Formation Of Mixed Micelles Research Articles

Comparative study on lipid digestion and carotenoid bioaccessibility of emulsions, nanoemulsions and vegetable-based in situ emulsions

Emulsions (d43 0.484–0.897 μm) and nanoemulsions (d43 0.279–0.226 μm) based on lipid fractions enriched with carotenoids from different sources were produced by increasing the amount of a non-ionic surfactant (Tween 80) at surfactant-to-oil ratio (SOR) 0.1 or 2, respectively. Alternatively, carrot purée in situ emulsions were fabricated by processing carrot purée with a non-enriched oil fraction and Tween 80 (SOR 0.1). Emulsions, nanoemulsions and in situ emulsion were subjected to in vitro digestion conditions with focus on the kinetics of lipid digestion, micelle formation and carotenoid bioaccessibility during the small intestinal phase. Nanoemulsions produced with a high amount of surfactant showed an almost complete lipid hydrolysis inhibition probably due to hindered lipase adsorption at oil-water interfaces. Emulsions presented a gradual lipid digestion during the intestinal phase and subsequent incorporation of lipid digestion products into mixed micelles. Interestingly, the in situ emulsion presented a similar lipid digestion trend than emulsions, yet a lower concentration of lipid digestion products incorporated into mixed micelles, which resulted in a lower carotenoid bioaccessibility. Overall, this study highlights the potential of using nanoemulsions to modulate lipid digestion. Moreover, it also indicates possible interactions between food components and lipid digestion products during the formation of mixed micelles and its implications on the bioaccessibility of lipophilic bioactives such as carotenoids.

Synergism in cationic surfactant and triton X-100 mixtures: Role of enthalpic interactions and conformation changes

Investigation of fundamental properties of colloidal systems of cationic surfactants is important for practical and theoretical reasons. In this work, the nonideal behavior in cetyltrimethylammonium bromide and Triton X-100 mixtures was studied, during the mixed micelle formation. In order to study the aggregation behavior, excess Gibbs energy (GE), excess enthalpy (HE) and excess entropy (SE) were determined. The thermodynamic parameters were monitored as a function of the molar ratio, at different temperatures (283.15–328.15 K). GE, HE and SE were obtained based on the critical micelle concentration values of investigated mixtures. The results of the study indicate existence of synergism in the surfactant mixtures. The increase of the stability of the mixed micelles (GE < 0) is explained as the result of enthalpic interactions and/or entropy changes during the formation of the aggregates.

Aqueous electrophoretic deposition of drugs using bile acids as solubilizing, charging and film-forming agents

A new electrophoretic deposition (EPD) technique is reported for the fabrication of composite coatings containing drugs, which have low solubility in water. We present a conceptually new approach, which is based on the use of bile salts (BS) as solubilizing, charging and film forming agents. Composite films are obtained using tetracycline and ibuprofen as model drugs with commercial BS, such as cholic acid sodium salt and deoxycholic acid sodium salt. The suggested mechanism involves the solubilization of drugs in BS solutions, followed by the formation of mixed charged micelles, containing drugs and charged BS, which allow electrophoretic transport of the drugs to the electrode surface. The EPD mechanism involves pH decrease at the anode surface, protonation of carboxylic groups of BS and formation of water insoluble films. The approach can be used for deposition of other functional organic molecules and inorganic nanoparticles.

Effect of Oleic Acid on the Dynamic Interfacial Tensions of Surfactant Solutions

The dynamic interfacial tensions (IFTs) of three different types of surfactant solutions, cationic surfactant hexadecyl trimethylammonium (CTAB), anionic surfactant sodium dodecyl benzenesulfonate (SDBS), and non-ionic surfactant Triton X-100 (TX-100), against model oil containing oleic acid (OA) have been investigated by a spinning drop interfacial tensiometer. The influences of the OA concentration and addition of alkali have been studied. The experimental results show that OA added to the oil phase has a significant effect on interfacial behaviors of surfactant solutions. An obvious synergistic effect on reducing IFT can be observed for low-concentration cationic surfactant CTAB solution because the tight mixed adsorption films will be formed through electrostatic attraction between CTAB and OA molecules. However, the synergistic effect will disappear at a high surfactant concentration by the formation of mixed micelles. For the anionic surfactant SDBS system, the mixed adsorption of SDBS and OA molecu...

Synergistic effect of an antipsychotic drug chlorpromazine hydrochloride with pluronic triblock copolymer: A physicochemical study

Chlorpromazine is a psychiatric medication that belongs to the class of drugs called phenothiazine antipsychotics. It works by helping to restore the balance of certain natural substances in the brain. In the current study, the micellization behavior of chlorpromazine hydrochloride (CPZ) drug with Pluronic F-127 in the absence/presence of 0.1M NaCl at 298K temperature, has been studied. Tensiometric profile, for pure and mixtures of CPZ+F-127, generally consists of two breaks, whereas a single break is presents in pure CPZ tensiometric isotherm. Various bulk and interfacial parameters, such as critical micelle concentration (cmc), interaction parameter (β), maximum surface excess concentration (Γmax), effectiveness of surface tension reduction (πcmc), and other surface parameters at the interface have been estimated using a surface tension technique. Theories of Clint, Rosen, and Rubingh have been used to determine the theoretical cmc, interaction between amphiphiles at interface and micellar state of aggregation, and the composition of aggregates. The experimental cmc values of the mixed systems are lower than that of ideal one (cmcideal) over the entire mole fraction range of CPZ. A negative departure of cmc values from the ideal indicate that the mixed micelle formation occurs due to favorable mixing (synergistic interactions) in the mixed states. Several thermodynamic parameters have also been predicted from those theories. The interaction between the CPZ and Pluronic F–127 was also determined by extensive steady–state fluorescence measurements.

Study on a Ternary Surfactant System of Disodium Hexadecyl Diphenyl Ether Disulfonate, Dodecyldimethyl Hydroxyethyl Ammonium Chloride and Fatty Alcohol Polyoxyethylene Ether

AbstractThis study was focused on a ternary complex system composed of hexadecyl diphenyl ether disulfonate (C16-MADS), dodecyl dimethyl hydroxyethyl ammonium chloride (K3) and fatty alcohol polyoxyethylene ether (AEO9). The binary mixture C-K consisting of C16-MADS and K3 was investigated and the maximum synergism was obtained when the mole ratio was 0.33:0.67. Subsequently, AEO9 was added to the binary C-K complex system to investigate whether the behavior of the new complex system, in which C-K is thought to be a surfactant, can be explained by the theory of regular solution. The maximum synergism was achieved when the molar ratio of C-K and AEO9 was 7:3. The regular solution theory could well explain the synergism in mixed micelle formation and the synergism in surface tension reduction efficiency, but the synergism in surface tension reduction effectiveness did not match with the experimental data. So, there still exist some deficiencies in the application of the regular solution theory to a complex system of three surfactants.

The importance of membrane microdomains for bile salt-dependent biliary lipid secretion.

ABSTRACTAlternative models explaining the biliary lipid secretion at the canalicular membrane of hepatocytes exist: successive lipid extraction by preformed bile salt micelles, or budding of membrane fragments with formation of mixed micelles. To test the feasibility of the latter mechanism, we developed a mathematical model that describes the formation of lipid microdomains in the canalicular membrane. Bile salt monomers intercalate into the external hemileaflet of the canalicular membrane, to form a rim to liquid disordered domain patches that then pinch off to form nanometer-scale mixed micelles. Model simulations perfectly recapitulate the measured dependence of bile salt-dependent biliary lipid extraction rates upon modulation of the membrane cholesterol (lack or overexpression of the cholesterol transporter Abcg5–Abcg8) and phosphatidylcholine (lack of Mdr2, also known as Abcb4) content. The model reveals a strong dependence of the biliary secretion rate on the protein density of the membrane. Taken together, the proposed model is consistent with crucial experimental findings in the field and provides a consistent explanation of the central molecular processes involved in bile formation.

Aggregation study in mixture surfactant system TX-100+SDS in heavy water solutions by SANS method

The mixing of amphiphiles in water may lead to the formation of mixed micelles which often present new properties with respect to the pure component solutions [1,2]. The mixture system of classic surfactants SDS (sodium dodecyl sulfate)+TX-100(p-(1,1,3,3- tetramethyl) poly(oxyethylene) (anionic + non-ionic) in heavy water solutions was investigated at temperatures 30°, 50°, 70°C for compositions 1:1, 2:1, 3:1 by the small-angle neutron scattering(SANS) method on spectrometer (‘YuMO’) at the IBR-2 pulsed neutron source at FLNP, JINR in Dubna (Russia). Measurements have covered Q range from 8x10-3 to 0.4 Å-1. From the measured dependence of the scattered intensity on the scattering angle, we derived the size, shape of micelles, aggregation number at various compositions and temperatures. The size of mixed micelle is a weak function of the mixing ratio between the two components.

Conductometric Probe Analysis of the Effect of Benzyldimethylhexadecylammonium Chloride on the Micellization Behavior of Dodecyltrimethylammonium Bromide in Aqueous/Urea Solution: Investigation of Concentration and Temperature Effect

AbstractSurfactant mixtures are used in many different industrial formulations. In this study, the mixed micelle formation behavior of 2 different cationic surfactants, namely dodecyltrimethylammonium bromide (DTAB) and benzyldimethylhexadecylammonium chloride (BDHAC), in the absence and presence of urea at various temperatures (298.15–318.15 K) was studied using the conductometric method. The attractive interaction between DTAB and BDHAC was estimated from the values of critical micelle concentration (CMC) and the CMC for ideal mixing (CMCid). Urea increases the CMC value as a result of the enrichment in the surface charge of the micelles/mixed micelles. The values of micellar mole fraction (X1Rub [Rubingh], X1M [Motomura], X1Rod [Rodenas]) and ideal micellar (X1id) of surfactant BDHAC were obtained by different models and are shown to exhibit the high contribution or effective involvement of BDHAC in mixed micelles and increase with increasing BDHAC mole fraction (α1). Activity coefficients (f1 and f2) were also evaluated from the relevant formula given in the literature. The negative values of the interaction parameters (β) show the attractive interaction among the studied components. Excess Gibbs free energy (∆Gex) of micellization revealed that the stability of mixed micelles is higher in aqueous solution than in urea solution. The thermodynamic parameters, namely the Gibbs free energy change, enthalpy change, and entropy change (∆Gom, ΔHom, and ∆Som, respectively), were also calculated from the conventional standard equations.

Micellisation of Binary Mixtures of Surfactants Na-Deoxycholate–Na-Decyl Sulfate and Na-Hyodeoxycholate–Na-Decyl Sulfate in Water Solutions: Rational Development of the Thermodynamic Model for the Excess Gibbs Energy (GE)

Micellisation of two binary mixtures, sodium decyl sulfate (NaDS)–sodium deoxycholate (NaDC) and sodium decyl sulfate (NaDS)–sodium hyodeoxychlolate (NaHDC), is examined, in consideration with the different molar fractions of the monomers (x = 0.1–0.9). In bile acids, the OH groups attached to steroid skeletons are spatially ecraned, while the sulfate group of NaDS is not ecraned. Thus, NaDC and NaHDC in relation to NaDS have different coordination numbers in the micelle pseudophase. On the basis of stereochemical analysis, rational design shows that the excess Gibbs energy (GE) of formation of the binary mixed micelle throughout the whole composition range can be described with a two-parameter Margules function, while the GE function from the regular solution theories is applicable only in the narrow interval of the bile salts molar fractions (x = 0.5– 0.8). Coefficient of activity (hypothetical) of the infinitely diluted mixed micelle of NaDS in the presence of NaDC shows significant thermodynamic stabi...

Unveiling the pH dependent interaction between bolaamphiphiles (dicarboxylic acids) and C10TAB (decyltrimethylammonium bromide) in aqueous medium

Development of stable self-assembled nanostructures (especially vesicles and liposomes), and understanding their physicochemical behaviors in aqueous solution is a long-standing topic of interest in chemical and biochemical research. In this progressive area, we report for the first-time formation of mixed micelles (at pH 12), and vesicles of anionic bolaamphiphiles (dicarboxylic acids viz. HOOC-(CH2)n-COOH, with moderate values of n 10, 11, 12, and 14) in combination with a cationic surfactant decyltrimethylammonium bromide (C10TAB) in buffered aqueous medium at different pH (6.0, 6.5, 6.8, and 8.0 for bola 10, 11, 12, and 14, respectively). Three pH dependent states of the solutions are observed: clear (high pH > 8), turbid and translucent (mid pH ≈ 6–8), and viscous inhomogeneous oil-like state (low pH < 6). The micelle size varies from 6.24 to 7.43 nm at pH 12, for vesicles the values are large (220–296 nm), and small (∼30–70 nm) in the pH range of 6.0–8.0. The self-assembly formation properties of their mixtures are herein investigated using different techniques viz. UV–vis spectroscopy, fluorescence spectroscopy, dynamic light scattering, laser confocal scanning microscopy, and isothermal titration calorimetry. The formed vesicles are fairly polydisperse, and show overall spherical shape. Formation of the bilayer assemblies, and their conversion to mixed micelles by the temperature effect are observed from steady state fluorescence anisotropy measurements. Micellization of the mixed bola-C10TAB species is endothermic and fairly entropy controlled; their formation/deformation is pH sensitive. They have spherical morphologies, and once formed at the right pH they are found to be very stable in terms of time. Thus, these vesicles have prospects for encapsulation and delivery of materials like drugs, and other substrates by controling the acid-base conditions of the system environment. The formed mixed micelles, and vesicles are expected to be low toxic, and thus green materials in nature with wider application possibilites.

A gemini surfactant-containing system with abundant self-assembly morphology and rheological behaviors tunable by photoinduction.

Photoresponsive micellar systems with adjustable aggregate morphologies and rheological properties may be useful in a number of fields such as in microfluidics, controlled release, and sensors. However, the complexity and great difficulty of synthesising photosensitive molecules hamper their practical applications to a significant degree. In this study, we constructed a novel photoinduced self-assembly system by introducing the photoresponsive derivative trans-2-methoxy-cinnamate (trans-OMCA) into the gemini surfactant N,N′-bis(dodecyldimethyl)-1,2-ethane diammonium dibromide (12-2-12·2Br−) solutions. The system displays abundant phase behaviors, and the long worm-like micelles, vesicles, as well as an aqueous two-phase system (ATPS) are observed in the 12-2-12·2Br−/trans-OMCA mixed system even at lower surfactant concentrations. The UV-responsive behavior of the formed vesicles and the worm-like micelles is investigated systematically. The results have shown that OMCA undergoes photoisomerization from the trans-form to the cis-form through UV light irradiation that alters the molecular packing at the micellar interface and thus leads to the transformation of micellar morphologies. The long worm-like micelles will turn into much shorter units when the sample is exposed to 365 nm UV light accompanied by a decrease in solution viscosity by more than an order of magnitude. The formed vesicle system, however, can be utilized to generate a multi-state self-assembly structure, including a worm-like micelle and a small spherical micelle, depending on the UV irradiation time. The morphologies of micelles in a 12-2-12·2Br−/trans-OMCA mixed system can be tailored by adjusting the system composition and the duration of UV light irradiation. Correspondingly, the rheological behavior of the 12-2-12·2Br−/trans-OMCA mixed system can be purposely tuned. The light-induced system with abundant self-assembly behaviors and tunable rheological properties would widen the potential application of gemini surfactants in drug delivery, smart fluids, and materials science.

Synergism and aggregation behaviour in an aqueous binary mixture of cationic-zwitterionic surfactants: physico-chemical characterization with molecular simulation approach.

Aqueous interactions between a cationic surfactant benzyl dimethylhexadecylammonium chloride (BDHAC) and alkyldimethylammoniopropane sulfonates (CnDAPS) based three zwitterionic surfactants n = 10, 12, and 14 (abbreviated as C10DAPS, C12DAPS and C14DAPS, respectively) were studied using tensiometry, and fluorescence spectrophotometry techniques. The critical micelle concentration degree of synergism and various other parameters such as interaction parameter (β), activity coefficients (fm) and interfacial parameters such as surface pressure (πCMC), packing parameter (P), surface excess concentration (Γmax), surface tension at CMC (γCMC), and minimum area per molecule (Amin) were evaluated using the Regular Solution Theory (RST) of mixed systems. The results indicate a strong dependency on the mixed system and their composition. For the quantitative prediction, the molecular architecture of the surfactants in mixed systems and their synergistic interactions were investigated by computational simulation using Spartan'14 V1.1.8. The structural optimization results obtained were found to be in good agreement with the estimations made using RST. The reduction in surface tension indicates a certain efficiency in mixed micelle formation owing to electrostatic attraction between the cationic and zwitterionic surfactants. In addition, the binary surfactant systems evaluated by Maeda's approach infer the mixed micelles are thermodynamically stable. The aggregation number (Nagg) appeared to be larger at the composition point where the efficiency of mixed micelle formation is greatest. The strength of the interaction between BDHAC and CnDAPS followed the order: C14DAPS > C12DAPS > C10DAPS indicating a greater synergism at 0.25 molar ratio of zwitterionic surfactants to cationic surfactants in the aqueous solution at 303.15 K.

Tuning Nanoparticle-Micelle Interactions and Resultant Phase Behavior.

The evolution of the interaction between an anionic nanoparticle and a nonionic surfactant and their resultant phase behavior in aqueous solution in the presence of electrolyte and ionic surfactants have been studied. The mixed system of anionic silica nanoparticles (Ludox LS30) with nonionic surfactant decaethylene glycol monododecylether (C12E10) forms a highly stable clear phase over a wide concentration range of surfactant. Small-angle neutron scattering (SANS) and dynamic light scattering data show that the surfactant micelles adsorb on the surface of the nanoparticle, resulting in micellar-decorated nanoparticle structures. With the addition of a small amount of electrolyte into this system, the stability gets disturbed substantially and turns to a two-phase (turbid) system. The evolution of interaction in this system has been examined, and it was found that micelle-induced long-range depletion attraction (modeled by a double Yukawa potential) between nanoparticles leads to their aggregation. Interestingly, the addition of anionic surfactant sodium dodecyl sulfate (SDS) in this two-phase system transforms it to a transparent one-phase state, suppressing the depletion-mediated aggregation of nanoparticles. This is attributed to the formation of anionic C12E10-SDS mixed micelles, and it is their repulsive micelle-micelle interaction that disrupts the depletion phenomenon. On the other hand, the addition of cationic surfactant dodecyl trimethylammonium bromide (DTAB) to the turbid LS30-C12E10 electrolyte system shows no change in the turbidity arising from an aggregated nanoparticle system. The driving interaction, in this case, is different from that of the surfactant-mediated depletion attraction; it is due to the attraction between the nanoparticles mediated by the presence of oppositely charged DTAB micelles between them, resulting in a charge-driven bridging aggregation of nanoparticles. Each of these multicomponent systems has been investigated using contrast variation SANS measurements for different contrast conditions where the role of individual components (nanoparticle or surfactant) in the mixed system has been selectively studied. These results thus show that nanoparticle-surfactant micelle interactions can be tuned by the presence of electrolyte and/or choice of surfactant combination.

P-Sulfonatocalix[6]arene-dodecyltrimethylammonium Supramolecular Amphiphilic System: Relationship between Calixarene and Micelle Concentration.

In this work, the formation of supramolecular mixed micelles from a hexamethylated p-sulfonatocalix[6]arene (SC6HM) derivative and a conventional cationic surfactant (dodecyltrimethylammonium bromide, C12TAB) was investigated by surface tension and using pyrene as a micropolarity fluorescent probe to gain insights into the role of the calixarene concentration on the aggregation behavior. The formation of micelles at a concentration well below the critical micelle concentration of pure surfactant was observed in the presence of very low concentrations of SC6HM (below the micromolar range). Interestingly, the critical micelle concentration of the mixed system was shown to be rather insensitive to the concentration of SC6HM. On the other hand, the concentration of mixed micellar aggregates was demonstrated to be highly dependent on the macrocycle concentration and less dependent on the C12TAB concentration in the range between the critical micelle concentrations of the mixed systems and pure surfactant.

Effects of 1-alkyl-3-methylimidazolium bromide ionic liquids on the micellar properties of [butanediyl-1,4-bis(dimethyldodecylammonium bromide)] gemini surfactant in aqueous solution

The micellar properties of the cationic gemini surfactant [butanediyl-1,4-bis (dimethyldodecylammonium bromide)] (12-4-12) were investigated at 298.15 K in the presence of 1-R-3-methylimidazolium bromide ionic liquids (ILs, R = 1-butyl, 1-(1-butyl)-3-methylimidazolium bromide (BuMeImBr); 1-hexyl, 1-(1-hexyl)-3-methylimidazolium bromide (HxMeImBr); and 1-octyl, 1-(1-octyl)-3-methylimidazolium bromide (OcMeImBr)), present as additives. The emphasis was on c(IL) (0.2 to 1 wt%) and the length of R. The following techniques were employed: conductivity, surface tension, UV-Vis, fluorescence spectroscopy of solubilized compounds, and dynamic light scattering (DLS). For the ILs, values of the critical micelle concentration, critical micellar concentration (cmc) of 12-4-12 decreased between 0.2 and 0.8 wt% and then increased at 1 wt%. The increased cmc may be due to the formation of mixed micelles. For (inorganic additive) NaBr, cmc decreased continuously as a function of c(NaBr). Other parameters calculated include those related to the adsorption of the surfactant at solution/air interface, micelle aggregation number, and hydrodynamic radius. ILs behave as electrolyte and cosurfactants based on c(IL).

Thermodynamic Characterization of Temperature‐ and Composition‐Dependent Mixed Micelle Formation in Aqueous Medium

AbstractThis article provides a detailed description of micellization of a binary nonionic surfactant system (Lutensol AP9 and AP20) at different compositions and temperatures in an aqueous medium. Critical micelle concentrations of the individual components (CMCi) and the mixed surfactants (CMCm) were determined via surface tension measurements in the temperature range of 288.15–318.15 K. On the basis of the magnitude and temperature dependence of CMC values, changes in the Gibbs energy (ΔmicG), enthalpy (ΔmicH), and entropy (ΔmicS) of micellization were evaluated. At 288.15 K the thermodynamic parameters were confirmed by isotherm titration calorimetry (ITC). Regular solution theory (RST) was used to estimate the compositions of the mixed micelles (XM) and molecular interaction parameters (βM) for all molar ratios and temperatures. The results of evaluations based on the closed association model and RST were compared.

BILOSOME: A BILE SALT BASED NOVEL CARRIER SYSTEM GAINING INTEREST IN PHARMACEUTICAL RESEARCH

The human body has long provided pharmaceutical science with biomaterials of interesting applications. Bile salts (BSs) are biomaterials reminiscent of traditional surfactants with peculiar structure and self-assembled topologies. Most of the new drugs, biological therapeutics (proteins/peptides) and vaccines have poor performance after oral administration due to poor solubility or degradation in the gastrointestinal tract (GIT). Though, vesicular carriers exemplified by liposomes or niosomes can protect the entrapped agent to a certain extent from degradation. Nevertheless, the harsh GIT environment i.e, low pH, the presence of bile salts and enzymes limits their capabilities by destabilizing them. In response to that, more resistant bile salts-containing vesicles (BS-vesicles) were developed by the inclusion of bile salts into lipid bilayers constructs. Tremendous research in the last decade has made bilosomes a potential carrier system. Bilosomes with its name derived from bile salts (which is one of its major constituents), is a ‘niosome-like’ colloidal carrier. Here, we focus on different aspects of bile salt based drug delivery systems including their composition, developmental techniques, characterization, comparative advantages of BS-integrated nanomedicines over conventional nanocarriers, stability, transitional modifications and scale-up – emphasizing their biomedical potential in oral immunization against various diseases and delivery of peptide drugs. Bile acid-based amphiphiles, in the form of mixed micelles, bilosomes, drug conjugates and hybrid lipid-polymer nanoparticles are critically discussed as delivery systems for anticancer drugs, antimicrobial agents and therapeutic peptides/proteins, including vaccines. Current pitfalls, future perspectives, and opinions have also been outlined. Bile acid-based nanoparticles are a growing research area therefore, multifaceted pharmaceutical and biomedical applications of bile salts are to be expected in the near future. Key words: Bile acids, bilosomes, vaccines, protein and peptides, bile salt containing liposomes, M-cellÂ

Triblock-Copolymer-Assisted Mixed-Micelle Formation Results in the Refolding of Unfolded Protein.

The present work reports a new strategy for triblock-copolymer-assisted refolding of sodium dodecyl sulfate (SDS)-induced unfolded serum protein human serum albumin (HSA) by mixed-micelle formation of SDS with poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer EO20PO68EO20 (P123) under physiological conditions. The steady-state and time-resolve fluorescence results show that the unfolding of HSA induced by SDS occurs in a stepwise manner through three different phases of binding of SDS, which is followed by a saturation of interaction. Interestingly, the addition of polymeric surfactant P123 to the unfolded protein results in the recovery of ∼87% of its α-helical structure, which was lost during SDS-induced unfolding. This is further corroborated by the return of the steady-state and time-resolved fluorescence decay parameters of the intrinsic tryptophan (Trp214) residue of HSA to the initial nativelike condition. The isothermal titration calorimetry (ITC) data also substantiates that there is almost no interaction between P123 and the native state of the protein. However, the mixed-micelle formation, accompanied by substantial binding affinities, removes the bound SDS molecules from the scaffolds of the unfolded state of the protein. On the basis of our experiments, we conclude that the formation of mixed micelles between SDS and P123 plays a pivotal role in refolding the protein back to its nativelike state.

Phospholipase D Activity in Relation to the Size of Substrate Micelles

AbstractThe activity of phospholipase D (PLD) is strongly dependent on the aggregation state of its substrate. Artificial substrates, such as phosphatidyl‐p‐nitrophenol (PpNP), are preferably used in a mixture with Triton® X‐100 (Serva) and sodium dodecyl sulfate (SDS) in the form of mixed micelles. In this paper the activities of PLD from cabbage (PLDcab), which needs Ca2+ ions for its activity, and PLD from Streptomyces sp. (PLDStr), which is independent of Ca2+ ions, are compared as a function of the detergent and substrate concentrations. While the variation of the Triton® X‐100 and PpNP contents changed the activities of both enzymes in similar way, SDS showed activation effects on plant PLD but inactivation effects on microbial PLD. The activity decreased to 50% at a 7‐tenfold molar excess of Triton® X‐100 related to PpNP. SDS induced a strong activation (up to fourfold) of PLDcab but decreased the activity of PLDStr with increasing concentration. Activity‐substrate profiles of both PLDs passed optima at 3–4 mM PpNP. In all experiments, the activity changes correlated with changes of the micelle sizes determined by dynamic light scattering. These results highlight the immense importance of the micelle structure, which is not considered in most studies.