Md. Farid Ahmedab,
Mohammad Robel Mollaab,
Mousumi Sahac,
Imrul Shahriarc,
Mohammad Saidur Rahmanad,
Mohammad A. Halimc,
Malik Abdul Rubef,
Md. Anamul Hoque*a and
Abdullah M. Asirief
aDepartment of Chemistry, Jahangirnagar University, Savar, Dhaka-1342, Bangladesh. E-mail: ahoque_ju@yahoo.com; ahoque_ju@juniv.edu; Fax: +880-2-7791052; Tel: PABX: +880-2-7791045-51 extn 1437
bBangladesh Council of Scientific and Industrial Research (BCSIR), Dhaka, Bangladesh
cDivision of Quantum Chemistry, The Red-Green Research Centre, BICCB, Dhaka, Bangladesh
dDepartment of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603, Kuala Lumpur, Malaysia
eChemistry Department, Faculty of Science, King Abdulaziz University, Jeddah-21589, Saudi Arabia
fCenter of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah 21589, Saudi Arabia
First published on 25th February 2019
Herein, we have investigated the interaction of bovine serum albumin (BSA), the most abundant globular protein, with a conventional cationic surfactant, cetyldimethylethylammonium bromide (CDMEAB), through a conductivity technique in the absence/presence of electrolyte solutions at various temperatures (298.15–323.15 K). The interaction of the protein with drugs/surfactants and other additives plays a crucial role in the body. Hence, the main concern of the study is to extract the impact of BSA on surfactant molecules and vice versa. From the specific conductivity versus concentration of surfactant plots, three different noticeable critical micelle concentration (c*) values were obtained for pure CDMEAB and its mixture with protein/protein + salts. The presence of BSA and electrolytes altered the c* values of CDMEAB revealing interactions among the studied constituents where the salt solutions reduced the c* values and created a convenient environment for favorable micellization. The negative magnitudes achieved for standard free energy changes (ΔG0m) suggest spontaneity of micellization while the values of ΔH0m and ΔS0m signified the existence of some electrostatic and hydrophobic interactions. The values of molar heat capacity (ΔC0m) were positive as well as small which was an indication of less structural deformation. Molecular Dynamics (MD) simulation for all atoms revealed that the salt ions promoted non-covalent interaction between BSA and CDMEAB, and such interactions were not observed in the absence of the salt. Protein structure remained nearly same in spite of strong interaction with CDMEAB as evident from the overall RMSD (root-mean-square deviation) values of the alpha carbons and backbone of the protein and RMSF (root-mean-square fluctuation) values of the amino acid residues present in BSA. In this work thermodynamic parameters of transfer (such as ΔG0m.tr., ΔH0m.tr., and ΔC0p.m.tr.) were also evaluated and the results are discussed in detail. Besides, contributions of enthalpy and entropy to free energy changes were also analyzed.
Assembly of amphiphilic molecules including surfactants/drugs etc. has a spontaneous propensity to form aggregates known as micelles due to the presence of weak non-covalent forces (i.e. van der Waals force) among the individual molecules. Micelles are formed at a particular concentration of that amphiphile known as “critical micelle concentration”.10–16 It is a unique feature of surfactant which helps to extract information about the interaction of drug and other biomolecules such as protein in the body. In the case of drug delivery, surfactants are important molecules to act as recipients/excipients. Hydrophobic nucleus/core of micelles is significant as it has the capability to enhance the solubility or bioavailability of hydrophobic drugs. Cationic surfactants are such surfactants that are utilized like anti-microbial substances in many types of antiseptics, disinfectants, perfumes as well as in cosmetics.11 Previous studies revealed that the presence of various additional substances exert their effects on the physiological feature of micelles including the extent of departure of opposite-ion binding, catalytic property, and reaction rate which are related to the phase-separation model.11,17
According to Deep and Ahluwalia, native globular protein's stability is noticeably influenced by pH, temperature, and insertion of small molecules like surfactants, co-enzymes, inhibitors and activators that bind specifically to the original state.18 The interaction between surfactant & globular proteins can help towards an understanding of the action of surfactant as a solubilizing agent for membrane proteins and lipids. Some excellent literatures are available on the study of the interaction between drugs/salts/polymers/polyols with surfactants; however, to the best of our knowledge, no study yet reported the interaction between BSA and CDMEAB.19–23 CDMEAB is a cationic surfactant which is used as disinfectant/laboratory purpose.24 Understanding the interaction between BSA and CDMEAB could be a key interest in the meadow study of protein-membrane chemistry model.
Therefore, in our present work, we have employed conductivity technique in order to investigate the interaction between BSA (Scheme 1(I)) and CDMEAB (Scheme 1(II)). Numerous studies revealed that when the aggregates are exposed to a variety of additives, the physiological properties (aggregate's stability, reaction rate, degree of ionization, and clouding phenomenon) could be affected due to the presence of additives.25–27 In a biological system, sodium ion plays a vital role to control the sodium pump and transmitting nerve systems.28 Salt ions have the unique property to decrease the critical micelle concentration (c*) by reducing the existing electrostatic repulsion force among the polar head groups of the surfactant, therefore, increasing the aggregation number (Nagg).29 Therefore, NaCl and Na2SO4 were utilized in this work to understand the effect of additives on different physicochemical parameters corresponding to BSA–CDMEAB interaction.
In recent years, MD simulation techniques are being implemented to understand the effect of surfactant on protein structure and on the protein folding phenomenon.30–32 Hoque et al. have studied the interaction of a cationic surfactant, cetylpyridinium chloride on BSA in presence and absence of electrolytes using MD simulation technique.33 MD simulation provided detailed information including non-covalent interactions between protein and surfactant, root-mean-square deviation of alpha carbon and backbone of the protein and root-mean-square fluctuation of each amino acid present in the protein. Delgado-Magnero and coworkers have utilized MD simulation method to understand the binding mechanism of nonionic surfactants with BSA and the results obtained suggest the potential use of polysorbates as excipients for minimizing the undesirable effects of protein adsorption and aggregation.34 In the present study, we investigate the nature and type of interactions present among the surfactant molecules and BSA employing MD simulation to support the experimental observations.
Fig. 1 Specific conductivity (κ) versus concentration of CDMEAB for (a) pure CDMEAB and (b) (BSA + CDMEAB) mixed system containing 0.03 mmol kg−1 BSA in water at 303.15 K. |
The obtained magnitudes of , , and of CDMEAB are found 0.74, 1.49, and 6.57 mmol kg−1 respectively at 303.15 K. On the other hand, the values of , , and in aqueous BSA (0.005–0.100 mmol kg−1) +CDMEAB mixed systems are found (0.75–1.05 mmol kg−1), (2.22–2.49 mmol kg−1), and (6.35–7.67 mmol kg−1) respectively (Table 1). The values of CDMEAB are in good agreement with literature values.44 In presence of BSA, values are increased with increasing the concentration of BSA. The and values are first increased, reach a maximum and then decreased. These variations in c* values signify the interaction between BSA and surfactant. The decrease in and values at higher concentration of BSA possibly due to the electrostatic interaction between BSA and the positively charged surfactant. However, interactions sustained through the nonpolar interactions of the hydrophobic portion of BSA and CDMEAB. The values of the degree of micelle ionization (α) were extracted by means of taking the corresponding ratios of the slopes relating to the closest straight lines before & after the c*.21,22,45,46 Then α1, α2, and α3 can be obtained from the ratios of S2/S1, S3/S1 and S4/S1 respectively, where S1, S2, and S3 are the first, second and third straight lines. The fraction of counter ion binding (β) can be obtained from the relation: β = 1 − α.
cBSA (mmol kg−1) | |||
---|---|---|---|
a Relative standard uncertainties (ur) is | |||
0.000 | 0.74 | 1.49 | 6.57 |
0.005 | 0.75 | 2.27 | 6.79 |
0.010 | 0.77 | 2.32 | 7.67 |
0.030 | 0.91 | 2.49 | 6.84 |
0.050 | 0.93 | 2.31 | 6.52 |
0.100 | 1.05 | 2.22 | 6.35 |
At a particular concentration of BSA (0.03 mmol kg−1) in the c* values for BSA + CDMEAB mixed systems (ESI Fig. 1†) are decreased with temperature up to a fixed limit and then increased as well as provided a U-contoured architecture. The ESI Fig. 1† demonstrates that micellization became favored with increasing temperature due to the dehydration of hydrophobic moiety which was predominated over the dehydration of hydrophilic moiety; therefore, decline the propensity of hydrophilic hydration, which is highly significant than the propensity of hydrophobic dehydration at higher temperatures and lessen the micellization process.
Medium | cBSA/mmol kg−1 | ISalt/mmol kg−1 | T/K | β1 | β2 | β3 | |||
---|---|---|---|---|---|---|---|---|---|
a Relative standard uncertainties (ur) are and ur(β1/β2/β3) = 0.04. | |||||||||
CDMEAB | |||||||||
H2O | 0.0 | 0.00 | 298.15 | 0.84 | 1.94 | 6.08 | 0.70 | 0.76 | 0.83 |
303.15 | 0.74 | 1.49 | 5.83 | 0.75 | 0.80 | 0.87 | |||
308.15 | 0.64 | 2.28 | 5.46 | 0.71 | 0.81 | 0.89 | |||
313.15 | 0.77 | 2.36 | 6.06 | 0.72 | 0.83 | 0.90 | |||
318.15 | 0.83 | 2.55 | 6.16 | 0.70 | 0.83 | 0.89 | |||
323.15 | 0.97 | 2.67 | 6.26 | 0.70 | 0.82 | 0.89 | |||
BSA + CDMEAB | |||||||||
H2O | 0.03 | 0.00 | 298.15 | 0.99 | 2.44 | 8.46 | 0.69 | 0.88 | 0.88 |
303.15 | 0.91 | 2.49 | 7.97 | 0.66 | 0.75 | 0.84 | |||
308.15 | 0.83 | 2.61 | 7.67 | 0.68 | 0.79 | 0.85 | |||
313.15 | 1.03 | 2.71 | 7.58 | 0.64 | 0.76 | 0.85 | |||
318.15 | 1.08 | 2.89 | 6.71 | 0.69 | 0.69 | 0.88 | |||
BSA + CDMEAB | |||||||||
H2O + NaCl | 0.03 | 0.50 | 303.15 | 0.88 | 3.26 | 7.24 | 0.28 | 0.79 | 0.84 |
1.00 | 303.15 | 0.78 | 3.08 | 7.31 | 0.69 | 0.76 | 0.82 | ||
1.50 | 303.15 | 0.71 | 2.86 | 7.52 | 0.54 | 0.67 | 0.77 | ||
2.00 | 303.15 | 0.85 | 2.67 | 7.41 | 0.58 | 0.69 | 0.84 | ||
3.00 | 303.15 | 0.89 | 2.63 | — | 0.58 | 0.69 | — | ||
BSA + CDMEAB | |||||||||
H2O + Na2SO4 | 0.03 | 0.50 | 303.15 | 0.79 | 1.96 | 5.87 | 0.57 | 0.72 | 0.81 |
1.00 | 303.15 | 0.71 | 2.67 | 6.56 | 0.57 | 0.67 | 0.77 | ||
1.50 | 303.15 | 0.65 | 3.09 | 6.68 | 0.60 | 0.69 | 0.77 | ||
2.00 | 303.15 | 0.67 | 3.78 | 7.91 | 0.60 | 0.62 | 0.72 | ||
3.00 | 303.15 | 0.79 | 4.82 | — | 0.57 | 0.56 | — | ||
BSA + CDMEAB | |||||||||
H2O + NaCl | 0.03 | 1.50 | 298.15 | 0.87 | 3.24 | 8.31 | 0.65 | 0.63 | 0.67 |
303.15 | 0.71 | 2.86 | 7.52 | 0.70 | 0.80 | 0.85 | |||
308.15 | 0.75 | 2.24 | 7.04 | 0.68 | 0.77 | 0.83 | |||
313.15 | 0.81 | 2.13 | 6.99 | 0.61 | 0.77 | 0.84 | |||
318.15 | 0.85 | 2.43 | 7.22 | 0.65 | 0.78 | 0.85 | |||
323.15 | 0.93 | 2.66 | 7.49 | 0.70 | 0.78 | 0.85 | |||
BSA + CDMEAB | |||||||||
H2O + Na2SO4 | 0.03 | 1.50 | 298.15 | 0.89 | 2.79 | 7.59 | 0.58 | 0.61 | 0.67 |
303.15 | 0.65 | 3.09 | 6.68 | 0.56 | 0.68 | 0.82 | |||
308.15 | 0.69 | 3.09 | 6.87 | 0.56 | 0.68 | 0.82 | |||
313.15 | 0.72 | 3.56 | 7.47 | 0.55 | 0.66 | 0.77 | |||
318.15 | 0.76 | 3.76 | 7.61 | 0.53 | 0.65 | 0.76 | |||
323.15 | 0.86 | 4.07 | 7.75 | 0.56 | 0.68 | 0.77 |
According to some previous study, chloride ion shows chaotropic nature along with reduction of the stability of hydrophobic moiety or associates of surfactants. Another anion is sulfate ion which carries high charge density and acts as water structure maker (a powerful kosmotrope). Therefore, raises the stability of hydrophobic aggregates of surfactants.47,48 Table 2 also indicates that Na2SO4 have better salting out effect than NaCl. The β magnitudes of CDMEAB & (BSA + CDMEAB) mixed systems in presence of electrolytes did not follow any hemolytic orientation while the aggregates stability is proved due to the high values of β (almost in all cases above 70% (±2), except ).
The effect of temperature on micellization of surfactant depends on the nature of surfactants whether it is ionic or non-ionic. Usually ionic amphiphiles exhibit U-shaped behavior,49,50 while non-ionic amphiphiles show regular trend;51–53 though it has been noted that non-ionic amphiphiles also provide the U-shaped curve.54 In our case, ESI Fig. 1† provides good harmony with literature. The alteration of critical micelle concentration due to the variation of temperatures may be analyzed by means of mode of hydration of the single surfactant and protein-surfactant mixtures. In the saturated solution of surfactant and its mixtures, both hydrophobic and hydrophilic hydration is possible while the aggregates of surfactant faced by hydrophilic hydration. However, reduction of both types of hydration is possible with an elevation of temperature and the de-solvation of polar head groups causes the favorable micellization. Many studies showed that elevation of temperature causes the disruption of H2O entities and rupturing of H-bonds around the hydrocarbon moiety of CDMEAB/BSA, stimulates the lessening of hydrophobic de-hydration, therefore, to decrease the micellization rate.11,17,50–52 Again, the β values of CDMEAB and (BSA + CDMEAB) mixed systems did not follow any regular bias with regard to the temperature in the presence and absence of sodium salts (Table 2).
ΔG0m = (1 + β)RTln(Xc*) | (1) |
ΔH0m = −(1 + β)RT2(∂lnXc*/∂T) | (2) |
ΔS0m=(ΔH0m − ΔG0m)/T | (3) |
Eqn (1) and (2) contains Xc* which denotes mole fraction of c*. ESI Fig. 1† depicts non-linear curve of ln(Xc*) versus T plot and these type of plots are used to calculate the magnitudes of ΔH0m. The values of tangents were measured as per temperature and then slopes are taken from the obtained tangents as the tantamount of ln(Xc*).59,60 The obtained magnitudes of different thermodynamic specifications for pure CDMEAB and (BSA + CDMEAB) mixed systems with and without salts (NaCl & Na2SO4) are summarized in Table 3.
System | Medium | ISalta, mmol kg−1 | T/K | ΔG01,m, kJ mol−1 | ΔG02,m, kJ mol−1 | ΔG03,m, kJ mol−1 | ΔH01,m, kJ mol−1 | ΔH02,m, kJ mol−1 | ΔH03,m, kJ mol−1 | ΔS01,m, J mol−1 K−1 | ΔS02,m, J mol−1 K−1 | ΔS03,m, J mol−1 K−1 | ΔC01,m, kJ mol−1 K−1 | ΔC02,m, kJ mol−1 K−1 | ΔC03,m, kJ mol−1 K−1 |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
a I = ionic strength of salts used.b Relative standard uncertainties (ur) limits are ur(ΔG0m), ur(ΔH0m), ur(ΔS0m), and (ΔmC0p) are 0.03, 0.03, 0.04 and 0.04 respectively. | |||||||||||||||
CDMEAB | H2O | 0.00 | 298.15 | −46.77 | −44.75 | −41.46 | −41.67 | 20.50 | −11.56 | 17.11 | 218.83 | 100.28 | 3.75 | 0.52 | 1.12 |
303.15 | −49.60 | −47.66 | −43.13 | −23.21 | 23.11 | −6.09 | 87.05 | 233.44 | 122.18 | 4.07 | 0.53 | 1.21 | |||
308.15 | −49.80 | −46.76 | −44.75 | −0.89 | 25.65 | 0.34 | 158.72 | 234.98 | 146.33 | 4.39 | 0.53 | 1.30 | |||
313.15 | −50.02 | −47.95 | −45.04 | 21.48 | 28.50 | 7.08 | 228.33 | 244.15 | 166.45 | 4.72 | 0.53 | 1.38 | |||
318.15 | −50.31 | −48.14 | −45.54 | 45.74 | 31.03 | 14.36 | 301.91 | 248.84 | 188.29 | 5.04 | 0.53 | 1.47 | |||
323.15 | −51.22 | −48.76 | −46.07 | 71.66 | 33.73 | 21.60 | 380.25 | 255.27 | 209.41 | 5.34 | 0.54 | 1.55 | |||
BSA + CDMEAB | H2O | 0.00 | 298.15 | −45.85 | −46.86 | −40.91 | −16.65 | 7.46 | −20.15 | 97.94 | 182.19 | 69.64 | 2.15 | 0.36 | 0.74 |
303.15 | −46.12 | −44.20 | −40.93 | −5.73 | 9.09 | −16.22 | 133.20 | 175.77 | 81.52 | 2.20 | 0.40 | 0.78 | |||
308.15 | −47.70 | −45.74 | −42.06 | 4.81 | 11.71 | −12.19 | 170.40 | 186.42 | 96.93 | 2.26 | 0.45 | 0.81 | |||
313.15 | −46.48 | −45.55 | −42.97 | 16.32 | 14.01 | −8.27 | 200.40 | 190.20 | 110.81 | 2.32 | 0.49 | 0.85 | |||
318.15 | −48.36 | −43.97 | −44.81 | 29.69 | 15.83 | −3.84 | 245.30 | 187.95 | 128.78 | 2.38 | 0.53 | 0.89 | |||
323.15 | −45.92 | −46.95 | −45.41 | 40.01 | 19.35 | 0.78 | 265.90 | 205.16 | 142.92 | 2.44 | 0.58 | 0.92 | |||
BSA + CDMEAB | H2O–NaCl | 1.50 | 298.15 | −43.71 | −39.32 | −36.52 | −78.09 | −65.10 | −27.83 | −115.29 | −86.48 | 29.17 | 5.16 | 3.59 | 1.52 |
303.15 | −48.40 | −44.68 | −41.48 | −52.95 | −49.20 | −21.50 | −14.99 | −14.91 | 65.91 | 5.59 | 4.13 | 1.76 | |||
308.15 | −48.25 | −45.84 | −42.06 | −22.62 | −25.84 | −10.86 | 83.17 | 64.91 | 101.25 | 6.05 | 4.69 | 2.00 | |||
313.15 | −46.82 | −46.95 | −42.93 | 9.56 | 1.41 | 0.28 | 180.05 | 154.43 | 137.98 | 6.52 | 5.24 | 2.26 | |||
318.15 | −48.31 | −47.30 | −43.82 | 42.28 | 28.05 | 11.93 | 284.72 | 236.83 | 175.23 | 6.98 | 5.80 | 2.50 | |||
323.15 | −50.17 | −47.69 | −44.33 | 78.35 | 57.03 | 24.38 | 397.70 | 324.06 | 212.62 | 7.41 | 6.35 | 2.74 | |||
BSA + CDMEAB | H2O–Na2SO4 | 1.50 | 298.15 | −41.42 | −39.62 | −36.76 | −92.92 | 15.58 | −13.08 | −172.71 | 185.16 | 79.42 | 5.81 | 0.60 | 1.14 |
303.15 | −43.80 | −41.38 | −39.94 | −64.48 | 18.77 | −7.69 | −68.21 | 198.40 | 106.39 | 6.25 | 0.63 | 1.26 | |||
308.15 | −45.38 | −42.13 | −41.98 | −31.23 | 22.21 | −0.80 | 45.93 | 208.81 | 133.64 | 6.68 | 0.66 | 1.37 | |||
313.15 | −45.53 | −41.72 | −41.03 | 4.40 | 25.22 | 6.66 | 159.45 | 213.74 | 152.30 | 7.14 | 0.69 | 1.52 | |||
318.15 | −45.46 | −41.84 | −41.52 | 39.90 | 28.52 | 14.42 | 268.32 | 221.16 | 175.81 | 7.58 | 0.72 | 1.62 | |||
323.15 | −46.57 | −43.01 | −42.30 | 79.05 | 32.66 | 22.48 | 388.75 | 234.17 | 200.47 | 8.00 | 0.75 | 1.73 |
The obtained magnitudes of ΔG0m (free energy changes) are conceived to be negative which provides strong evidence that the aggregation process is spontaneous in nature. The negative values of free energy changes are elevated with an elevation of temperatures. Such negative values of ΔG0m for the micellization of surfactants were also reported.19–22,55–58 These values are comparatively lower in presence of salts suggesting the slight reduction of dynamic forces for perfect interaction.
In the case of pure CDMEAB and (BSA + CDMEAB), the ΔH01,m, and ΔH03,m values are negative at the initial temperatures which suggest the process is exothermic in nature. These negative values are decreased with rising temperatures and revealed the endothermic process by altering the negative values to positive sign. The ΔH02,m values are positive (i.e. endothermic process) which are raised by means of rising temperatures. Again, all the values of entropy changes (ΔS01,m, ΔS02,m, and ΔS03,m) of pure CDMEAB and (BSA + CDMEAB) are positive and these values are raised by means of rising temperatures. Changes of entropy values point out that the aggregation process is entirely entropy controlled, especially at elevated temperatures. However, at the initial temperatures, the systems are both entropy and enthalpy controlled. These values of enthalpy and entropy also indicate the existence of hydrophobic (at the higher temperatures) and electrostatic (at the lower temperatures) interactions.
In presence of salts, the values of enthalpy (ΔH01,m, ΔH02,m, and ΔH03,m) for pure CDMEAB and (BSA + CDMEAB) are negative at the initial temperatures (except ΔH02,m values in presence of Na2SO4), which suggests the process is exothermic in nature. These negative values are decreased with rising temperatures and revealed the endothermic process by altering the negative values to positive sign. On the other hand, the values of entropy changes are positive except ΔS01,m and ΔS02,m (in presence of NaCl) & ΔS01,m (in presence of Na2SO4) values at 298.15 K and 303.15 K temperatures.
The negative values of ΔH0m demonstrate the existence of electrostatic interaction between the head groups, while the positive ΔH0m magnitudes demonstrate the disruption of water structure as well as the presence of hydrophobic interaction. Such negative and positive values of ΔH0m also reported by some groups previously.61–64
The positive magnitude of entropy changes (ΔS0m) of pure CDMEAB and (BSA + CDMEAB) mixed system can be analyzed by means of taking two considerable factors. Firstly, the disruption of iceberg structures as well as shifting of the hydrophobic moiety from H2O environment to hydrophobic environment of micelle cores. Secondly, the enhancement of free rotation of hydrophobic moiety inside the micelle core compared to H2O environment.65 On the other hand, a negative magnitude of entropy change (ΔS0m) may be possible only when the molecules of BSA and CDMEAB are affected due to the construction of iceberg structure around them.
ESI Fig. 2† shows the results of free energy changes (ΔG0m) which is form the enthalpic contribution (H01,m) together with entropic augmentation (−TΔS01,m). ESI Fig. 2† also depicts that enthalpy augmentation is gradually enhanced with rising temperatures while entropy augmentation showed opposite style in case of (BSA + CDMEAB) system in water. On the other hand, the supplementation of enthalpy along with entropy showed an interesting pattern in the salt medium. With an elevation of temperature, enthalpy contribution is increased and entropy contribution is decreased & showed a cross point. It is also clear from ESI Fig. 2† that both contributions are comparatively higher in salts medium than aqueous medium and provided the following pattern:
Na2SO4> NaCl > H2O |
Molar heat capacity is an important thermodynamic aspect through which we may noticeably understand the way by which drug–surfactant or protein–surfactant interaction get motion restriction along with structural rearrangement/response to binding of ligands. The value of changes of molar heat capacity (ΔC0m) for the micellization of pure CDMEAB and (BSA + CDMEAB) mixed system are summarized in Table 3. These values were achieved by taking the corresponding slopes from the plots of enthalpy changes (ΔH0m) vs. temperature (T) by utilizing the below-mentioned relation (4).66,67
ΔC0m = ((∂H0m)/∂T)P | (4) |
The achieved magnitudes of ΔC0m are positive in all cases and these values are clearly higher in the presence of salts than aqueous medium & followed the following trend (Table 3);
(BSA + CDMEAB + NaCl) > (BSA + CDMEAB + Na2SO4) > (BSA + CDMEAB) > CDMEAB |
The alteration of molar heat capacity in case of BSA & CDMEAB mixtures can be considered to associate with motion restriction event. It is also nearly related to the molecular surface area as well as staying in the similar horizon of change in the solvent accessible surface area.68 The little positive molar heat capacity values, as well as entropy values, indicate the less structural changes in studied surfactant's micelle over the period of binding interaction.43
ΔG0m.tr. = ΔG0m(aq. additive) − ΔG0m(aq.) | (5) |
ΔH0m.tr. = ΔH0m(aq. additive) − ΔH0m(aq.) | (6) |
ΔC0p.m.tr. = ΔC0p.m.(aq. additive) − ΔC0p.m.(aq.) | (7) |
The evaluated values of free energy of transfer (ΔG0m.tr.), enthalpy of transfer (ΔH0m.tr.), & molar heat capacity of transfer (ΔC0p.m.tr.) for (BSA + CDMEAB) mixed systems in pure water and in salts solution are shown in ESI Table 1†. The calculated values of free energy of transfer (ΔG0m.tr.) are positive in the cases of (BSA + CDMEAB) mixed system in water and salts (except ΔG02.m.tr. at 298.15 K in water). The first transfer enthalpy values for (BSA + CDMEAB) system in water are positives at (298.15–308.15) K, while other values including second & third are negative. On the other hand, in the presence of electrolytes, the obtained values of transfer enthalpy are negative and the negative values are gradually lessened with rising temperatures and at the high temperature, it changed to positive magnitude. Including our previous study, some researchers also reported the negative as well as positive values of transfer enthalpy.3,26,71 Jha et al.72 reported that negative values of transfer enthalpy are possible about the driving of organic (amino acids) and inorganic (various salts) molecules to urea solution from the water. The transfer of amphiphilic molecules from water to (BSA + CDMAEB) and (BSA + CDMEAB + salts) medium creates exothermic as well as endothermic environment respectively. The molar heat capacity of transfer (ΔC0p.m.tr.) is negative (except at 318.15 & 323.15) of (BSA + CDMEAB) mixed system in water while the others values of (BSA + CDMEAB) mixed system in salts are positive. These positive values of ΔC0p.m.tr. signify that the micelles faced high hydration tendency which is determinable to achieve high H-bonding. On the contrary, negative magnitudes of ΔC0p.m.tr. signify that the hydration of the polar head group is more significant than structured water molecules about a non-polar group of monomeric surfactants.73 Apparently, the ΔC02,m.tr. value at 318.15 K is 0.00 kJ K−1 mol−1 which is not actually 0.00 kJ K−1 mol−1 except very low value. All the molar heat capacity of transfer (ΔC0p.m.tr.) are approximately constant which relates the noticeable structural changeover.
(8) |
In the eqn (8), and Tc signs are denoted as intrinsic enthalpy gain and compensation temperature respectively. The values of and Tc for (BSA + CDMEAB) mixtures in water and salts solution are summarized in Table 4. The intrinsic enthalpy values are found negative, suggesting the convenient and stable micelle formation even at ΔS0m = 020, therefore stable micelle formation in water compared to the salt solution is also noticed by means of reduction of values in salts medium.
The values demonstrate the solute–solute interaction along with an increase in hydrophobicity during micelle growth. The compensation temperature (Tc) values lie in the range of 278–363 K in the current study which shows good agreement with the literature values.10,20,48 Lumry et al.76 reported compensation temperature (Tc) from 270 to 300 K and suggested that it is the indication of H2O contributions in the protein solution as well as comparable to a biological fluid. Their finding supports our result with a good harmony. Ionic surfactants in water solution also showed same phenomena about compensation temperature.77
Fig. 2 Simulation box containing BSA + 40 CDMEAB in (A) no-salt and (B) 5% NaCl environments (sodium in purple and chloride in green). |
Fig. 3 (A) Gibbs free energy change over 5 ns molecular dynamics, comparative RMSD trajectory of (B) alpha carbon, (C) backbone of BSA, (D) RMSF trajectory of BSA. |
Fig. 3(B and C) shows that the root means square deviation (RMSD) values of the alpha carbons and backbones of BSA remains almost similar throughout the simulation timescale and fluctuates around an average value. Presence of salt causes no significant change in the RMSD values which infers that the salt ions cause no significant structural changes of the protein. On the other hand, Fig. 4(A) depicts that the Solvent Accessible Surface Area (SASA) of the protein is much smaller in presence of NaCl. An explanation for the phenomenon can be inferred for the 20 ns simulation snapshot shown in ESI Fig. 5†. It reveals that in presence of salt, the larger aggregates of surfactant molecules stay in close contact with the protein surface that makes the surface less accessible to solvent molecules.
Fig. 4 (A) Change in SASA over the simulation timescale (B) collision cross sections of BSA in salt (CDMEABs) and no salt (CDMEABn) environment. |
Fig. 3(D) shows that the root means square fluctuation (RMSF) of the amino acid residues (1–560) remains almost similar in the both aqueous and salt environment but the RMSF values of the residues 561–583 are significantly larger when salt ions are present. As proteins may undergo significant structural changes while interacting with other molecules, in recent years, the calculation of collision cross section (CCS) has received great attention for providing precise information about the conformation and size of a protein in different environments. Both the projection approximation (PA) and trajectory method (TJM) has been utilized to measure perform the CCS analysis of BSA in the presence and absence of salt.78,79 The CCS_PA and CCS_TJM values of BSA in presence of salt are 4000.59 Å2 and 5157.86 Å2 respectively which are slightly lower that when no salt ions are present (Fig. 4(B)). In spite of strong interaction between the protein and surfactant molecules, the overall collision cross section of BSA + CDMEAB system remains almost similar to the CCS value of native BSA.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra00070d |
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