gallbladder bile obtained from nine untreated cholesterol gallstone patients and eight cholesterol gallstone patients treated for 1 weeks with 600 mg/day of ursodeoxycholic acid. Bile obtained at cholecystectomy was ultracentrifuged for 2 h at 150 000 g to obtain isotrophic samples. The conventional cholesterol crystal observation time was 3.1 ± 4.1 (SD) days in controls and 19.0±1.9 days in the ursodeoxycholic acid-treated group ( p<0.001). Bile was analyzed by high-resolution gel-chromatography using 7 mM sodium taurocholate in the elution buffer. Biliary lipids eluted in four chromatographic zones: zone #I, corresponding to the column void volume, contained only minimal amounts of lipids; zone #II (apparent m.w. 100–220 kDa) comprised 29.1±12.4% of biliary cholesterol in the untreated group and 8.3 ±4.3% in the ursodeoxycholic acid-group ( p<0.001). At negative staining electron microscopy, this region was composed of roundish vesicles ranging from 7 to 20 nm in diameter. Zone #III (apparent m.w. 50–100kDa) carried 59.1±2.1% of cholesterol in untreated patients and 81.2±9.5% in ursodeoxychlic acid-rich biles, respectively ( p<0.001). At negative staining electron microscopy, this region was composed of lamellar stacks of variable length, usually with 5 nm interspaces and up to 30 nm width. In ursodeoxycholic acid-rich biles, lamellae often appeared in the form of concentric fingerprint-like images. Quasielastic light-scattering measurements in this region were compatible with the size estimates obtained at electron microscopy. Zone #IV (apparent m.w. 6–50 kDa) carried 11.8±9.4% and 11.6±9.0% of cholesterol, respectively (not significant). Since this region comprised a considerable fraction of endogeneous bile salts and had no distinct morphological structures, it was interpreted as mixed micelles. The cholesterol crystal observation time showed a significant inverse correlation (r–0.85, p<0.001) with percent cholesterol carried by vesicles (zone #II) and a direct correlation (R0.86, p <0.001) with percent cholesterol carried by lamellar bodies (zone #III). Vesicles and lamellae identical to those observed in isolated gel-chromatographic fractions were observed also on direct electron microscopic examination of unfractionated isotropic native biles. Similar findings were observed also in matched model biles. In conclusion, this study confirms previous work showing that lamellar bodies represent the predominant cholesterol carrier in gallbladder bile of cholesterol gallstone patients, and indicates that ursodeoxycholic acid feeding prevents cholesterol crystalization by increasing the lamellar carrier.

Dynamic critical phenomena in aqueous protein solutions.

An aqueous protein solution is a binary mixture of large particles (protein molecules) and small particles (water molecules). In contrast, a binary liquid mixture is a mixture of two different kinds of particles that are approximately the same size. Thus, a comparison of the critical behavior in these two systems allows one to determine the effect on critical behavior of a relative difference of size in the two kinds of particles in a binary mixture. The correlation length j, osmotic compressibility kT , and coexistence curve of aqueous protein solutions in the vicinity of the critical point for liquid-liquid phase separation have been reported recently [1‐3]. These experimental results established that the static critical properties of these solutions are consistent with the behavior of members of the static universality class of three-dimensional systems with short range interactions and scalar order parameters. This universality class includes binary liquid mixtures. In contrast to the equilibrium properties, the dynamic properties of aqueous protein solutions in the vicinity of the critical point have not been investigated as thoroughly. In this Letter, we report our use of quasielastic light scattering (QLS) to investigate the time decay of spontaneous concentration fluctuations, as a function of both wave number q of the fluctuations and temperature T, along the critical isochore of aqueous solutions of the bovine eye lens protein gII-crystallin. The only previous study of critical dynamics in a protein solution of which we are aware is the work of Ishimoto and Tanaka who made QLS measurements at a single fixed wave number of the fluctuations using a 19 channel digital correlator on aqueous solutions of lysozyme [4] along the critical isochore for sT 2 TcdyTc $ 5 3 10 23 (q j, 0.1), where Tc is the critical temperature. The present study is a significant experimental advance over the work of Ishimoto and Tanaka in that (i) we obtain data for fluctuations at up to 12 different wave numbers, (ii) we obtain data using a 144 channel digital correlator, and (iii) we obtain data much closer to the critical point [ sT 2 TcdyTc $ 1.56 3 10 24 and q j# 3.18]. The more thorough investigation of critical dynamics presented here has revealed new and unexpected behavior. Specifically, we find that the wave number and temperature dependence of the average rate of decay of the concentration fluctuations are consistent with the theory for critical dynamics in binary liquid mixtures [5] only if we allow both the background viscosity and the background contribution to the decay rate to be unusually large. Furthermore, in contrast to the behavior seen in binary liquid mixtures we find that the concentration fluctuations exhibit very significant deviation from exponential decay with time. Of course, these protein solutions may be viewed as

Cholesterol Thermodynamic Activity, Quasielastic Light Scattering, and Polarizing Microscopy Studies in Aqueous Taurocholate-Lecithin Solutions Supersaturated with Cholesterol

Abstract The thermodynamic activity, A T , of cholesterol (Ch), rather than the Ch saturation index (CSI), is the more appropriate measure of Ch supersaturation in bile. CSI has been based on the simple concept that the tendency for Ch dissolution or precipitation in bile should be directly related to the ratio of the actual Ch concentration to the equilibrium Ch solubility in bile. CSI, however, becomes inadequate when Henry's law breaks down. In the present study, data on Ch uptake into silicone polymer films were used to quantify A T in Ch-supersaturated taurocholate (TC)-lecithin (L) solutions. Quasielastic light scattering (QLS) experiments were conducted to quantify the nature of lipid particles (micelles and vesicles) and, together with microscopic observations, to determine Ch nucleation times. The results from the A T measurements, QLS measurements, and microscopic observations showed (a) that vesicles may be catalytic sites for Ch crystallite nucleation, (b) that mesophase formation may be a common occurrence in moderately supersaturated biles, and (c) that, at the same CSI, supersaturated systems with higher TC:L ratios require a higher A T for mesophase (vesicle) formation and have much shorter Ch nucleation times than those with lower TC:L ratios.