Interaction Mechanism of Different Surfactants with Casein: A

Article Cite This: J. Agric. Food Chem. 2019, 67, 6336−6349

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Interaction Mechanism of Different Surfactants with Casein: A Perspective on Bulk and Interfacial Phase Behavior Qing Tian,† Lu Lai,*,†,§ Zhiqiang Zhou,‡ Ping Mei,† Qingye Lu,§ Yanqun Wang,† Dong Xiang,† and Yi Liu‡,∥,⊥ †

College of Chemistry and Environmental Engineering, Yangtze University, Jingzhou 434023, P. R. China College of Chemistry and Material Sciences, Guangxi Teachers Education University, Nanning 530001, P. R. China § Department of Chemical and Petroleum Engineering, University of Calgary, Calgary T2N 1N4, Canada ∥ State Key Laboratory of Virology & Key Laboratory of Analytical Chemistry for Biology and Medicine (MOE), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, P. R. China ⊥ Key Laboratory of Coal Conversion and Carbon Materials of Hubei Province, College of Chemistry and Chemical Engineering, Wuhan University of Science and Technology, Wuhan 430081, P. R. China

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S Supporting Information *

ABSTRACT: Understanding the interaction mechanism between proteins and surfactants is conducive to the application of protein/surfactant mixtures in the food industry. The present study investigated the interaction mechanism of casein with cationic Gemini surfactant (BQAS), anionic Gemini surfactant (SGS), anionic single-chain surfactant (sodium dodecyl sulfate [SDS]), and two biosurfactants (rhamnolipid [RL] and lactone sophorolipid [SL]) at the interface and in bulk phase. BQAS/ casein and SDS/casein mixtures exhibit a strong synergistic effect on the surface activity. For SGS, RL, and SL, the formation of surfactant/casein complexes caused no improvement in surface activity. Dilational elasticity results indicate the displacement of casein by SGS, RL, and SL at the surface. However, the BQAS/casein complexes manifested varying dilational properties from pure casein surface. The strong electrostatic interaction between BQAS and casein produced large-size precipitate particles. For other surfactants, no precipitate particles formed. Determination of ζ-potential, UV−vis absorption spectra, and fluorescence spectra demonstrated the stronger interaction of BQAS and SDS with casein than that of SGS, RL, and SL. Addition of BQAS initially increased and then decreased the α-helix structure of casein. For SGS, RL, and SL, no noticeable change occurred in the casein structure. However, the formation of SDS/casein complexes was conducive to the casein structure. In conclusion, the interaction between BQAS and casein is similar to that of cationic single-chain surfactant. Furthermore, SGS exhibits a significantly different interaction mechanism from the corresponding monomer (SDS), possibly resulting from its excellent interfacial activity, low critical micelle concentration values, and strong self-assembly capability. For RL and SL, the weak interaction is attributed to the relatively complicated structure and less charged degree of hydrophilic headgroups. KEYWORDS: casein, gemini surfactant, biosurfactant, surface and interfacial properties, interaction mechanism Owing to their structural flexibility and importance in biological functions, intrinsically disordered proteins (IDPs) are considered an essential group of proteins. Although lacking an ordered three-dimensional structure, IDPs play a critically important role in crucial cellular processes, such as replication, signal transduction, and molecular recognition. Casein, which is one of the major proteins of mammalian milk, exists as micelles consisting of four separate polypeptides, namely, αs1-, αs1-, β-, and κ-caseins.6 These components are relatively similar in net charge, molecular mass, and size but differ in the degree of unfoldedness.7 As a disordered protein, casein exhibits a unique unfoldedness under natural conditions, contributing to its low intrinsic hydrophobicity and high net charge.8 Guo et al. investigated the interaction of a cationic surfactant, that is, dodecyltrimethylammonium bromide

1. INTRODUCTION Mixtures of proteins and surfactants are often used in the food industry as emulsifying ingredients and foam stabilizers.1,2 This utilization is attributed to the interaction behavior of protein and surfactant molecules at the phase interface, thus lowering interfacial tension of liquids. Therefore, studying the interaction between proteins and surfactants can provide additional insights into the performance of protein/surfactant mixtures in various food products.3 During the past few decades, protein−surfactant interactions have been widely investigated. To date, different research technologies have been employed to examine such interactions; examples of these technologies include the UV−vis absorbance spectra, circular dichroism (CD) spectra, fluorescence titration measurement, surface/interface tension determination,4 isothermal titration calorimetry, interfacial dilational rheology,5 and small-angle X-ray scattering. Consequently, numerous valuable findings have been gradually uncovered for protein−surfactant interactions. © 2019 American Chemical Society

Received: Revised: Accepted: Published: 6336

February 9, 2019 May 19, 2019 May 21, 2019 May 22, 2019 DOI: 10.1021/acs.jafc.9b00969 J. Agric. Food Chem. 2019, 67, 6336−6349

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Journal of Agricultural and Food Chemistry

Figure 1. Molecular structures of studied SGS (a), BQAS (b), SL (c), RL (d), and SDS (e).

is, CTAB, by spectroscopic methods. Addition of CTAB induces κ-casein to undergo two conformational transitions, including the change in disordered structure to a partially folded one and recovery of the disordered structure at high CTAB concentrations.12 These transitions are attributed to the electrostatic interaction between negative proteins and positive surfactants. Furthermore, dodecyl ammonium chloride, as a cationic surfactant, interacts with sodium caseinate through hydrophobic and hydrophilic groups, inducing the structural transition of self-aggregates, which are determined primarily by surfactant aggregation states, such as monomeric and micellar states.13 On the other hand, Guo et al. studied the interaction of ionic-liquid-type surfactants with β-casein micelles.14 In bulk solutions, the influences of surfactants on interfacial adsorption of protein molecules and properties of interfacial films have been extensively examined. For nonionic surfactants, Miller et al. studied the effects of nonionic surfactants on the adsorption of casein molecules. They observed that these

(DTAB), with casein and observed that DTAB binds to amino acid sites of casein chain at low surfactant concentrations. Then, micelles of DTAB attach to the chain of protein molecules, thereby generating insoluble surfactant/protein complexes. Finally, these complexes will be dissolved with a further increase in surfactant concentration.9 In another report, Guo et al. demonstrated that an anionic surfactant, that is, sodium dodecyl sulfate (SDS), first binds to casein micelles through hydrophobic interaction. Then, SDS micelles gradually generate molecular chains of casein at SDS concentrations higher than the critical aggregation concentration.10 These interaction models are specified as two types of the “necklace and bead” model. Basak et al. suggested that casein molecules adopt a more ordered structure after the addition of SDS and cetyltrimethylammonium bromide (CTAB).11 In a similar report about the conformational changes caused by the addition of cationic surfactants, Chakraborty et al. investigated the mixed behavior of κ-casein with a cationic surfactant, that 6337

DOI: 10.1021/acs.jafc.9b00969 J. Agric. Food Chem. 2019, 67, 6336−6349

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Figure 2. Equilibrium surface tension curves of individual surfactants and casein/surfactant mixtures [(a) BQAS/casein, (b) SGS/casein, (c) RL/ casein, (d) SL/casein, and (e) SDS/casein] at 298 K. The casein concentration is 0.05 g/L, while the cmc value of casein is 2.4 g/L determined using the plate method. The ratios are the molar ratios of surfactant to casein.

subjects. To elucidate the possible difference in the interaction mechanisms of Gemini surfactants and conventional surfactants with casein, we selected a traditional anionic surfactant, that is, SDS, as a control. By using UV−vis absorbance spectra, CD spectra, fluorescence spectra, and dynamic light scattering (DLS), we investigated the interaction mechanism of casein and surfactant in aqueous solutions. Meanwhile, the plate method, drop-shape analysis method, and interfacial dilational viscoelasticity were employed to study the surface/interface properties of casein/surfactant mixtures. The findings will help us to understand the mixing properties of surfactants and proteins.

surfactant/casein mixtures show a synergistic effect in decreasing surface tension at deficient concentrations.15 At the air/water interface, displacement of the β-casein film by a nonionic surfactant has been observed using Brewster angle microscopy and interfacial rheology.16 Yi et al. also suggested that competitive displacement of adsorbed caseinate by nonionic surfactant Tween 20 reduces protein oxidation but promotes lipid oxidation in food emulsion systems.17 However, for cationic ionic-liquid-type surfactants, Du et al. considered that conformational changes in β-casein would enable surfactant coadsorption to the interface, whereas lysozyme molecules would be desorbed by the surfactant from the interface as a result of competitive adsorption.18 Although several reports focus on the interaction between surfactants and casein, studies about the interaction of Gemini surfactants or biosurfactants with casein are scarce. Compared with conventional surfactants, Gemini surfactants feature better interfacial activity. Thus, a low dosage of Gemini surfactants is needed to obtain a surface/interface activity similar to that of conventional surfactants. Therefore, one cationic Gemini Surfactant (BQAS), one anionic Gemini surfactant (SGS), and two biosurfactants are selected as

2. EXPERIMENTAL SECTION 2.1. Materials. Casein was obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The cationic Gemini surfactant [dimethylene-1,2-bis(dodecyldimethylammonium bromide), BQAS] and anionic Gemini surfactant [1,2-bis(N-dodecyl-N-propanesulfonate sodium)-ethane, SGS] were synthesized and purified according to our previous literature.19,20 Figure 1 shows the molecular structures of BQAS and SGS. Lactone sophorolipid (SL) was obtained from Envgreen Biotechnology Co., Ltd. (Qingdao, China), and rhamnolipid (RL) was purchased from Rege Biotechnology Co., Ltd. (Xi’an, 6338


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Journal of Agricultural and Food Chemistry China). SDS was obtained from Aladdin Reagent (Shanghai, China) and washed thrice with diethyl ether to remove the residual dodecanol. Phosphate-buffered saline solution (0.02 mM, pH 7.4) was used as a solvent in all experiments. 2.2. Measurement of Equilibrium Surface Tension. The equilibrium surface tension (γeq) curves of individual samples were measured by K11 automatic tensiometer (Krüss, German) at 25.0 ± 0.1 °C using the plate method. The mean value was calculated from γeq values of three measurement results. 2.3. Measurement of Interfacial Dilational Viscoelasticity. The interfacial dilational viscoelasticity of individual surfactants or casein/surfactant mixtures was measured by the bubble profile tensiometer (Tracker, Teclis-IT Concept, France) at 25.0 °C. When interfacial tension reached equilibrium, the bubble volume oscillated with a sinusoidal deformation of 10% of the original drop volume. The dilational frequency was 0.1 Hz. Interfacial modulus of expansion (ε) was defined as the minimal change in interfacial tension (γ) to a specific interfacial area (A). ε=

dγ dInA

also the interfacial activity. Occasionally, surfactants and proteins form surface-active complexes through electrostatic or hydrophobic interaction. Figure 2 shows the γeq curves of individual surfactants and casein/surfactant mixtures observed in this study. The critical micelle concentrations (cmc) of different surfactants without and with casein are shown in Table S1. In these determinations, casein concentration was maintained at 0.05 g/L (the critical micelle concentration (cmc) of casein is 2.4 g/L as determined by the plate method), and the corresponding value of γeq reached 51.79 mN/m, which is depicted as a dotted line in Figure 2. As shown in Figure 2a, with a molar BQAS-to-casein ratio below 5:1, the γeq of BQAS/casein mixtures is slightly higher than that of pure casein solution. As BQAS concentration further increased, the γeq of BQAS/casein mixtures decreased rapidly and was consistently lower than that of either component. Compared with pure BQAS solution, the cmc of BQAS/casein mixtures has been reduced from 1.60 to 0.90 mol/L. Therefore, BQAS and casein exhibited a notable synergistic effect on surface activity. As a cationic Gemini surfactant, BQAS can bind to the negatively charged surface of bovine serum albumin molecules. At a low BQAS concentration, the bulk phase contains a small amount of free BQAS molecules, resulting from the combination of BQAS and casein molecules. Thus, the γeq of BQAS/casein mixtures approximates that of pure casein solution. With increasing surfactant concentration, the combination of BQAS to the surface of casein molecules through cationic headgroups of BQAS molecules significantly enhances the hydrophobicity of casein molecules. Therefore, the formation of BQAS/casein complexes benefits surface properties. As shown in Figure 2b, when nSGS:ncasein was lower than 0.8:1, the γeq of mixtures was slightly smaller than that of pure casein solution and considerably lower than that of pure SGS solution. Owing to a similar charge, a difficulty arose from the formation of surfactant/casein complexes through electrostatic interaction. If SGS binds to the surface of casein through hydrophobic alkyl chains, the hydrophilicity of casein molecules will increase and eventually reduce the adsorption capacity of casein. Consequently, the γeq of casein solution will increase. However, the SGS/casein mixtures yielded a lower γeq than pure SGS and casein, indicating that these molecules coadsorbed to the surface. The denser mixed surface layer reduced the γeq more effectively. When nSGS:ncasein increased to 0.8:1, the γeq of SGS/casein mixtures decreased significantly with increasing SGS concentration, and it was lower than that of either component. However, when nSGS:ncasein was 3:1, the γeq of mixture was higher than that of pure SGS, but the surface tension of mixtures remained constant at 8:1 nSGS:ncasein. Hence, competitive adsorption existed between SGS and casein at the surface. Owing to the highest surface activity of SGS among the surfactants and proteins studied in this research, the existence of casein at the surface will increase the surface tension of SGS/casein mixtures. With the increase in cSGS, the surface layer approached the pure SGS surface layer. As shown in Figure 2c, with a molar RL-to-casein ratio below 2.5:1, the surface tension of RL/casein mixtures was almost equal to that of pure casein solution, indicating that the interface film mainly consisted of protein molecules. When the molar ratio of RL to casein was above 2.5:1, surface tension decreased with increasing RL concentration, and the value was higher than that of pure surfactant solution. When nRL:ncasein increased to 70:1, a plateau occurred in the surface tension curve, implying that all added surfactants were bound to casein

(1)

Dilational modulus (ε) can also be expressed as a complex number through the following equation:

ε = εd + iωηd

(2)

where εd denotes the dilational elasticity, accounting for elastic energy storage in the surface, and ηd refers to the dilational viscosity representing the energy loss in the relaxation process. 2.4. Measurement of Dynamic Interfacial Tension (DIFT) Curves. The DIFT of individual surfactants or casein/surfactant mixtures was measured by a drop profile tensiometer (Tracker, TeclisIT Concept, France) at 25.0 ± 0.1 °C. Then, 4 μL of n-decane was injected into the surfactant solution through a stainless-steel needle. Images of drops were captured through a charge-coupled device camera. The images were analyzed using the Laplace equation by the instrument Soft. 2.5. UV−vis Absorbance Spectra and CD Spectra. Different volumes of surfactant solution were titrated into 3 g/L casein solution. The UV−vis spectra of mixed solutions were measured using a UV− vis spectrophotometer (TU-1810, Puxi Analytic Instrument Co., Ltd., Beijing, China) equipped with 1.0 cm quartz cells. The turbidity of casein and surfactant mixed solution, reported as100-%T, was measured at 670 nm. A CD Photomultiplier (Applied Photophysics Limited, U.K.) was used to examine CD spectra at room temperature and absorbance wavelengths of 220−260 nm. The path length of quartz cells equaled 0.1 cm, whereas scanning speed was 200 nm/min. Data were expressed in terms of mean residue ellipticity. Under the same conditions, the spectra of buffer solution were subtracted from the sample spectra. 2.6. Fluorescence Titration Measurement. By using a LS-55 fluorophotometer (Perkin−Elmer Corporation, U.S.A.), fluorescence quenching of proteins was detected. A certain amount of surfactant solutions was successively titrated into 3 mL protein solution (3 g/L). The excitation wavelength for the casein solution measured 295 nm. Excitation and emission slits were 13.0 and 3.0 nm, respectively. 2.7. Measurement of Dynamic Light Scattering (DLS). By using Zetasizer Nano ZS (Malvern Co., U.K.), we examined the hydrodynamic diameters of casein/surfactant mixtures at the scattering angle of 173° at 25.0 ± 0.1 °C. The concentration of casein solution was 3 g/L. The ζ-potential values of surfactant/casein mixtures were also determined using this instrument. The corresponding hydrodynamic diameters and ζ-potential values were the mean value of three experimental results.

3. RESULTS 3.1. γeq Curves of Surfactants and Casein/Surfactant Mixtures. In general, the interaction of surfactants with proteins affects not only the properties of the bulk phase but 6339


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Figure 3. Air/liquid interfacial dilational elasticity and phase angle of the surfactant/casein mixtures as a function of surfactant concentration (a) BQAS, (b) SGS, (c) RL, and (d) SL at 298 K. The casein concentration is 0.05 g/L.

without adsorption at the surface. This plateau yielded a final molar ratio of 100:1. Beyond this point, the surface tension decreased again, and the association process concluded. Thus, the subsequently added surfactant molecules were adsorbed at the surface. The RL/casein mixtures exhibited higher cmc values than pure RL. For SL/casein mixtures, the surface tension approached that of pure casein solution when the molar ratio was lower than 4:1. Subsequently, the increase in SL concentration induced the progressively gradual reduction of surface tension. However, the decrement rate was slightly smaller than that of pure SL solution, whereas the cmc of SL/ casein mixtures was slightly higher than that of pure SL solution, indicating the weak association between SL and casein. To elucidate the interaction mechanism, we have also examined the γeq curves of SDS/casein mixtures. SDS and casein exhibited a significant synergistic effect caused by the hydrophobic interaction between SDS and casein. 3.2. Air/Liquid Interfacial Dilational Elasticity of Individual Surfactants and Casein/Surfactant Mixtures. In the study of surfactant−protein interaction, the rheological properties of interfacial films can provide information about competitive adsorption, protein conformation changes, and interfacial film properties; such information is extremely important for emulsification and foaming performance.21,22 Interfacial dilatation modulus (E), which is the source of energy change caused by the deviation of the interfacial molecular disturbance from the equilibrium state, is closely related to the interaction between molecules. Moreover, phase angle (θ) is a quantitative representation of viscoelastic characteristics of interfacial film, reflecting the ratio of viscous to the elastic part.23 Figure 3 shows the effects of adding different concentrations of BQAS, SGS, RL, and SL on dilational elasticity and phase angle of casein solution. As depicted in Figure 3a, as BQAS concentration increased the dilational elasticity of BQAS/casein mixtures decreased

gradually. When cBQAS < 100 μmol/L (nBQAS:ncasein = 50:1), the dilational elasticity of the interfacial film decreased slightly and neared that of pure casein. This finding is attributed to the negative charge of casein at pH 7.4. This negative charge is distributed at the region of hydrophilic residues, and the relatively hydrophobic part is almost electrically neutral. The ζpotential of pure casein is −24.4 mV, as shown in Figure 4(1). When BQAS was added to the casein solution, the ζ-potential of BQAS/casein mixtures gradually increased. When the molar ratio of BQAS to casein was 50:1, the ζ-potential increased to 0.80 mV, indicating that BQAS completely neutralized the negative part of the casein molecule. Thus, the formation of BQAS/casein complexes was mainly attributed to electrostatic interaction. Given that BQAS/casein complexes occupied the interfacial layer, the dilational elasticity decreased slowly but was close to that of pure casein film. When cBQAS > 200 μmol/ L (nBQAS:ncasein = 100:1), the dilational elasticity decreased sharply to 6.67 mN/m, indicating that binding of BQAS with casein reached saturation. Thus, free BQAS molecules increased the diffusive exchange of molecules between the interface and bulk phase and then increased the contribution of the relaxation process, as confirmed by an increase in phase angle. As presented in Figure 3b, the dilational elasticity of SGS/ casein mixtures increased rapidly with increasing cSGS. When cSGS equaled 8 mol/L (nSGS:ncasein = 4:1), the dilational elasticity of the interface film approached that of pure SGS, indicating the competitive adsorption between casein and SGS molecules. The increase in dilational elasticity suggests that casein film is displaced by SGS. For biosurfactants, namely RL and SL, with the increase in RL concentration, the dilational elasticity of RL/casein mixtures decreased gradually and eventually approached that of pure RL film (Figure 3c). Given the weak interaction between RL and casein, the casein film was also displaced by 6340


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Figure 4. Variation of ζ-potential values of the surfactant/casein mixtures as a function of the concentration of surfactants [(a) BQAS, (b) SGS, (c) RL, (d) SL, and (e) SDS] at 298 K. The casein concentration is 3 g/L.

BQAS to casein reached saturation. For anionic Gemini surfactants, the addition of SGS slightly decreased the ζpotential values of casein micelles, suggesting the extremely weak interaction force between SGS and casein. Conversely, the addition of single-chain anionic surfactant SDS led to a visible decrease in ζ-potential of casein micelles. Consequently, the interaction between SDS and casein became significantly stronger than that of SGS and casein. As a result, the anionic Gemini surfactant (SGS) exhibited a markedly different interaction mechanism from the corresponding monomer (SDS), possibly resulting from the excellent interfacial activity, low cmc values, and strong self-assembly capability. When the SDS concentration reached 13 mmol/L, the binding of SDS to casein also reached saturation. For biosurfactants, the addition of RL and SL showed a slight influence on the ζ-potential value of casein micelles, indicating the tremendously weak interaction between these biosurfactants and casein. Such interaction is attributed to the relatively complicated structure and less charged degree of hydrophilic headgroups. 3.4. Dynamic Interfacial Properties of Individual Surfactants and Surfactant/Casein Mixtures. DIFT curves will provide information on the adsorption kinetics of

RL. On the other hand, the dilational elasticity of SL/casein mixtures initially increased and then decreased. When the concentration of SL was lower than 100 μmol/L (nSL:ncasein = 50:1), the generation of SL/casein complexes increased the elasticity of interfacial film. When the concentration of SL increased to 100 μmol/L (nSL:ncasein = 50:1), SL/casein complexes and free SL molecules were adsorbed competitively to the interfacial film, leading to a decrease in dilational elasticity of the interfacial film compared with that of pure SL. 3.3. ζ-Potential Measurement of Surfactant/Casein Mixtures. To understand the interaction between casein and surfactant, we measured the ζ-potential of surfactant/casein mixtures. Figure 4a shows that the ζ-potential values of casein micelles slowly increased after the addition of BQAS and then rapidly increased. When cBQAS > 5 mmol/L, the ζ-potential value of casein micelles reached 0.90 mV, indicating that BQAS/casein complexes are electroneutral. When cBQAS > 5 mmol/L (nBQAS:ncasein = 50:1), the ζ-potential values continually increased. At this time, BQAS interacted with casein via hydrophobic interaction force. When BQAS concentration reached 12 mmol/L, the ζ-potential values of BQAS/casein complexes totaled 14.6 mV, which is close to that of pure BQAS micelles (14.9 mV). Thus, binding of 6341


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Figure 5. Dynamic interfacial tension curves (n-decane/water interface) of pure casein solution (the black line), pure surfactant solution (the brown line), and surfactant/casein mixtures [(a) BQAS/casein, (b) SGS/casein, (c) RL/casein, and (d) SL/casein] at 298 K. The concentration of casein is 0.002 g/L, and the ratios are the molar ratios of surfactant to casein.

mixtures are similar to those of SGS/casein mixtures. However, a slight synergistic effect was observed on the dynamic surface activity of RL/casein mixtures (black, brown, and green lines in Figure 5c). Hence, the free RL molecules and RL/casein complexes underwent competitive adsorption. Finally, the RL molecule occupied the interface. Meanwhile, SL and casein mixtures showed an evident synergistic effect on the reduction of interface tension (black, brown, and green lines in Figure 5d). These results agree with those of dilational elasticity. 3.5. Effects of the Addition of Different Surfactants on UV−vis Absorption Spectra of Casein. UV spectrophotometry is a rapid and straightforward method to examine microenvironmental changes of aromatic amino acid residues in proteins. In general, the change in peak intensity in the UV spectra indicates a strong or weak interaction between surfactants and proteins, and the shift in peak position is generally attributed to conformational change caused by changing microenvironment of hydrophobic amino acid residues of protein macromolecules.24 Considering the absorption of aromatic heterocycles in amino acids, such as tryptophan (Trp) and tyrosine (Tyr), most protein molecules present an absorption peak near the wavelength of 270 nm.25,26 The change in the microenvironment of aromatic amino acid residues in protein molecules will lead to shifting absorption wavelength of proteins. Thus, changes in protein structure can be preliminarily studied by using the UV−vis absorption spectra of proteins. As shown in Figure 6, the characteristic absorption peak of casein solution appeared at 275 nm, and it was mainly caused by the π → π* transition of indole ring and Tyr on the peptide chain.27 The redshift in maximum absorption wavelength indicated the increased hydrophilicity and polarity of aromatic amino acid residues. The blue change in maximum absorption wavelength suggests the increased hydrophobicity of aromatic amino acid residues and decreased

interface-active materials at the interface. Figure 5 shows the DIFT curves of different surfactant/casein mixtures at the ndecane/water interface obtained through drop profile analysis. As shown in Figure 5a, the addition of a small amount of BQAS slightly decreased the dynamic interfacial activity. However, further increasing cBQAS reduced the γ of BQAS/ casein mixtures faster, indicating a more rapid interface adsorption process. Similar to γeq curves, the formation of BQAS/casein complexes is beneficial to the interface adsorption resulting from higher hydrophobicity of these complexes compared with pure casein. If we perform a comparative analysis of the DIFT curves of pure BQAS solution (brown line), pure casein solution (black line), and BQAS/casein mixtures (100:1, blue line), the BQAS/casein mixtures will exhibit a remarkable synergistic effect on dynamic interface activity. At the same time, similar to pure casein solution, the DIFT of these mixtures more easily attains a constant state compared with that of pure BQAS solution, which still notably decreased with increasing interface age (brown line). Therefore, we suppose that the interface film mainly consisted of BQAS/casein complexes rather than BQAS molecules. As presented in Figure 5b, contrary to BQAS/ casein mixtures, we have observed no decreased interfacial activity resulting from the addition of a small amount of SGS. When nSGS:ncasein < 2:1, the dynamic interfacial activity of SGS/ casein mixtures increased with cSGS. However, at a molar ratio above 2:1, the profile of DIFT curves gradually approaches that of pure SGS solution (brown line) rather than pure casein solution (black line). The significant difference between the DIFT curves of these pure components is the interface age at which interfacial tension attains equilibrium. Therefore, the component of interface film gradually changed from protein molecules to surfactants as cSGS increased. Furthermore, the effects of the addition of SL on the DIFT of SL/casein 6342


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Figure 6. Variation of UV−vis absorption spectra of casein as a function of surfactant concentration (a) BQAS, (b) SGS, (c) RL, (d) SL, and (e) SDS at 298 K, respectively. The casein concentration is 3 g/L.

polarity. Figure 4 shows the effects of different concentrations of BQAS, SGS, RL, SL, and SDS on UV absorption spectra of casein. Figure 6a shows that BQAS slightly reduced the intensity of the UV peak of casein; the intensity then rapidly increased and noticeably decreased afterward. When cBQAS < 0.1 mmol/L, the absorbance of the characteristic absorption peak decreased slowly, but the maximum absorption wavelength remained unchanged. These findings are attributed to the insufficient interaction of BQAS with casein molecules to change the microenvironmental polarity of Tyr and Trp residues. When BQAS concentration was higher than that of 0.1 mmol/L, the intensity of the characteristic absorption peak increased, and a large amount of precipitation formed in the solution, consistent with the result shown in Figure 6a. As a result, high amounts of BQAS and casein formed large precipitates, which increased the turbidity and absorbance of the solution, whereas maximum absorption wavelength remained unchanged. When cBQAS > 1 mmol/L, absorbance decreased, and the wavelength red-shifted from 275.5 to 279.0 nm. A large amount of precipitated substances was dissolved. Consequently, the Trp and Tyr residues buried deep within the protein were gradually exposed to the aqueous solution. Considering SGS, Figure 6b shows that the characteristic

absorption peak decreased with increasing SGS concentration, indicating that formation of SGS/casein complexes caused deep embedment of Trp and Tyr residues in protein molecules. However, the maximum absorption wavelength showed no change, indicating the minimal effect on the microenvironment of Trp and Tyr residues. The cmc of SGS was extremely low. Thus, weak binding of SGS with casein was observed. For RL and SL, as shown in Figure 6c,d, respectively, the absorbance continuously decreased, whereas the maximum absorption wavelength has shown no considerable change. The results show that the microenvironment of Trp and Tyr residues remained unchanged. Meanwhile, compared with Figure 6c,d, the addition of SL led to a more evident decrease in the intensity of absorbance peak compared with the addition of RL. Therefore, SL exhibited a stronger interaction with casein than RL. For the single-chain anionic surfactant SDS, a blue shift in maximum absorption wavelength occurred, indicating the increased hydrophobicity of Trp and Tyr residues in casein. This phenomenon differs from that observed with SGS, which yielded a lower cmc value. 3.6. Turbidity Titration Curves. Figure 7a shows the turbidity titration curves of casein with BQAS. At four different casein concentrations, all turbidity titration curves show that 6343


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Figure 7. (a) Turbidity titration curves of 0.05, 0.25, 1, and 3 g L−1 casein with BQAS solution at 298 K. (b) Turbidity and differential turbidity curves of BQAS with 3 g L−1 casein at 298 K. (c) Phase boundaries of the BQAS/casein mixtures at different casein concentrations.

turbidity first reached a maximum and then declined, and the maximal value of turbidity increased with ccasein. In the following discussion, the turbidity titration curve of 3 g/L casein with BQAS was selected for thorough analysis. As shown in Figure 7b, at low BQAS concentrations, turbidity was almost undetectable. Above 0.1 mM, turbidity increased sharply to a maximum value at 0.6 mM and then decreased slightly with the further addition of BQAS. When BQAS concentration was higher than 2 mM, turbidity decreased sharply. Three critical concentrations (C1, C2, and C3) were determined using the differential turbidity curve. C1 was remarkably lower than the cmc of BQAS. When cBQAS < C1, turbidity was almost undetectable, indicating that no precipitate particles were produced by BQAS and casein. When cBQAS > C1, more BQAS molecules were bound on the casein chain through electrostatic interaction, and BQAS and casein assembled into large precipitate particles. As BQAS concentration further increased to C2, turbidity sharply increased, indicating that the association of large precipitate particles resulted in liquid−liquid phase separation. When cBQAS > C3, turbidity gradually decreased, indicating that large precipitate particles were dissolved into BQAS/casein complexes with excess BQAS molecules. Figure 7c shows the effects of casein concentration on the three critical concentrations. When ccasein increased from 0.05 to 3 g/L, the values of C1, C2, and C3 considerably increased given that more surfactant molecules were needed to form and dissolve precipitate particles with increasing casein concentration. However, as shown in Figure S1, titration of other surfactants, such as SGS, RL, SL, and SDS, caused no significant increase in turbidity. Thus, no surfactant/protein precipitate particles were formed. 3.7. Effects of the Addition of Different Surfactants on the Fluorescence Spectra of Casein. Fluorescence titration is generally used to calculate the associate constants and analyze the interaction mechanism of drug molecules, nanoparticles, and surfactant molecules with protein molecules

in aqueous solutions.28,29 The intrinsic fluorescence of protein molecules mainly generates from Trp, Tyr, and phenylalanine (Phe) residues.30 To neglect the fluorescence of Tyr and Phe, we have set the excitation wavelength at 295 nm.31 As reported, αs1-casein and αs2-casein each possesses two Trp residues, that is, (Trp-164 and Trp-199) and (Trp-109 and Trp-193), respectively, whereas β-casein contains a single Trp143 residue in the hydrophobic C-terminal region.14,31 Figure 8 exhibits the fluorescence spectra of casein with different concentrations of BQAS, SGS, RL, SL, and SDS at 298 K. As shown in Figure 8a, BQAS caused a slight decrease in fluorescence spectra of casein, followed by a sharp increase. Specifically, when cBQAS increased to 0.1 mmol/L, C1 in the turbidity titration curve and fluorescence intensity of casein reached the minimum. Hence, binding of a small amount of BQAS molecules to casein caused fluorescence quenching of casein. With the further increase in BQAS concentration, the increase in fluorescence intensity was accompanied by a gradual blue shift of the maximum emission wavelength (λem). As reported, the blue shift indicates that the microenvironment surrounding Trp residues became more hydrophobic. According to the turbidity titration curve, above C1, the BQAS/casein complexes assembled into larger aggregates, further enhancing the hydrophobicity of microenvironment surrounding Trp residues. When cBQAS increased to 3 mmol/L, the fluorescence intensity of casein sharply increased and λem showed an evident redshift. Thus, the dissociation of BQAS/casein complexes with excess BQAS molecules increased the fluorescence intensity of casein. Meanwhile, the Trp residues buried in the hydrophobic zone of casein molecules were exposed to more hydrophilic microenvironment.32 For SGS, the fluorescence intensity of casein initially decreased and then increased with increasing SGS concentration; this result was accompanied by a notable redshift of λem from 352.0 to 361.0 nm. The varying degrees of fluorescence intensity induced by the addition of SGS were the smallest among the five surfactants studied in this paper. 6344


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Figure 8. Fluorescence emission spectra of casein as a function of surfactant concentration (a) BQAS, (b) SGS, (c) RL, (d) SL, and (e) SDS at 298 K, respectively (λem = 295 nm). The casein’s concentration is 3 g/L.

Table 1. Stern−Volmer Quenching Constants for the Interaction of Surfactants with Casein at 298 K RL Ksv (mM) R jS.D.

SL

SDS

0.01−0.56 mM

0.001−0.08 mM

0.08−0.64 mM

1−4 mM

5−13 mM

0.2350 0.9979 0.0028

0.7200 0.9979 0.0232

0.3400 0.9969 0.0101

0.0138 0.9951 0.0009

0.0040 0.9996 0.0005

from the formation of surfactant/casein complexes, and no considerable effect has occurred on the microenvironment surrounding Trp residues. For fluorescence quenching, the decrease in intensity is generally described by the well-known Stern−Volmer equation:34

However, as the monomer of SGS, SDS can gradually decrease fluorescence emission of casein and cause a slight blue shift in λem (Figure 8e). As shown in the inset in Figure 8e, the plots of F0/F versus cSDS are linear above or below SDS cmc, respectively, but the slopes are different. Above the cmc of SDS, the addition of SDS solution induced a slower decrease in fluorescence intensity. Compared with those of SDS, the effects of SGS on the fluorescence intensity of casein were more modest, and such results were attributed to the small cmc of Gemini surfactants. The redshift of λem indicates that SGS induced dissociation of casein micelles. Thus, Trp residues have been exposed to a more hydrophilic environment.33 For biosurfactants RL and SL, a gradual decrease in fluorescence intensity has been observed, but λem has shown no noticeable shift. Hence, the reduction of fluorescence intensity resulted

F0 = 1 + KSV[Q ] F

(3)

where F0 and F denote the steady-state fluorescence intensities without or with a quencher, respectively, Ksv refers to the Stern−Volmer quenching constant, and [Q] represents the concentration of the quencher. Hence, eq 3 was applied to determine Ksv by linear regression of the plot of F0/F against [Q]. The resultant Stern−Volmer plots are shown in the inset 6345


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Figure 9. Variation of CD spectra of casein as a function of surfactant concentration (a) BQAS, (b) SGS, (c) RL, (d) SL, and (e) SDS at 298 K, respectively. The casein concentration is 3 g/L.

of Figure 6. In Table 1, the Ksv values are in the following order: SL > RL > SDS. 3.8. Conformational Changes of Casein Molecules Induced by the Addition of Surfactants. The CD spectra can reflect the secondary structure of protein or polypeptide chains.10 Figure 9 shows the CD spectra of the casein solution at different concentrations of BQAS, SGS, RL, SL, and SDS. Casein solution exhibited a negative peak at 236 nm, generally because of the n → π* transition of the peptide bond.35 The decreased intensity of the negative peak is due to the loss of αhelical structure.36 Figure 9a shows that the addition of BQAS caused an initial increase in the negative peak followed by a considerable decrease. When BQAS concentration equaled 0.1 mmol/L, the intensity of the negative peak at 236 nm increased slightly, indicating that the binding of a small amount of BQAS to casein increases the α-helix structure. As the concentration of BQAS further increased, the intensity of the negative peak decreased rapidly and gradually shifted to 239 nm. Thus, high amounts of BQAS result in the loss of α-helix structure of casein. For SGS, RL, and SL, the intensity of the negative peak showed no significant change. Thus, these surfactants exert no noticeable effect on the structure of casein, conforming to the UV and fluorescence spectra. However, for

SDS, the intensity of negative peak increased with SDS concentration, indicating that the α-helix structures of casein increased, and the formation of SDS/casein complexes favored the secondary structure of casein. When SDS concentration reached 13 mmol/L, the intensity at 236 nm exhibited no change. Binding of SDS to casein reached saturation, and the structures of SDS/casein complexes remained unchanged. 3.9. Changes in the Size of Casein Micelles Induced by the Addition of Surfactants. DLS is an intuitive method used for determining particle size distribution. Figure 10 shows the DLS results of casein micelles after the addition of different concentrations of BQAS, SGS, RL, and SL. Figure 10a shows that the average particle size of casein micelles approximated 165 nm at 3 g/L. When BQAS concentration was in the range of 0.01−0.07 mmol/L, the average particle size of BQAS/ casein mixtures fluctuated around 160 nm. The results show that binding of a small amount of BQAS to casein induced a slight decrease in the size of casein micelles and a more compact structure of proteins, consistent with the results of CD spectra. When cBQAS continually increased, the particle size of casein micelles increased rapidly and reached the maximum when cBQAS was 1 mmol/L (C3). At the same time, the solution became turbid, indicating the formation of large BQAS/casein 6346


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Figure 10. Variation of the size distribution of casein micelles as a function of surfactant concentration (a) BQAS, (b) SGS, (c) RL, (d) SL, and (e) SDS at 298 K, respectively. The casein concentration is 3 g/L.

precipitates, consistent with the findings obtained by turbidity titration. When cBQAS measured higher than 1 mmol/L (C3), the aggregates in the solution dissolved, and the particle size decreased. When the concentration of BQAS reached 9 mmol/ L, a peak appeared at 5.01 nm, which is close to the size of pure BQAS micelles (5.28 nm). This result is attributed to the appearance of free BQAS micelles in the solution. If the concentration of BQAS further increased, the particle size distribution showed no change. Thus, the interaction between BQAS and casein reached saturation. For SGS, Figure 10b shows that with the increase in SGS concentration, the particle size distribution of casein micelles increased slightly, suggesting the formation of SGS/casein complexes. Meanwhile, a size peak of ∼30 nm was obtained, and such value approximates that of pure SGS micelles. Thus, the displacement of casein

molecules by SGS molecules at the interface possibly occurred due to the existence of pure SGS micelles. For SDS, as depicted in Figure 10e, the addition of SDS to the solution increased the number of aggregates with a size of 20 nm, indicating the depolymerization and structural change of casein. This result agrees with those of UV and fluorescence spectra. When cSDS > 13 mmol/L, the size of aggregates (4.82 nm) was close to that of pure SDS micelles, showing that free SDS micelles appeared in the solution, and binding of SDS with casein reached saturation. From Figure 10c, as RL concentration increased, the size of aggregates decreased slightly. When cRL reached 3 mmol/L, the aggregates measuring approximately 10 nm appeared in the solution, featuring a similar size with pure RL micelles. Hence, a few free RL micelles also appeared in the solution. For SL, as cSL 6347


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SGS, SDS can gradually decrease fluorescence emission of casein and cause a slight blue shift in λem. Compared with those of SDS, the effects of SGS on the fluorescence intensity of casein were more modest, and such results were attributed to the small cmc of Gemini surfactants. The red shift of λem indicates that SGS induced dissociation of casein micelles. For biosurfactants RL and SL, a gradual decrease in fluorescence intensity has been observed, but λem has shown no noticeable shift. Hence, the reduction of fluorescence intensity resulted from the formation of surfactant/casein complexes, and no considerable effect has occurred on the microenvironment surrounding Trp residues. High amounts of BQAS result in the loss of α-helix structure of casein. For SGS, RL, and SL, the intensity of the negative peak showed no significant change. Thus, these surfactants exert no noticeable effect on the structure of casein. However, for SDS, the intensity of the negative peak increased with SDS concentration, indicating that the α-helix structures of casein increased, and the formation of SDS/casein complexes favored the secondary structure of casein.

increased, the size of aggregates also slightly increased, indicating the formation of SL/casein complexes.

4. DISCUSSION BQAS/casein and SDS/casein mixtures exhibited a notable synergistic effect on surface activity. As a cationic Gemini surfactant, the combination of BQAS to casein molecules through cationic headgroups significantly enhanced the hydrophobicity of casein molecules. However, for SGS, it was difficult to form the surfactant/casein complexes through electrostatic interaction. There was competitive adsorption between SGS and casein at the surface. With the increase in cSGS, the surface layer approached the pure SGS surface layer. This result differed from that of the corresponding monosurfactant SDS. For biosurfactants namely, RL and SL, the cmc values of SL/casein and RL/casein mixtures were higher than those of pure biosurfactant solutions, indicating the weak association. For dynamic interfacial activity, the formation of BQAS/ casein complexes is beneficial to the interface adsorption resulting from higher hydrophobicity of these complexes compared with pure casein. Contrary to BQAS/casein mixtures, the profile of DIFT curves of SGS/casein mixtures gradually approaches that of pure SGS solution. The effects of the addition of SL on the DIFT of SL/casein mixtures are similar to those of SGS/casein mixtures. However, a slight synergistic effect was observed on the dynamic surface activity of RL/casein mixtures. Given that BQAS/casein complexes occupied the interfacial layer, the dilational elasticity decreased slowly but was close to that of pure casein film. When the binding of BQAS with casein reached saturation, free BQAS molecules increased the diffusive exchange of molecules between the interface and bulk phase and then increased the contribution of the relaxation process. For SGS/casein mixtures, the dilational elasticity of the interface film approached that of pure SGS, indicating the competitive adsorption between casein and SGS molecules. The increase in dilational elasticity suggested that casein film was displaced by SGS. Given the weak interaction between RL and casein, the casein film was also displaced by RL. To further demonstrate the above results, the ζ-potential values of surfactant/casein mixtures were measured. The addition of BQAS or SDS led to a visible decrease in ζpotential of casein micelles. This indicates the formation of surfactant/casein complexes. The addition of SGS slightly decreased the ζ-potential values of casein micelles, suggesting the extremely weak interaction. Conversely, SGS exhibited a markedly different interaction mechanism from the corresponding monomer (SDS), possibly resulting from the excellent interfacial activity, low cmc values, and strong selfassembly capability. For biosurfactants, the addition of RL and SL showed a slight influence on the ζ-potential value of casein micelles, indicating the tremendously weak interaction between these biosurfactants and casein. Such interaction is attributed to the relatively complicated structure and less charged degree of hydrophilic headgroups. For the BQAS/casein mixture, the turbidity first reached a maximum and then declined. However, other surfactants, such as SGS, RL, SL, and SDS, caused no significant increase in turbidity. Thus, no surfactant/protein precipitate particles were formed. The varying degrees of fluorescence intensity induced by the addition of SGS were the smallest among the five surfactants studied in this paper. However, as the monomer of

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jafc.9b00969. Turbidity titration curves (Figure S1) and critical micelle concentrations of different surfactants (Table S1) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86 716 8060654. Fax: +86 716 8060087. ORCID

Lu Lai: 0000-0002-1254-7711 Qingye Lu: 0000-0002-4157-7602 Yi Liu: 0000-0001-7626-0026 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21403017 and 21473125).

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REFERENCES

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