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Bile salts (BSs) are important for the digestion and absorption of fats and fat-soluble vitamins in the small intestine. In this work, we scrutinized, with small-angle X-ray scattering (SAXS), the crucial functions of bile salts beyond their capacity for the interfacial stabilization of submicrometer-sized lipid particles. By studying a wide compositional range of BS-lipid dispersions using two widely applied lipids for drug-delivery systems (one a monoglyceride being stabilizer-sensitive and the other an aliphatic alcohol being relatively stabilizer-insensitive), we identified the necessary BS to lipid ratios to guarantee full emulsification. A novel ad hoc developed global small-angle-X-ray scattering analysis method revealed that the addition of BS hardly changes the bilayer thicknesses in bicontinuous phases, while significant membrane thinning is observed in the coexisting fluid lamellar phase. Furthermore, we show that a BS strongly decreases the average critical packing parameter. At increasing BS concentration, the order of phases formed is (i) the bicontinuous diamond cubic ( Pn3 m), (ii) the bicontinuous primitive cubic ( Im3 m), and (iii) the fluid lamellar phase ( L α ). These distinctive findings on BS-driven "emulsification" and "membrane curvature reduction" provide new molecular-scale insights for the understanding of the interfacial action of bile salts on lipid assemblies.

Drugs for treating Leishmaniasis, a parasitic tropical orphan disease, currently have several limitations on their use, which topical treatments could alleviate. Topical treatment requires penetration of drugs deep into the skin, which is aided by encapsulation within ultradeformable liposomes. Penetrability depends on the flexibility of the lipid membrane, which may be affected by the drugs. We have studied the biophysical effects of four anti-Leishmania drugs (miltefosine (Milt), amphotericin B (AmpB), indole (Ind), and imiquimod (Imiq)) on a soy phosphatidylcholine/sodium cholate membrane. Using diffuse X-ray scattering techniques, we determined bending modulus ( K C ) and chain order parameter ( S X-ray ) of the membrane at several drug concentrations. Form factor scattering data allowed construction of electron density profiles, which yielded bilayer thickness and area per lipid. Results show that AmpB had the largest effect on K C and S X-ray , causing the bilayer to lose integrity at high concentrations. Imiq and Ind induced slight membrane stiffening, whereas Milt had little effect. Imiq also notably decreased chain order at high concentrations. These results will aid in the design of new topical treatments, where Milt, Ind, and Imiq could be used at any concentration without affecting liposome integrity or physical properties, whereas AmpB should not be used at high concentrations.

The role of vesicle-to-micelle transition has been investigated in the interactions of phospholipid vesicles, phospholipid/sodium cholate (NaC) mixed vesicles, and phospholipid/NaC mixed micelles with curcumin in aqueous solution. The addition of NaC causes phospholipid vesicles to transit into phospholipid/NaC mixed vesicles and phospholipid/NaC mixed micelles. Turbidity measurement reveals that the presence of curcumin increases the NaC concentration for the solubilization of phospholipid vesicles, which indicates that the bound curcumin tends to suppress the vesicle-to-micelle transition. The pyrene polarity index and curcumin fluorescence anisotropy measurements suggest that phospholipid/NaC mixed micelles have a more compact structure than that of phospholipid vesicles and phospholipid/NaC mixed vesicles. Curcumin associated with phospholipid vesicles, phospholipid/NaC mixed vesicles, and phospholipid/NaC mixed micelles often results in higher intensities of absorption and fluorescence than those of free curcumin. However, phospholipid/NaC mixed vesicles lead to the highest values of absorption and fluorescence intensities, binding constant, and radical-scavenging capacity with curcumin. The different structures in the phospholipid bilayer of phospholipid/NaC mixed vesicles and the hydrophobic part of phospholipid/NaC mixed micelles where curcumin located are discussed to explain the interaction behaviors of phospholipid/NaC mixed systems with curcumin.

Interaction of endogenous sodium cholate (SC) with dietary amphiphiles would induce structural evolution of the self-assembled aggregates, which inevitably affects the hydrolysis of fat in the gut. Current work mainly focused on the interaction of bile salts with classical double-layered phospholipid vesicles. In this paper, the thermodynamics and structural evolution during the interaction of SC with novel unilamellar vesicles formed from vitamin-derived zwitterionic bolaamphiphile (DDO) were characterized. It was revealed that an increased temperature and the presence of NaCl resulted in narrowed micelle-vesicle coexistence and enlarged the vesicle region. The coexistence of micelles and vesicles mainly came from the interaction of monomeric SC with DDO vesicles, whereas micellar SC contributed to the total solubilization of DDO vesicles. This research may enrich the thermodynamic mechanism behind the structure transition of the microaggregates formed by amphiphiles in the gut. It will also contribute to the design of food formulation and drug delivery system.

Lipid metabolism is accompanied by the solubilization of lipid bilayer membranes by bile salts. We use Brownian dynamics simulations to study the solubilization of model membranes and vesicles by sodium cholate. The solubilization pathways of small and large vesicles are found to be different. Both results for small and large vesicles can be compared with predictions of a macroscopic theoretical description. The line tension of bilayer edges is an important parameter in the solubilization process. We propose a simple method to determine the line tension by analyzing the shape fluctuations of planar membrane patches. Macroscopic mechanical models provide a reasonable explanation for processes observed when a spherical vesicle consisting of lipids and adsorbed bile salt molecules is transformed into mixed lipid-bile salt micelles.

We report a novel green chemical approach for the synthesis of blue light-emitting and water-soluble Ag subnanoclusters, using sodium cholate (NaC) as a template at a concentration higher than the critical micelle concentration (CMC) at room temperature. However, under photochemical irradiation, small anisotropic and spherically shaped Ag nanoparticles (3-11 nm) were obtained upon changing the concentration of NaC from below to above the CMC. The matrix-assisted laser desorption ionization time-of-flight and electrospray ionization mass spectra showed that the cluster sample was composed of Ag4 and Ag6. The optical properties of the clusters were studied by UV-visible and luminescence spectroscopy. The lifetime of the synthesized fluorescent Ag nanoclusters (AgNCs) was measured using a time-correlated single-photon counting technique. High-resolution transmission electron microscopy was used to assess the size of clusters and nanoparticles. A protocol for transferring nanoclusters to organic solvents is also described. Toxicity and bioimaging studies of NaC templated AgNCs were conducted using developmental stage zebrafish embryos. From the survival and hatching experiment, no significant toxic effect was observed at AgNC concentrations of up to 200 μL/mL, and the NC-stained embryos exhibited blue fluorescence with high intensity for a long period of time, which shows that AgNCs are more stable in living system.

The present study demonstrates a detailed characterization of the interaction of a series of bile salts, sodium deoxycholate (NaDC), sodium cholate (NaC), and sodium taurocholate (NaTC), with a model transport protein, human serum albumin (HSA). Here, steady-state and time-resolved fluorescence spectroscopic techniques have been used to characterize the interaction of the bile salts with HSA. The binding isotherms constructed from steady-state fluorescence intensity measurements demonstrate that the interaction of the bile salts with HSA can be characterized by three distinct regions, which were also successfully reproduced from the significant variation of the emission wavelength (λ(em)) of the intrinsic tryptophan (Trp) moiety of HSA. The time-resolved fluorescence decay behavior of the Trp residue of HSA was also found to corroborate the steady-state results. The effect of interaction with the bile salts on the native conformation of the protein has been explored in a circular dichroism (CD) study, which reveals a decrease in α-helicity of HSA induced by the bile salts. In accordance with this, the esterase activity of the protein-bile salt aggregates is found to be reduced in comparison to that of the native protein. Our results exclusively highlight the fact that it is the hydrophobic character of the bile salt that governs the extent of interaction with the protein. Isothermal titration calorimetry (ITC) and molecular docking studies further substantiate our other experimental findings.

Achieving oxygen reduction at high positive potentials with fast heterogeneous electron transfer is desirable for the biocathode of fuel cells based on enzymes. Here, we present an effective interface for obtaining direct electron transfer from a laccase (Lac)-based cathode for O2 reduction, starting from a potential very close to the redox equilibrium potential of the oxygen/water couple. The interface between Lac and single-walled carbon nanotubes was improved by modification with the steroid biosurfactant sodium cholate. The heterogeneous electron-transfer rate between the type-1 Cu site of Lac and the modified electrode was determined to be 3000 s(-1). The electron-transfer rate was sensitive to the side chain of the steroid biosurfactant, and the rate decreased more than 10-fold when sodium deoxycholate was used as the side chain.

Surfactant mediated exfoliation of multilayer graphene and its effects on phenanthrene adsorption were investigated using a passive dosing technique. In the absence of surfactant (sodium cholate, NaC), multilayer graphene had higher adsorption capacity for phenanthrene than carbon nanotube and graphite due to the higher surface area and micropore volume. The observed desorption hysteresis is likely caused by the formation of closed interstitial spaces through folding and rearrangement of graphene sheets. In the presence of NaC (both 100 and 8000 mg/L), phenanthrene adsorption on graphene was decreased due to the direct competition of NaC molecules on the graphene surface. With the aid of sonication, multilayer graphene sheets were exfoliated by NaC, leading to better dispersion. The degree of dispersion depended on the graphene-NaC ratio in aqueous solution rather than critical micelle concentration of NaC, and the good dispersion occurred after reaching adsorption saturation of NaC molecules on graphene sheets. In addition, exfoliation weakened the competition between phenanthrene and NaC and enhanced the adsorption capacity of graphene for phenanthrene due to exposed new sites. The findings on exfoliation of graphene sheets and related adsorption properties highlight not only the potential applications of multilayer graphene as efficient adsorbent but also its possible environmental risk.

The photophysical behavior of 8-anilino-1-naphthalenesulphonate (ANS) in vesicles of dipalmitoylphosphatidylcholine (DPPC), a pulmonary surfactant, has been carried out in a detailed manner. ANS shows notable variations in fluorescence intensity, lifetime, and anisotropy parameters as it gets into the vesicle. It was found that ANS partitions well into the DPPC bilayer membrane with an estimated partition coefficient of ~2.0 × 10(5). Among the various fluorescence parameters of ANS, fluorescence anisotropy was found to be most responsive to the temperature induced phase change of the bilayer membrane. These interesting fluorescence parameters of ANS were then used to study the hydration of lipid bilayer membrane by submicellar concentration of bile salts. From the steady-state fluorescence intensity and dynamic fluorescence lifetime analyses it is clear that ANS is able to probe the submicellar concentration (≤1 mM) of bile salt induced hydration of lipid bilayer membrane that accompanies expulsion of ANS from the bilayer to the aqueous bulk phase. Lower-temperature shift in the phase transition of DPPC bilayer indicates that fluorescence anisotropy of ANS is sensitive enough to the bile salt induced perturbation in the packed acyl chains of DPPC bilayer and modification in the membrane fluidity. In presence of sodium deoxycholate (NaDC) and sodium cholate (NaC) in DPPC vesicles, ANS experiences restriction in rotational mobility which is evident from the variation in steady-state fluorescence anisotropy and fluorescence anisotropy decay parameters.

Two-dimensional diffusion ordered spectroscopy (2D DOSY) NMR was used to probe the micellar structure of sodium dodecyl sulfate (SDS) and sodium cholate (SC) in aqueous solutions with and without semiconducting and metallic single-walled carbon nanotubes (SWCNTs). The solutions contain SDS and SC at weight ratios of 1:4 and 3:2, the ratios commonly used to isolate semiconducting and metallic SWCNTs through density gradient ultracentrifugation (DGU). These results show that the coverage of surfactant on the semiconducting and metallic SWCNTs is nearly identical in the 1:4 surfactant mixture, and a lower degree of bundling is responsible for the greater buoyancy of semiconducting SWCNTs. In the 3:2 surfactant mixture, the metallic SWCNTs are only encapsulated in SC while the semiconducting SWCNTs remain encapsulated in a poorly packed two-surfactant micelle, leading to a large buoyant density difference between the electronic species. This work provides insight into future directions to increase the purity of semiconducting and metallic SWCNTs sorted through DGU and demonstrates the utility of 2D DOSY NMR in probing SWCNT-surfactant complexes.

Mutual diffusion coefficients have been measured for several average compositions of the system sodium cholate-sodium deoxycholate-water at 25 °C. The experiments have been grouped in different sets having constant concentration of one component and variable concentration of the other one. Following this approach, it has been found that the trends of the main- and cross-term diffusion coefficients can be interpreted on the basis of the diffusion and equilibrium results of similar experiments performed on the two binary systems sodium cholate-water and sodium deoxycholate-water. Implications of the presented results in the transport of lipids operated by bile salt aggregates are mentioned. The method proposed in this work, able to connect the diffusivities of an n-component system to those of the related n-1 subsystems, can be extended to obtain qualitative prediction on the diffusion coefficient trends for mixtures of other surfactants, of both industrial and biological interest.

The two enantiomers of the nonsteroidal antiinflammatory drug naproxen and of its methyl ester have been selected as representative probes with markedly different hydrophobicity to assess the potential of cholic acid aggregates as drug carriers by means of photophysical techniques. The different distribution of the probes between bulk solution and aggregates has been assessed by quenching of their singlet and triplet excited states by iodide and nitrite anions, respectively. This straightforward photophysical methodology can, in principle, be extended to a variety of drugs containing a photoactive chromophore.

The conformation of a cholate hexamer with a clicked tether in between two tricholate units and pyrene groups at the chain ends was studied by fluorescence spectroscopy. In contrast to the parent cholate hexamer that folded in all micelles investigated, the folding of the clicked hexamer was highly dependent on the type of surfactant used to solubilize the compound. The clicked oligocholate folded in the Brij 35 micelle, possibly due to the latter's small size and strong internal hydrophobicity. The oligocholate formed intermolecular aggregates in SDS solutions below the CMC of the surfactant. The aggregates were dissociated by the SDS micelles but the individual oligocholate stayed unfolded. In Triton X-100 and sodium cholate solutions, the aggregated, unfolded, and folded oligocholates coexisted and gradual unfolding occurred with an increasing concentration of the surfactant. The conformation of the clicked oligocholate was sensitive to the nonideal mixing of ionic/nonionic micelles and to the unconventional aggregation of sodium cholate.

We report a facile method to synthesize water-soluble gold nanoparticles (AuNPs) using a biosurfactant sodium cholate as reducing reagents and protective groups in aqueous solution at ambient temperature. The diameters (13-70 nm) of uniform AuNPs can be readily adjusted by changing the initial molar ratio of sodium cholate to chloroauric acid (HAuCl(4)). Also, the alkaline condition of preparative solution is found to affect the size of as-synthesized AuNPs. This synthetic approach is one-step and "green". The obtained AuNPs exhibit a good electrocatalytic activity toward methanol oxidation. Meanwhile, the AuNPs thin films can serve as an efficient substrate for surface-enhanced Raman scattering (SERS). Furthermore, platinum nanoparticles (PtNPs) are also prepared by reducing sodium tetrachloro platinate hydrate with sodium cholate., (© 2011 American Chemical Society)

Bile salts, including sodium cholate (NaCh), are amphiphilic molecules with a concave hydrophilic side and a convex hydrophobic side. By forming aggregates in aqueous solution, these natural surfactants fulfill vital biological roles in the solubilization of cholesterol, lipids, and fat-soluble vitamins and thus are involved in the transport and absorption of important biological molecules. Following our success with the encapsulation of fluorescent protein chromophore (FP) analogs by synthetic hydrophobic and hydrophilic hosts, based upon substitution patterns, we now report the binding and turn on of other analogs by bile salt aggregates, observations which may lead to new tools for studying trafficking in these important systems.

An aqueous solution containing photosynthetic reaction centers (RCs), membrane scaffold proteins (MSPs), phospholipids, and single-walled carbon nanotubes (SWCNTs) solubilized with the surfactant sodium cholate (SC) reversibly self-assembles into a highly ordered structure upon dialysis of the latter. The resulting structure is photoelectrochemically active and consists of 4-nm-thick lipid bilayer disks (nanodisks, NDs) arranged parallel to the surface of the SWCNT with the RC housed within the bilayer such that its hole injecting site faces the nanotube surface. The structure can be assembled and disassembled autonomously with the addition or removal of surfactant. We model the kinetic and thermodynamic forces that drive the dynamics of this reversible self-assembly process. The assembly is monitored using spectrofluorimetry during dialysis and subsequent surfactant addition and used to fit a kinetic model to determine the forward and reverse rate constants of ND and ND-SWCNT formation. The calculated ND and ND-SWCNT forward rate constants are 79 mM(-1) s(-1) and 5.4 × 10(2) mM(-1) s(-1), respectively, and the reverse rate constants are negligible over the dialysis time scale. We find that the reaction is not diffusion-controlled since the ND-SWCNT reaction, which consists of entities with smaller diffusion coefficients, has a larger reaction rate constant. Using these rate parameters, we were able to develop a kinetic phase diagram for the formation of ND-SWCNT complexes, which indicates an optimal dialysis rate of approximately 8 × 10(-4) s(-1). We also fit the model to cyclic ND-SWCNT assembly and disassembly experiments and hence mimic the thermodynamic forces used in regeneration processes detailed previously. Such forces may form the basis of both synthetic and natural photoelectrochemical complexes capable of dynamic component replacement and repair.

Very recently, bile salt biosurfactants have been utilized extensively to disperse individual single-walled carbon nanotubes (SWNTs) in aqueous solution with high weight fractions, as well as to sort SWNTs according to their electronic properties with the aid of ultracentrifugation. To help elucidate the role of bile salts in the SWNT dispersion process, we report the first detailed large-scale all-atomistic molecular dynamics (MD) simulation study of the adsorption and surface self-assembly of a common bile salt surfactant, sodium cholate (SC), on a SWNT in aqueous solution. We find that the cholate ions wrap around the SWNT like a ring and have a small tendency to orient perpendicular to the cylindrical axis of the SWNT, a unique feature that has not been observed for conventional linear surfactants such as sodium dodecyl sulfate (SDS). In addition, we carry out a series of simulations to compute the potential of mean force (PMF) between two parallel SC-covered SWNTs as a function of the intertube separation. By comparing our simulated PMF profile of SC with the PMF profile of SDS reported in the literature, we found that, at the saturated surface coverages, SC is a better stabilizer than SDS, a finding that is consistent with the widespread use of SC to disperse SWNTs in aqueous media. Indeed, the superior dispersion-induced stability of SC over SDS results from a higher repulsive energy barrier and a shallower attractive energy well induced by SC in the PMF profile. In particular, we found that the shallower attractive energy well induced by SC is due to the rigid, bean-like structure of SC which allows this bile salt surfactant to more effectively accommodate the intertube gap.

In this study, 1-naphthol has been used as a sensitive ESPT fluorescent molecular probe to investigate the interaction of submicellar concentrations of two physiologically important bile salts, sodium deoxycholate and sodium cholate, with dipalmitoylphosphatidylcholine small unilamellar vesicles in solid gel and liquid crystalline phases. Steady-state and time-resolved fluorescence of the two excited state prototropic forms of 1-naphthol indicate that the incorporation of monomeric bile salt molecules in the lipid bilayer membrane induces appreciable wetting of the bilayer up to the hydrocarbon core region, even at very low (≤1 mM) concentrations of the bile salts.

Nascent high-density lipoproteins (HDLs), which are also known as discoidal HDLs, are formed by the interaction of apolipoprotein A-I (apoA-I) with transmembrane ATP-binding cassette transporter A1 (ABCA1). However, the molecular mechanism governing disc formation is not fully understood. Here, we evaluated the thermodynamic and kinetic stability of disc formation from mixtures of 1-palmitoyl-2-oleoylphosphatidylcholine and apoA-I by quantifying the discs and vesicles produced. Sodium cholate dialysis experiments revealed that the discs are thermodynamically more stable than the vesicle/apoA-I mixture (Delta*G = -52 kJ/disc mol at 37.0 degrees C) because the decrease in enthalpy (Delta*H = -620 kJ/disc mol) exceeds the decrease in entropy (TDelta*S = -570 kJ/disc mol). Circular dichroism spectral measurements ascribed 68% of the decrease in enthalpy during disc formation to the formation of helices in apoA-I. Fluorescence measurements suggested that phospholipids enclosed in the discs are more closely packed than those in the vesicles so that they are entropically destabilized. To determine if the disc could be spontaneously produced from vesicles, we measured the decrease in the turbidity of vesicles in response to the addition of apoA-I. However, the rate of disc formation was very slow, suggesting that the large kinetic barrier against disc formation makes the vesicle/apoA-I mixtures metastable. These results raise the possibility that ABCA1 may act to lower the activation energy, thereby facilitating disc formation.

Bile salt micelles can be employed as a pseudostationary phase in micellar electrokinetic capillary chromatography (MEKC) separations of chiral analytes. To improve MEKC separations of chiral analytes, a molecular level understanding of micelle aggregation in the presence of analyte is needed. Here, aggregation of sodium cholate has been observed by exploiting the presence of a model analyte molecule. The 31P and 1H nuclear magnetic resonance spectroscopy (NMR) chemical shifts of (R,S)-1,1'-binaphthyl-2,2'-diylhydrogenphosphate ((R,S)-BNDHP), a model analyte in chiral MEKC separations, are demonstrated to be very sensitive to the aggregation state of the bile salt sodium cholate. In addition to probing micellar aggregation, the NMR spectral resolution of enantiomeric species is also stronglycorrelated with chiral separations in MEKC. In this work, the aggregation of sodium cholate in basic solutions (pH 12) has been observed over the concentration range 0-100 mM. The primary critical micelle concentration (cmc) was found to be 14 +/- 1 mM for basic solutions of sodium cholate. In addition, a primitive aggregate is clearly observed to form at 7 +/- 1 mM sodium cholate. The data also show pseudo-cmc behavior for secondary aggregation observed in the regime of 50-60 mM cholate. Finally, the H5-H7 edge of BNDHP is shown to be sensitive to chirally selective interactions with primary cholate micelles.

The hydrodynamic properties of surfactant encapsulated single-walled carbon nanotubes (SWNTs) have been characterized by optically measuring their spatial and temporal redistribution in situ in an analytical ultracentrifuge. The measured redistribution profiles are fit to the Lamm equation, thus determining the sedimentation, diffusion, and hydrodynamic frictional coefficients of the surfactant encapsulated SWNTs. For sodium cholate encapsulated SWNTs, we demonstrate that the technique of analytical ultracentrifugation can be utilized to determine the linear packing density of surfactant molecules along the length of the SWNTs, 3.6 +/- 0.8 nm(-1), and the anhydrous molar volume of the surfactant molecules on the SWNT surfaces, 270 +/- 20 cm(3) mol(-1). Additionally, analytical ultracentrifugation is used to measure and compare the sedimentation rates of bundled and isolated carbon nanotubes. This study should serve as a guide for designing centrifuge-based processing procedures for preparing samples of SWNTs for a wide variety of applications and studies. Additionally, the results obtained here should aid in understanding the hydrodynamic properties of SWNTs and the interactions between SWNTs and surfactants in aqueous solution.

Many envision a future where hydrogen is the centerpiece of a sustainable, carbon-free energy supply. For example, the energy in sunlight may be stored by splitting water into H2 and O2 using inorganic semiconductors and photoelectrochemical approaches or with artificial photosynthetic systems that seek to mimic the light absorption, energy transfer, electron transfer, and redox catalysis that occurs in green plants. Unfortunately, large scale deployment of artificial water-splitting technologies may be impeded by the need for the large amounts of precious metals required to catalyze the multielectron water-splitting reactions. Nature provides a variety of microbes that can activate the dihydrogen bond through the catalytic activity of [NiFe] and [FeFe] hydrogenases, and photobiological approaches to water splitting have been advanced. One may also consider a biohybrid approach; however, it is difficult to interface these sensitive, metalloenzymes to other materials and systems. Here we show that surfactant-suspended carbon single-walled nanotubes (SWNTs) spontaneously self-assemble with [FeFe] hydrogenases in solution to form catalytically active biohybrids. Photoluminescence excitation and Raman spectroscopy studies show that SWNTs act as molecular wires to make electrical contact to the biocatalytic region of hydrogenase. Hydrogenase mediates electron injection into nanotubes having appropriately positioned lowest occupied molecular orbital levels when the H2 partial pressure is varied. The hydrogenase is strongly attached to the SWNTs, so mass transport effects are eliminated and the absolute potential of the electronic levels of the nanotubes can be unambiguously measured. Our findings reveal new nanotube physics and represent the first example of "wiring-up" an hydrogenase with another nanoscale material. This latter advance offers a nonprecious metal route to the design of new biohybrid architectures and building blocks for hydrogen-related technologies.

The electrophoretic mobilities of single-walled carbon nanotubes (SWNTs) in agarose gels subjected to negatively charged covalent functionalization and noncovalent anionic surfactant adsorption are compared using a simplified hydrodynamic model. Net charges are calculated on the basis of estimated friction coefficients for cylindrical rodlike particles. The effects of functionalization with negatively charged 4-hydroxybenzene diazonium and anionic sodium cholate are quantified and compared with model predictions. The adsorption of Na+ counterions into the nonionic surfactant layer adsorbed on SWNTs (Triton-X-405) is shown to induce a positive charge and reverse the mobility under select conditions. This effect has not been identified or quantified for nanoparticle systems and may be important in the processing of these systems.

Effect of sodium cholate (NaC) bile salt on the absorption and fluorescence properties of berberine cation was studied in aqueous solution and water-cosolvent mixtures. The alteration of the fluorescent behavior with increasing NaC concentration showed an entirely different trend from that found previously in the presence of sodium dodecylsulfate. Binding to bile salt agglomerates led to significant fluorescence intensity enhancement, and the fluorescence lifetime of berberine proved to be highly sensitive to the structure and size of the aggregates. The dual exponential decay kinetics above 10 mM NaC concentration showed that the probe resided in two totally different binding sites. At 2-10 mM NaC concentrations, only primary aggregates were detected. The aggregate disrupting power of cosolvents decreased in the series of dimethylformamide, acetonitrile, formamide, and methanol. These compounds enhanced the water accessibility of berberine bound to aggregates and diminished the number of secondary aggregates.

The kinetics of single-walled carbon nanotube rebundling have been investigated by photoluminescence (PL) spectroscopy. The rate of loss of PL intensity was measured for 12 different nanotubes in three common aqueous surfactants (sodium dodecyl sulfate, SDS; sodium dodecylbenzene sulfonate, SDBS; and sodium cholate, SC) as the surfactant suspensions were diluted to promote nanotube rebundling, quenching of semiconductor nanotube PL, and precipitation. The rate of PL decay was first-order in the concentration of isolated nanotubes, as expected if surfactant desorption is rate-limiting in the rebundling process. Temperature-dependent measurements permitted an Arrhenius analysis from which diameter-dependent activation energies were determined. SDS was found to have very strong diameter dependence for activation energy, with stronger binding to smaller-diameter nanotubes, whereas SDBS displayed a weaker diameter dependence. SC was found to bind strongly to certain nanotubes and weakly to the (10,2) nanotube. The PL emission red shifted with time after dilution as surfactant desorption proceeded. This effect is attributed to an increase in the micropolarity at the nanotube surface.

We examine the Breit-Wigner-Fano (BWF) line shape in the Raman spectra of carbon single-wall nanotubes (SWNTs) dispersed in aqueous suspensions. Bundling and electronic effects are studied by comparing undoped SWNTs (C-SWNTs) to boron-doped nanotubes (B-SWNTs) in a variety of different surfactant solutions. For SWNTs dispersed with nonionic surfactants that are less effective in debundling than ionic surfactants, the Raman spectra retain a large BWF feature. However, we demonstrate that even for SWNTs dispersed as isolated nanotubes by ionic surfactants the BWF feature may be present and that the intensity of the BWF is highly sensitive to the specific surfactant. In particular, surfactants with electron-donating groups tend to enhance the BWF feature. Also, modification of the SWNT electronic properties by boron doping leads to enhanced surfactant dispersion relative to undoped C-SWNTs and also to modification of the BWF feature. These observations are in agreement with reports demonstrating an enhancement of the BWF by bundling but also agree with reports that suggest electron donation can enhance the BWF feature even for isolated SWNTs. Importantly, these results serve to caution against using the lack or presence of a BWF feature as an independent measure of SWNT aggregation in surfactant dispersions.

Muscarinic cholinergic receptors can appear to be more numerous when labeled by [(3)H]quinuclidinylbenzilate (QNB) than by N-[(3)H]methylscopolamine (NMS). The nature of the implied heterogeneity has been studied with M(2) receptors in detergent-solubilized extracts of porcine atria. The relative capacity for [(3)H]NMS and [(3)H]QNB was about 1 in digitonin-cholate, 0.56 in cholate-NaCl, and 0.44 in Lubrol-PX. Adding digitonin to extracts in cholate-NaCl increased the absolute capacity for both radioligands, and the relative capacity increased to near 1. The latency cannot be attributed to a chemically impure radioligand, instability of the receptor, an irreversible effect of NMS, or a failure to reach equilibrium. Binding at near-saturating concentrations of [(3)H]QNB in cholate-NaCl or Lubrol-PX was blocked fully by unlabeled NMS, which therefore appeared to inhibit noncompetitively at sites inaccessible to radiolabeled NMS. Such an effect is inconsistent with the notion of functionally distinct, noninterconverting, and mutually independent sites. Both the noncompetitive effect of NMS on [(3)H]QNB and the shortfall in capacity for [(3)H]NMS can be described quantitatively in terms of cooperative interactions within a receptor that is at least tetravalent; no comparable agreement is possible with a receptor that is only di- or trivalent. The M(2) muscarinic receptor therefore appears to comprise at least four interacting sites, presumably within a tetramer or larger array, and ligands appear to bind in a cooperative manner under at least some conditions.

The extent to which bovine cytochrome c oxidase (COX) dimerizes in nondenaturing detergent environments was assessed by sedimentation velocity and equilibrium. In contrast to generally accepted opinion, the COX dimer is difficult to maintain and is the major oligomeric form only when COX is solubilized with a low concentration of dodecylmaltoside, i.e., approximately 1 mg/mg protein. The dimer form is intrinsically unstable and dissociates into monomers with increased detergent concentration, i.e., >5 mg/mg protein. The structure of the solubilizing detergent, however, greatly alters detergent effectiveness by inducing either monomerization or aggregation. Triton X-100 is most effective at solubilizing COX, but it destabilizes COX dimers, even at low concentration. Undecylmaltoside, decylmaltoside, and octaethyleneglycolmonododecyl ether (C(12)E(8)) are less effective at solubilizing COX. Each prevents COX aggregation at high detergent concentration, but also destabilizes the COX dimer. Other detergents, e.g., Tween 20, sodium cholate, sodium deoxycholate, CHAPS, or CHAPSO, are completely ineffective COX solubilizers and do not prevent aggregation even at 10-40 mg/mL. The transition from dimers to monomers depends on many factors other than detergent structure and concentration, e.g., protein concentration, phospholipid content and pH. We conclude that the intrinsic dimeric structure of COX can be maintained only after solubilization with low concentrations of dodecylmaltoside at near neutral pH, and even then precautions must be taken to prevent its dissociation into monomers.

The PufX membrane protein is essential for photosynthetic growth of Rhodobacter sphaeroides wild-type cells. PufX is associated with the reaction center-light harvesting 1 (RC-LH1) core complex and plays a key role in lateral ubiquinone/ubiquinol transfer. We have determined the PufX/RC stoichiometry by quantitative Western blot analysis and RC photobleaching. Independent of copy number effects and growth conditions, one PufX molecule per RC was observed in native membranes as well as in detergent-solubilized RC-LH1 complexes which had been purified over sucrose gradients. Surprisingly, two gradient bands with significantly different sedimentation coefficients were found to have a similar subunit composition, as judged by absorption spectroscopy and protein gel electrophoresis. Gel filtration chromatography and electron microscopy revealed that these membrane complexes represent a monomeric and a dimeric form of the RC-LH1 complex. Since PufX is strictly required for the isolation of dimeric core complexes, we suggest that PufX has a central structural role in forming dimeric RC-LH1 complexes, thus allowing efficient ubiquinone/ubiquinol exchange through the LH1 ring surrounding the RC.