Thermodynamic Characterization of 3-[(3-Cholamidopropyl

Article pubs.acs.org/Langmuir

Thermodynamic Characterization of 3-[(3-Cholamidopropyl)dimethylammonium]-1-propanesulfonate (CHAPS) Micellization Using Isothermal Titration Calorimetry: Temperature, Salt, and pH Dependence Ana Kroflič, Bojan Šarac, and Marija Bešter-Rogač* Faculty of Chemistry and Chemical Technology, Aškerčeva 5, University of Ljubljana, SI-1000 Ljubljana, Slovenia S Supporting Information *

ABSTRACT: A systematic investigation of the micellization process of a biocompatible zwitterionic surfactant 3-[(3-cholamidopropyl)dimethylammonium]-1-propanesulfonate (CHAPS) has been carried out by isothermal titration calorimetry (ITC) at temperatures between 278.15 K and 328.15 K in water, aqueous NaCl (0.1, 0.5, and 1 M), and buffer solutions (pH = 3.0, 6.8, and 7.8). The effect of different cations and anions on the micellization of CHAPS surfactant has been also examined in LiCl, CsCl, NaBr, and NaI solutions at 308.15 K. It turned out that the critical micelle concentration, cmc, is only slightly shifted toward lower values in salt solutions, whereas in buffer media it remains similar to its value in water. From the results obtained, it could be assumed that CHAPS behaves as a weakly charged cationic surfactant in salt solutions and as a nonionic surfactant in water and buffer medium. Conventional surfactants alike, CHAPS micellization is endothermic at low and exothermic at high temperatures, but the estimated enthalpy of micellization, ΔH0M, is considerably lower in comparison with that obtained for ionic surfactants in water and NaCl solutions. The standard Gibbs free energy, ΔG0M, and entropy, ΔS0M, of micellization were estimated by fitting the model equation based on the mass action model to the experimental data. The aggregation numbers of CHAPS surfactant around cmc, obtained by the fitting procedure also, are considerably low (nagg ≈ 5 ± 1). Furthermore, some predictions about the hydration of the micelle interior based on the correlation between heat capacity change, Δc0p,M, and changes in solvent-accessible surface upon micelle formation were made. CHAPS molecules are believed to stay in contact with water upon aggregation, which is somehow similar to the micellization process of short alkyl chain cationic surfactants.

1. INTRODUCTION With rise of proteomics and its numerous applications, the purification of proteins, especially membrane proteins, became a crucial step to master. However, the isolation of membrane proteins relies on the ability of surfactant used in the purification procedure to extract the protein in a nondenatured state. The charge properties of the protein should not be affected by the surfactant, and the latter should not cause an artificial aggregation of the protein under study. A biocompatible surfactant, 3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulfonate (CHAPS), has proven to overcome these problems. CHAPS is a derivative of cholic acid and combines properties of sulfobetaine-type surfactants and bile salts. It has been widely used in biochemistry for membrane protein and receptor isolation and purification since the 1980s.1−8 In contrast to naturally occurring bile salts and other anionic surfactants, CHAPS is composed of a zwitterionic structure and is thus applicable in many fractional techniques commonly used for protein separation.1,3,7 It has been also used in the extraction process of proteins for further proteomic analysis8 and reconstitution of membrane proteins into lipid vesicles (proteoliposomes).4,9,10 Because of its biocompatibility, noncharged character, nondenaturing, and disaggregating properties, CHAPS is very convenient for use in pharmacy and © 2012 American Chemical Society

medicine and thus also a subject of many investigations. Numerous studies have been devoted to aggregation and catalytic activity of various enzymes in CHAPS solution.3,11−17 Recently, the effect of CHAPS on the nucleosome structure and dynamics has been also investigated.18 The structure of cholic acid surfactants differs strongly from conventional surfactants (hydrophilic headgroup and hydrophobic tail) we have been studying intensively.19 The surfactant under investigation is composed of a rigid steroid structure (bean-like structure; concave and convex side of a skeleton) possessing three hydroxyl groups and a zwitterionic aliphatic tail (Figure 1). Instead of a common micellization process, different aggregation patterns in solution have been predicted for this type of surfactant.20−25 It seems that aggregation of bile salt derivatives proceeds stepwise over a broad concentration range and with a great portion of polydispersity. It is why Seret and Bahri24 suggested that, instead of critical micelle concentration, cmc, usually used to describe the micellization process, the concept of micellar dissociation concentration, defined as a Received: January 15, 2012 Revised: June 10, 2012 Published: June 11, 2012 10363

dx.doi.org/10.1021/la302133q | Langmuir 2012, 28, 10363−10371

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Aldrich, Ultrapure grade, ≥99.9%) and glycine (Merck, p.a., ≥99%) were used as received. The 4 M HCl solution was used to adjust the pH of the buffers. All of the solutions were prepared by weights of pure compounds and demineralized distilled water, obtained from a quartz bidestillation apparatus (Destamat Bi 18E, Heraeus). 2.2. Isothermal Titration Calorimetry (ITC). The heat changes upon micellization were measured using a VP-ITC microcalorimeter from MicroCal, LLC (Northampton, MA, USA). Periodical calibration of the instrument has been proposed electrically by administering a known quantity of power through a resistive heater element located on the cell.37 The precision of the instrument has been also validated through binding the cytidine 2′-monophosphate to the ribonuclease A active site. The standard chemical reaction is described elsewhere.38 Degassed water, NaCl solution (0.1 M, 0.5 M, 1 M), 1 M LiCl, CsCl, NaBr, or NaI solution, or 50 mM buffer solution (pH = 3.0, 6.8, 7.8) in the titration cell was titrated with the degassed surfactant solution in the corresponding solvent from a 0.300 cm3 syringe at temperatures between 278.15 K and 328.15 K. Successive aliquots of surfactant solution were added into the calorimeter cell (V = 1.386 cm3) under constant stirring through a motor driven syringe. Some experiments were carried out in duplicate to check the repeatability of the procedure (Figure A in the Supporting Information). The experimental data were analyzed using the Origin 7.0 software provided by MicroCal. A detailed description of the procedure is given in our previous work.39

Figure 1. (a) Structure of CHAPS surfactant with its hydrophobic cholesterol-like skeleton possessing three hydroxyl groups and a zwitterionic aliphatic tail and (b) schematic representation of CHAPS aggregate.

concentration below which any dilution leads to disruption of the aggregates, should be applied. The efficiency of protein stabilization is highly related to the surfactant concentration. Therefore, it is very important to predict the best nondenaturing conditions. The influence of temperature and added salt (NaCl) on thermodynamic parameters of bile salt aggregation in aqueous solutions has been already investigated by isothermal titration calorimetry (ITC).26,27 It has been found that the hydrophobic effect does not play the expected role in the aggregation of cholic acid derivatives as in the case of classical surfactant micellization.27,28 However, many discrepancies between micellization properties of bile salts can be found in the literature.26,27,29−33 A survey of literature data on micellization process of zwitterionic surfactants reveals that it is strongly affected by the composition of aqueous media.34−36 We thus decided to carry out a systematic investigation of the influence of added electrolyte, pH, and temperature on the aggregation properties of zwitterionic surfactant CHAPS in aqueous solution by ITC as one of the most valuable techniques for thermodynamic analysis. ITC experiments were performed within a broad temperature range of 278.15−328.15 K in 10 K steps with CHAPS solutions in water, 0.1 M, 0.5 M, and 1 M NaCl, in 50 mM tris(hydroxymethyl)aminomethane (TRIS) buffer (pH = 6.8 and 7.8), and in 50 mM glycine buffer (pH = 3.0). To obtain more information about the influence of co-ion on CHAPS aggregation and to check the importance of electrostatic interactions at the process investigated, measurements were also performed in 1 M LiCl, CsCl, NaBr, and NaI solutions at 308.15 K. The corresponding standard thermodynamic parameters of micellization (enthalpy, ΔH0M; Gibbs free energy, ΔG0M; entropy, ΔS0M; heat capacity change, Δc0p,M) were estimated by fitting the model equation based on the mass action model to the experimental data. For comparison, an additional analysis using the simplified pseudophase separation model and mass action model with a presumption of large aggregation number of CHAPS micelles (nagg > 50) was also carried out and is represented in the Supporting Information. Δc0p,M was further correlated to the changes in the solvent accessible surface upon micelle formation.

3. THERMODYNAMIC ANALYSIS It should be pointed out that the completely ionized zwitterionic aggregates could be treated as nonionic surfactant micelles since the net charge of such aggregates equals zero. Still, we assumed that the zwitterionic surfactant micelles could be slightly ionized in salt solutions. Therefore, the model equation for the micellization of cationic surfactant was used for fitting to the experimental data. A detailed explanation is given in the Supporting Information. According to the mass action model,40,41,46,47 the micellization process of a cationic surfactant at constant temperature, T, and pressure, p, may be described as nS+ + (n − z)C− ⇌ Mz

(1) −

+

where S represents the monomeric state of surfactant, C the corresponding counterions, and Mz the micellar aggregate of n surfactant monomers with an effective charge of z. The equilibrium between species can be expressed by the apparent equilibrium constant, KM, m KM = n Mn − z mS mC (2) where mS is the molality of surfactant ions in monomeric form, mC the molality of counterions, and mM the molality of surfactant in micellar form. Surfactant solution with a concentration greater than the cmc can be considered as a mixed electrolyte solution with the total surfactant molality, m, m = mS + n·mM (3) and the degree of micellization, αM = n·mM/m. In the presence of added salt, the molality of free counterions, mC, is given as

2. EXPERIMENTAL SECTION 2.1. Materials. The zwitterionic surfactant 3-[(3-cholamidopropyl)dimethylammonium]-1-propanesulfonate (CHAPS) was purchased from Sigma-Aldrich (98%) in a hydrate form and used without further purification. The exact amount of water was determined by Karl Fischer titration, and the correction of CHAPS concentration was made afterward. Sodium chloride (Merck, p.a., ≥99.5%) was kept in a desiccator before use. Tris(hydroxymethyl)aminomethane (TRIS) (Sigma-

mC = m − (n − z)mM + mX

(4)

where mX represents the molality of added electrolyte. From the above equations, it follows that KM = 10364

αMm1 − n n(1 − αM)n (m(1 − αM(1 − z /n)) + mX )n − z

(5)


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Tr + ΔH0M,Tr[1/T − 1/Tr] + Δc0p,M[1 − Tr/T − ln(T/Tr)]), the ITC model function (eq 9) may be described in terms of n, ΔH0M,Tr, ΔG0M,Tr, Δc0p,M, and the coefficients A′(T) and B′(T) at any temperature, T, and surfactant concentration, c2 (eqs p−t in the Supporting Information). ΔH0M,Tr and ΔG0M,Tr are the standard enthalpy and Gibbs free energy of micellization at some reference temperature, Tr. All of these values were obtained by fitting of eq 9 to the experimental data points using the Levenberg−Marquardt nonlinear regression algorithm43 and further employed to calculate the corresponding ΔH0M, ΔG0M, and TΔS0M. At this point, it must be emphasized that eqs 2−10 and consequently the standard Gibbs free energy and entropy are based on the molarity scale. Therefore, the last two values were converted to the mole fraction scale to become useful for comparison with other thermodynamic parameters obtained. Two classical approaches of fitting were performed: individual fitting of the model equation to the experimental data obtained from a single temperature experiment and global fitting of the model equation to the corresponding experimental curves at all of the examined temperatures simultaneously. The advantage of the latter procedure over the former is a greater confidence in the proposed model and consequently in all of the adjustable parameters.44 The main presumption concerning the global fitting was the temperature independence of the aggregation number, which turned out to be acceptable since the individual best-fit values and the global-fit value for n of the chosen system were nearly the same. In the paper, only the results obtained by the global fit procedure are represented for CHAPS in water, NaCl, and buffer solutions. The degree of micelle ionization was assumed to be small for all of the systems investigated (in the model equation z/n was taken to be 1 in water and buffer solutions and 0.9 in all of the salt solutions).34,45

where n is the aggregation number and z/n the degree of micelle ionization. The equilibrium between monomeric surfactant ions, their micelles, and corresponding simple counterions can be also described in terms of the composition variables n1 (the number of moles of solvent), nS (the number of moles of surfactant ions in monomeric form), nC (the number of moles of free counterions), and nM (the number of moles of surfactant ions in micellar form) and the corresponding partial molar enthalpies H̅ 1, H̅ S, H̅ C, and H̅ M. The enthalpy of solution in the titration cell obtained after ith injection, Hi, can be thus expressed as Hi = n1H1̅ + nSH̅ S + nCH̅ C + nMH̅M

(6)

Taking into account the total number of moles of surfactant, n2 = nS + nM, and counterions, nC = nS + (z/n) nM + nX, eq 6 becomes Hi = n1H1̅ + n2(H̅ S + H̅ C) + nMΔHM + nX H̅ C

(7)

where the enthalpy of micelle formation, ΔHM, is defined as ΔHM = (H̅M − H̅ S − (1 − z /n)H̅ C)

(8)

By taking the partial derivative of enthalpy with respect to n2 at constant p, T, and n1 in combination with the Gibbs−Duhem equation (a more detailed explanation is given in the Supporting Information), one can obtain the relation for measured relative molar enthalpy, ΔH, including the enthalpy of micellization, ΔHM, which is assumed to be concentration-independent and thus equal to its value in the standard state (ΔHM = ΔH0M) ⎞ ⎛ 0 ∂nM ΔH = (H̅ S + H̅ C − const) + ΔHM ⎟ ⎜ ⎝ ∂n2 ⎠n , p , T 1

(9)

Using the Gibbs−Helmholtz relation and the Guggenheim approximation for the surfactant mean activity coefficient,42 the (H̅ S + H̅ C) contribution below cmc can be expressed as ⎛ A′(T ) m ⎞ + B′(T )m⎟ H̅ C + H̅ S = H̅ 20 + 2RT 2⎜ ⎝1 + m ⎠

4. RESULTS AND DISCUSSION The experimental heat of dilution, ΔH, dependence on surfactant concentration (enthalpogram) for the titration of CHAPS in water at six different temperatures is shown in Figure 2. Similar enthalpy

(10)

where H̅ 02 represents the partial molar enthalpy of surfactant in its standard state. The coefficients A′(T) and B′(T) are temperature derivatives of the constants A(T) and B(T), that reflect solvent properties and ion−ion interactions, respectively.42 Above cmc the (H̅ S + H̅ C) contribution is assumed to remain constant and equal to its value at the cmc due to the small changes in concentrations of S+ and C− ions. The (∂nM/∂n2)n1,p,T derivative in eq 9 represents the fraction of surfactant molecules in micellar state M and can be derived from the apparent equilibrium constant considering the massbalance relations as ⎛ ∂nM ⎞ n[u + g ] = ⎟ ⎜ 1 + n[u + (1 − z /n)g ] ⎝ ∂n2 ⎠n , p , T 1

(11)

where the parameters u and g are functions of the degree of micellization (αM) and the degree of micelle ionization (z/n) u=

αM 1 − αM

g=

Figure 2. Temperature-dependent enthalpograms for CHAPS in water. Solid lines represent the fits according to the mass action model (eq 9). Insets: (a) the model-independent enthalpy of micellization and (b) cmc determination according to the Phillips’s criterion. Pentagon, 328.15 K; ★, 318.15 K; ◇, 308.15 K; ▲, 298.15 K; ○, 288.15 K; ■, 278.15 K.

(1 − z /n)αM 1 − (1 − z /n)αM + mx /m (12)

By help of the well-known relation ΔG0M = (RT/n) ln KM, the Kirchoff’s law for ΔH0M, ΔH0M = ΔH0M,Tr + Δc0p,M(T − Tr), and the integrated Gibbs−Helmholtz equation ΔG0M = T(ΔG0M,Tr/

patterns were also obtained for CHAPS in 0.1 M, 0.5 M, and 1 M NaCl, 50 mM TRIS buffer, pH = 6.8 and 7.8, and 50 mM glycine buffer, pH = 3.0, at the same temperatures (Figures B and C in the 10365


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Figure 3. Represented titration curves show the effect of (a) NaCl concentration and (b) different co-ions on self-association behavior of CHAPS at 308.15 K. Solid lines represent the fits according to the mass action model (eq 9). Inset: the logarithm of critical micelle concentration, log(cmc/ mM), and enthalpy of micellization, ΔH0M, dependence on concentration of added NaCl. A linear fit according to eq 13 is also presented.

corresponds to the model-independent enthalpy of micellization, ΔHM = ΔH(final) − ΔH(initial) ≃ ΔH0M (inset a in Figure 2).19,46,47 According to the Phillips’s criterion, cmc can be determined from a second derivative of the reaction heat against total surfactant concentration (inset b in Figure 2).41 Both procedures were described in detail in our previous work.19,39 However, the values of cmc and ΔH0M were simultaneously determined by the fitting procedure and are listed in Table 1 for all of the systems and temperatures investigated. For comparison with the values, obtained by the above-mentioned procedures, they are also presented in Table A in the Supporting Information. The temperature dependence of cmc shows typical U-shaped form (Figure 4) reaching the minimum at a temperature T* ≈ (295 ± 2) K for all of the systems investigated as it has been usually found for diverse surfactant systems.48 Our findings are also supported by a similar behavior observed already for CHAPS and other resembling cholic acid surfactants as sodium cholate (NaC) and sodium deoxycholate (NaDC).26,27,29 Moreover, T* coincides well with a temperature T0 at which ΔH0M = 0 (Figure 5), which is typical of the surfactant micellization process.48 However, the absence of such a minimum observed by proton NMR spectroscopy for CHAPS in D2O at different temperatures and NaCl concentrations by Qin and co-workers30 can be the consequence of too narrow temperature range investigated. It is evident from Figures 3a (inset) and 4 and from data in Tables 1 and A in the Supporting Information that the addition of NaCl shifts cmc toward lower values. Still, this effect is much less pronounced as it has been observed for ionic surfactants TTAC, DTAC, and DeTAC where screening of the repulsion between cationic head groups of surfactant molecules by the excess Cl− ions can be assumed.19,39 A simple empirical law, which describes the effect of added salt on cmc, was formerly developed for nonionic 49 and also applied on ionic surfactants19,48

Supporting Information). Enthalpograms for the titration of CHAPS surfactant at different NaCl concentrations and the comparison of titration curves for CHAPS in water, 1 M NaCl, LiCl, CsCl, NaBr, and NaI solutions at 308.15 K are presented in Figure 3. If one describes the experiment, CHAPS undergoes demicellization initially, which is followed by monomer dilution (at surfactant concentrations in the calorimetric cell below cmc). Thus, at the beginning of experiment the heat changes measured result from the demicellization of surfactant micelles, dilution of monomers, electrolytes, and/or buffer components and their mutual interactions. After surfactant concentration in the calorimetric cell reaches the cmc, solution undergoes micellar dilution only (and no further demicellization) with an appreciable change in enthalpy that remains nearly constant with increasing surfactant concentration. Therefore, the whole enthalpy curve (Figures 2, 3 and B, C in the Supporting Information) consists of three different regions: two more or less horizontal or at least linear parts connected by a declining or increasing one. Evidently, CHAPS micellization in general occurs in a broader concentration range as it is usually observed for classical surfactants and represented in our previous work for dodecyltrimethylammonium chloride (DTAC) and tetradecyltrimethylammonium chloride (TTAC).19,39 However, the CHAPS enthalpograms obtained can be compared to the titration curves of decyltrimethylammonium chloride (DeTAC), which is a conventional short alkyl chain surfactant.19 It can be thus concluded, from all of the ITC experiments, that CHAPS aggregation can be regarded as micellization process in all of the media investigated. As can be seen in Figures B and C in the Supporting Information, the initial parts of titration curves at 278.15 K do not always follow a common sigmoidal behavior, as it is characteristic for other temperatures. Even more, from duplicate titrations at 278.15 K (Figure A in the Supporting Information) it can be concluded that the process observed in the initial region of titration curves is not consistent. At this stage, we do not have any reliable explanation for such a behavior that has been also partially noticed for DeTAC in 1 M NaCl at 278.15 K.19 As a matter of fact, the process lacking in the specificity cannot be included in the proposed model, which assumes the reproducibility of modeled aggregation process. Therefore, during the fitting procedure deviations from purely sigmoidal behavior in the initial region of some titration curves at 278.15 K were neglected. In case of a single-step micellization process, the difference between the final and the initial linear part of the enthalpogram

log(cmc/mM) = constant −ks(csalt /M )

(13)

where csalt represents the NaCl concentration and ks a salt− surfactant specific parameter. Recently, the value of 0.66 ± 0.05 was found for ks of DeTAC,19 which is far away from the estimate determined for CHAPS surfactant (ks = 0.21 ± 0.05). Nevertheless, this value is comparable with the reported value of 0.28 for nonionic surfactant n-decyldimethylphosphine oxide (APO10) in NaCl solutions.50 Evidently, the repulsive electrostatic forces hinder CHAPS micellization to a lesser extent as at ionic surfactants. Thus, 10366


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Table 1. Thermodynamic Parameters of CHAPS Micellization in Water, NaCl, LiCl, CsCl, NaBr, NaI, and Buffer Solutions at all Temperatures Investigated: Critical Micelle Concentration, cmc, Standard Enthalpy, ΔH0M, Gibbs Free Energy, ΔG0M, and Entropy, ΔS0M, of Micellization, Standard Heat Capacity Change upon Micelle Formation, Δc0p,M, and Aggregation Number of CHAPS Micelles, nagg, as Obtained by the Fitting Procedurea,b T

cmc

ΔH0M

ΔG0M

278.15 288.15 298.15 308.15 318.15 328.15

7.0 6.9 6.8 6.8 7.2 7.7

± ± ± ± ± ±

0.7 0.7 0.7 0.7 0.7 0.8

3.53 1.45 −0.64 −2.72 −4.81 −6.89

± ± ± ± ± ±

0.04 0.04 0.04 0.04 0.04 0.04

−15.9 −16.6 −17.2 −17.7 −18.2 −18.5

± ± ± ± ± ±

278.15 288.15 298.15 308.15 318.15 328.15

7.1 6.9 6.9 7.1 7.4 8.0

± ± ± ± ± ±

0.7 0.7 0.7 0.7 0.7 0.8

3.28 1.32 −0.64 −2.60 −4.55 −6.51

± ± ± ± ± ±

0.05 0.05 0.05 0.05 0.05 0.05

−17.5 −18.2 −18.9 −19.5 −20.0 −20.4

± ± ± ± ± ±

278.15 288.15 298.15 308.15 318.15 328.15

5.2 5.2 4.9 5.2 5.4 6.1

± ± ± ± ± ±

0.5 0.5 0.5 0.5 0.5 0.6

3.98 1.62 −0.75 −3.11 −5.47 −7.84

± ± ± ± ± ±

0.06 0.06 0.06 0.06 0.06 0.06

−17.0 −17.7 −18.4 −18.9 −19.4 −19.8

± ± ± ± ± ±

278.15 288.15 298.15 308.15 318.15 328.15

4.3 4.2 4.1 4.2 4.5 4.9

± ± ± ± ± ±

0.4 0.4 0.4 0.4 0.5 0.5

3.53 1.35 −0.82 −3.00 −5.17 −7.35

± ± ± ± ± ±

0.07 0.07 0.07 0.07 0.07 0.07

−17.0 −17.7 −18.4 −18.9 −19.4 −19.8

± ± ± ± ± ±

308.15

5.5 ± 0.6

−2.7 ± 0.4

Water 0.2 0.2 0.2 0.2 0.2 0.2 0.1 M NaCl 0.3 0.3 0.3 0.3 0.3 0.3 0.5 M NaCl 0.4 0.4 0.4 0.4 0.4 0.4 1 M NaCl 0.2 0.2 0.2 0.2 0.2 0.2 1 M LiCl

−18 ± 1

TΔS0M

Δc0p,M

ASAexp

nagg

19.5 18.0 16.5 15.0 13.4 11.7

± ± ± ± ± ±

0.2 0.2 0.2 0.2 0.2 0.2

−208 ± 3

142

6.4 ± 0.2

20.8 19.6 18.3 16.9 15.4 13.9

± ± ± ± ± ±

0.3 0.3 0.3 0.3 0.3 0.3

−196 ± 3

134

6.7 ± 0.7

21.0 19.4 17.6 15.8 13.9 12.0

± ± ± ± ± ±

0.4 0.4 0.4 0.4 0.4 0.4

−236 ± 3

161

5.6 ± 0.2

20.6 19.1 17.6 15.9 14.2 12.5

± ± ± ± ± ±

0.2 0.2 0.2 0.2 0.2 0.2

−218 ± 3

149

5.4 ± 0.1

16 ± 1

5.4 ± 0.6

1 M CsCl 308.15

4.5 ± 0.5

−3.3 ± 0.5

308.15

4.6 ± 0.5

−3.1 ± 0.5

308.15

4.8 ± 0.5

−4.7 ± 0.7

278.15 288.15 298.15 308.15 318.15 328.15

6.8 6.5 6.3 6.5 6.8 7.6

± ± ± ± ± ±

0.7 0.7 0.6 0.7 0.7 0.8

3.70 1.57 −0.57 −2.70 −4.83 −6.97

± ± ± ± ± ±

0.06 0.06 0.06 0.06 0.06 0.06

278.15 288.15 298.15 308.15 318.15 328.15

6.8 6.6 6.6 6.8 7.3 7.6

± ± ± ± ± ±

0.7 0.7 0.7 0.7 0.7 0.8

3.60 1.44 −0.73 −2.89 −5.05 −7.22

± ± ± ± ± ±

0.05 0.05 0.05 0.05 0.05 0.05

278.15 288.15 298.15 308.15 318.15 328.15

7.0 6.8 6.8 7.0 7.3 7.8

± ± ± ± ± ±

0.7 0.7 0.7 0.7 0.7 0.8

3.47 1.36 −0.75 −2.86 −4.97 −7.09

± ± ± ± ± ±

0.05 0.05 0.05 0.05 0.05 0.05

−19 ± 1

16 ± 1 1 M NaBr −18.9 ± 0.9 15.8 ± 0.9 1 M NaI −19.1 ± 0.7 14.4 ± 0.7 50 mM glycine, pH = 3.0 −15.8 ± 0.2 19.5 ± 0.2 −16.4 ± 0.2 18.0 ± 0.2 −17.0 ± 0.2 16.5 ± 0.2 −17.5 ± 0.2 14.8 ± 0.2 −18.0 ± 0.2 13.2 ± 0.2 −18.4 ± 0.2 11.4 ± 0.2 50 mM TRIS, pH = 6.8 −15.9 ± 0.2 19.5 ± 0.2 −16.5 ± 0.2 18.0 ± 0.2 −17.1 ± 0.2 16.4 ± 0.2 −17.6 ± 0.2 14.7 ± 0.2 −18.1 ± 0.2 13.0 ± 0.2 −18.5 ± 0.2 11.2 ± 0.2 50 mM TRIS, pH = 7.8 −15.9 ± 0.4 19.4 ± 0.4 −16.6 ± 0.4 17.9 ± 0.4 −17.2 ± 0.4 16.4 ± 0.4 −17.7 ± 0.4 14.8 ± 0.4 −18.1 ± 0.4 13.2 ± 0.4 −18.5 ± 0.4 11.4 ± 0.4

5.5 ± 0.8 5.5 ± 0.3 5.8 ± 0.5 −214 ± 3

146

6.0 ± 0.2

−216 ± 3

148

6.2 ± 0.2

−211 ± 3

144

6.4 ± 0.3

Units: cmc, mM; ΔH0M, ΔG0M, TΔS0M, kJ mol−1; Δc0p,M, J mol−1 K−1; ASAexp, Å2. bThe reported errors correspond to the precision of the experimental data and the fitting procedure assuming the proposed model is correct. a

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in water and all of the buffer solutions investigated are nearly the same, although the experiments were performed in a broad pH range (3−7.8) (Table 1 and Table A, Figure D in the Supporting Information). It could be thus assumed that CHAPS behaves practically as nonionic surfactant under these conditions. On the contrary, the accompanied protonation effect would manifest in enthalpy change being measured by ITC upon demicellization since it would depend on buffer pH. This fact has been already reported in the literature as a positive characteristic of CHAPS surfactant.53 Enthalpies of micellization obtained by the fitting procedure are listed in Table 1. ΔH0M values as acquired directly from the titration curves (inset a in Figure 2) are also collected in Table A in the Supporting Information for comparison. Evidently, some differences can be found between the applied procedures. The values obtained directly from the enthalpograms are believed to be more subjective and less precise. Due to the broad micellization passage, it is sometimes hard to determine the inflection point of titration curve and the correct level of purely micellar dilution. On the other hand, the model equation applied for fitting to the experimental data (eq 9) is able to describe the process investigated in the whole concentration range (with some exceptions in the initial region of enthalpograms at 278.15 K, which were neglected during the fitting procedure). This is why these enthalpies seem to be more reliable. In general, all of the enthalpies are much lower than those reported for some ionic surfactants19,39 but are actually in good agreement with the recently reported data by Naskar et al. for CHAPS in water (ΔH0M(303 K) = −1.54 kJ mol−1)32 and Giacomelli et al. for CHAPS in 10 mM phosphate buffer (pH = 8.1, ΔH0M(295 K) = −0.4 kJ mol−1 and ΔH0M(309 K) = −1.6 kJ mol−1) determined by ITC.29 Surprisingly, all of these values represent only approximately one tenth of the values determined by proton NMR spectroscopy for CHAPS in D2O by Qin and co-workers.30 The observed distinction could obviously arise from the influence of different medium (D2O versus water). Interestingly, their cmc and ΔG0M values (obtained from the cmc dependence on temperature) are comparable to ours, but still quite a big discrepancy between the determined enthalpies can be observed. Although the calorimetric and van’t Hoff enthalpies from our experiments are almost the same also, they differ substantiality from those reported by Qin et al. (Figure E in the Supporting Information). From all of the collected data (Figure 5, Table 1 and Figure C, Table A in the Supporting Information) it is evident that the addition of NaCl and buffer compounds in solution does not affect ΔH0M of CHAPS surfactant substantially. As already mentioned, CHAPS behaves as a weakly charged surfactant in salt solutions and as a nonionic one in water and buffer solutions with ability to maintain zero net charge over a broad pH range.53 That is why ionic interactions are of lesser importance at CHAPS micellization, although anion binding to the zwitterionic chain cannot be excluded. Anyway, the micellization process of CHAPS surfactant is endothermic at low temperatures and becomes exothermic at high temperatures in all of the solutions investigated (Figures 5). A similar change in the sign of ΔH0M with increasing temperature has been already observed for number of ionic surfactants also.19,39,48,54 As is typical for surfactant micellization, the temperature T0 at which ΔH0M = 0 is in the range of room temperature, T0 = (295 ± 1) K (Figure 5). It can be seen in Figure 5 that ΔH0M exhibits a linear dependence on temperature, so Δc0p,M, assuming to be constant over the temperature range investigated, can be determined from the

Figure 4. Temperature dependence of cmc for CHAPS in all of the media investigated. ■, water; □, 0.1 M NaCl; ●, 0.5 M NaCl; ○, 1 M NaCl; △, pH = 3.0; ◆, pH = 6.8; ◇, pH = 7.8.

the cmc dependence on salt concentration is also less expressed and comparable to that of nonionic surfactants. Therefore, it is not surprising that the cmc of CHAPS surfactant does not change significantly in the presence of different anions (Cl−, Br−, and I−) or cations (Na+, Cs+, and Li+) in solution as can be deduced from Figure 3b, which is not quite true for ΔH0M also. It can be seen in Figure 3a (inset) that ΔH0M is decreasing with increasing NaCl concentration. Figure 3b reveals that (at 308.15 K) it actually remains constant in 1 M NaCl, CsCl, and NaBr media. Further, ΔH0M is found just slightly lower in 1 M LiCl (probably due to the highly hydrated Li+ cation which hardly interacts with the sulfate group) but is considerably higher in 1 M NaI solution as it is evident from Figure 3b. So it can be concluded that the Cs+ and Li+ cation and Br− anion affect CHAPS micellization to a similar extent as Na+ and Cl−, which cannot be claimed for I− in comparison with Cl− and Br− also. To summarize, the effect of anions on CHAPS micellization is slightly more pronounced compared to the effect of cations. As it has been already reported also, anions bind stronger to the quaternary amine group in the micelle interior as cations to the negatively charged group at the end of the surfactant side chain (lower charge density).34,45 Therefore it follows that our presumption of a small degree of micelle ionization (or no ionization) for all of the systems investigated is correct and that the zwitterionic CHAPS molecule can be treated like a slightly charged cationic surfactant in salt solutions. The micellization process is always treated as the interplay between opposing effects (attractive and repulsive forces). Micelle formation is primarily driven by the hydrophobic effect, which denotes the positive entropy change accompanying the release of ordered water molecules at the nonpolar surfaces in the bulk.51 Since the solubility of hydrophobic moiety decreases with increasing salt concentration, the cmc of the surfactant lowers with salt addition (Figures 3 and 4). For zwitterionic surfactants, ion binding upon micellization is also assumed, which is accompanied with a negative enthalpy change. As it was already published for proteins, the I− anion binds stronger to the quaternary amine group as Cl−; therefore a more exothermic micellization of CHAPS in NaI could be assigned to stronger anion−NR3+ interaction after micellization.52 On the other hand, conformational restriction of surfactant molecules and counterions bound upon micellization opposes the aggregation process (negative entropy change). Interestingly, the titration curves and corresponding thermodynamic parameters of CHAPS micellization (cmc and ΔH0M) 10368


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hydroxyl groups on the hydrocarbon skeleton of CHAPS molecule, which can oppose the formation of denser aggregates because of a favorable hydrogen bonding with the surrounding water. Actually, the very low aggregation numbers, nagg, of the rigid CHAPS surfactant obtained by the fitting procedure (Table 1) even support the above presumption of loose aggregates with lots of water molecules still in contact with hydrocarbon skeleton even after micelles are formed. Moreover, this assumption can explain the differences in ΔG0M and ΔS0M obtained by the pseudophase separation model or mass action model with the presumption nagg > 50 and by the fitting procedure (Table A in the Supporting Information). The presumption nagg > 50 in the derivation of relation ΔG0M = RT ln Xcmc is obviously not valid for CHAPS surfactant micellization. Since ΔS0M is derived from ΔH0M and ΔG0M using the Gibbs−Helmholtz equation (eq c in the Supporting Information), these values also differ from those obtained by the fitting procedure. Nevertheless, the tendency in ΔG0M and ΔS0M, estimated by both procedures, is the same. ΔG0M is negative in the whole temperature range investigated and only moderately temperature dependent. It is slightly more negative at high temperatures as can be seen in Tables 1 and A in the Supporting Information. All of the ΔS0M values are positive and decrease with increasing temperature. Such behavior has been observed for ionic surfactants also.48 The ΔG0M and ΔS0M values for CHAPS micellization and their temperature dependence are comparable in all of the media investigated.

Figure 5. Temperature dependence of enthalpy of micellization, ΔH0M, for CHAPS in water, NaCl, and buffer solutions. The corresponding heat capacity changes, Δc0p,M, are represented with the slopes. ■, water; □, 0.1 M NaCl; ●, 0.5 M NaCl; ○, 1 M NaCl; △, pH = 3.0; ◆, pH = 6.8; ◇, pH = 7.8.

slope of the linear fit of corresponding data points. On the other hand, Δc0p,M was simultaneously determined by the fitting procedure also as it follows from the eq v in the Supporting Information. The Δc0p,M values obtained from both procedures are in reasonable agreement as it can be revealed from Table B in the Supporting Information. All of the Δc0p,M values are negative, which can be ascribed to the removal of water molecules from contact with the nonpolar surface area upon micelle formation.55 However, the absolute values are considerably lower from those reported for the micellization of classical surfactants in water and NaCl solutions.19 Kresheck recently derived a simple relationship between the heat capacity change and the water accessible surface area removal upon burial of nonpolar groups to determine the number of buried methylene groups along with the methyl group for a series of nonionic surfactants.50 The experimental water accessible surface area, ASAexp, can be thus obtained as ASA exp/Å2 =

5. CONCLUSIONS The micellization behavior of zwitterionic surfactant CHAPS in water, 0.1 M, 0.5 M, and 1 M NaCl, in 50 mM TRIS buffer (pH = 6.8 and 7.8), and 50 mM glycine buffer (pH = 3.0) has been investigated by ITC in the temperature range between 278.15 K and 328.15 K. Additional experiments were also performed in 1 M LiCl, CsCl, NaBr, and NaI solutions at 308.15 K to obtain more information about the influence of co-ion on CHAPS aggregation and mainly the importance of electrostatic interactions at the process investigated. Critical micelle concentration decreases only slightly with increasing NaCl concentration, indicating that screening of the repulsion between charged groups of surfactant molecules is less expressed at CHAPS as cationic surfactants studied before. Moreover, cmc values in water and buffer solutions are nearly the same at all of the temperatures investigated, and the cmc for CHAPS in 1 M NaCl at 308.15 K is close to those in 1 M LiCl, CsCl, NaBr, and NaI at the same temperature. The values of enthalpy of micellization are very similar in all of the media studied. The ΔH0M dependence on temperature passes zero enthalpy at approximately same temperature in all of the investigated systems. This temperature agrees well with the temperature at which cmc dependence on temperature reaches the minimum value also. This observation can be ascribed to the compensation effect between enthalpy and entropy of micellization, arising from the hydrophobic effect. Thus, the aggregation of CHAPS can be treated as the micellization process of classical surfactants. As already mentioned, ΔH0M does not change substantially with salt or buffer addition and is only slightly dependent on the presence of different co-ions also. In comparison with the ΔH0M values found for the micellization of CHAPS in NaCl, CsCl, and NaBr aqueous solutions, the micellization of CHAPS in NaI solution is more exothermic. This can be ascribed to the stronger binding of the I− anion from the surrounding medium to the zwitterionic tail as it is in

Δcp0,M −1.463 J mol−1 K−1

(14)

This method has turned out to be a useful tool for modeling the micellization process of classical ionic surfactants (especially DTAC and TTAC) recently and is described in detail in our previous work.19 All of the ASAexp values as assessed by help of the eq 14 are listed in Table 1. Overall, ASAexp is practically independent of the solvent medium and could be reported as ASAexp = (150 ± 20) Å2. In comparison with conventional ionic surfactants, the structure of CHAPS molecule is more complex. Therefore, the nonpolar part of molecule cannot be simply ascribed as a methyl group along with number of methylene groups like a classical surfactant tail. The theoretical value for water accessible surface area, ASAth ≈ 900 Å2, was thus estimated from the optimized geometries using the Winmostar program,56 applying the value of 0.14 nm for the radius of water molecule. The enormous discrepancy between ASAexp and ASAth may lead us to the assumption that there are still water molecules in the aggregate interior which are in contact with nonpolar surface of CHAPS monomers. Presumably, only a very small part of the cholesterol-like skeleton of bile salt derivative is fully removed from contact with water molecules upon micellization. This observation suggests the formation of looser CHAPS aggregates in comparison with classical surfactant micelles. Namely, there are three 10369


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the case of Cl− and Br−. In contrast, a slightly less exothermic effect of micellization observed in 1 M LiCl can be assigned to the pronounced hydration of the Li+ cation. The Gibbs free energy and entropy of CHAPS micellization are model-dependent quantities. Obviously, CHAPS is a surfactant forming low aggregation number micelles. The pseudophase separation model and the presumption nagg > 50 are thus not appropriate for modeling its micellization process, although the temperature-dependent trend of derived thermodynamic parameters can be determined correctly. It should be also pointed out that the proposed model equation for fitting to the experimental data is based on the possibly incorrect presumption of a single-step process, since CHAPS aggregation is somehow believed to proceed stepwise. Nevertheless, such a behavior is, at least for now, unpredictable and cannot be taken into account in the modeling of the process investigated. The heat capacity changes of CHAPS micellization are negative in all of the solutions investigated, showing the relation to the loss of water accessible nonpolar surface area of the surfactant molecule upon micellization. However, their absolute values are much lower than those determined for a series of ionic and nonionic surfactants,19,39,50 indicating that there are still water molecules in contact with the nonpolar part of CHAPS monomers in the micelle interior. This finding is in agreement with the very low aggregation number of CHAPS micelles, nagg ≈ 6 ± 1, as obtained by fitting of the model equation to the experimental data. CHAPS micelles can be thus regarded as small and loose aggregates with lots of water molecules still in contact with the hydrocarbon skeleton. However, it can be concluded that our findings agree with those obtained by Qin et al. by NMR spectroscopy30 in part that CHAPS aggregation is obviously not sensitive to the changes of surrounding medium. In this context, the present thermodynamic study contributes to the understanding of aggregation behavior of CHAPS surfactant in various media and thus enlightens its ability to solubilize membrane proteins in similar manner under different conditions.

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ASSOCIATED CONTENT

S Supporting Information *

Thermodynamic analysis, titration curves, enthalpograms, thermodynamic parameters, and standard heat capacity changes. This material is available free of charge via the Internet at http://pubs. acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +386 1 2419 410. Fax: +386 1 2419 425. E-mail address: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support by the Slovenian Research Agency through Grant No. P1-0201 is gratefully acknowledged. The work was partially supported by COST Actions D43 and CM1101.

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REFERENCES

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