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Interaction Between Calcium and Casein Hydrolysates: Stoichiometry, Binding Constant, Binding Sites and Thermal Stability of Casein Phosphopeptide Complexes

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Interaction Between Calcium and Casein Hydrolysates: Stoichiometry, Binding Constant, Binding Sites and Thermal Stability of Casein Phosphopeptide Complexes International Dairy Journal 88 (2019) 25e33 Contents lists available at ScienceDirect International Dairy Journal journal homepage: www.elsevier.com/locate/idairyj Interaction between calcium and casein hydrolysates: Stoichiometry, binding constant, binding sites and thermal stability of casein phosphopeptide complexes * Ricardo Troncoso Recio a, , Nelson P. Guerra a, Ana Torrado a, Leif H. Skibsted b a Department of Analytical and Food Chemistry, Faculty of Science, Ourense Campus, University of Vigo, Ourense, Spain b Department of Food Science, University of Copenhagen, Rolighedsvej 30, DK-1958, Frederiksberg C, Denmark article info abstract Article history: The ability of a casein hydrolysate rich in casein phosphopeptides (CPPs) to form calcium complexes was À Received 18 June 2018 studied. An association constant of 125 ± 32 L mol 1 at 25 C was determined electrochemically, forming Received in revised form 1:1 complexes according to conductivity measurements. Chloride anions increased the affinity for cal- 15 August 2018 cium and number of CPPs bound to calcium in solution to more than one. From temperature dependence Accepted 15 August 2018 of the association constant, the enthalpy of binding of calcium chloride to CPPs was determined to Available online 27 August 2018 À be À24 kJ mol 1. Fourier transform infrared spectroscopy confirmed solid state binding of chloride and calcium to CPPs, further showing a positive cooperativity induced by calcium due to opening of the alpha-helix structure of CPPs. In addition, calcium chloride complexation was shown to increase the thermal stability of CPPs according to the results of thermogravimetry differential scanning calorimetry. In conclusion, hydrolysed casein could be considered as a potential enhancer of calcium availability in food. © 2018 Elsevier Ltd. All rights reserved. 1. Introduction the basis of novel calcium fortified foods and food supplements with high calcium availability. Casein phosphopeptides (CPPs) are phosphorylated peptides Much research has been conducted to characterise the Ca-CPP from hydrolysis of casein, the main protein of milk, which is interaction using purified CPP products (Berrocal et al., 1989; distributed through aS1-, aS2-, b- and k-casein fractions in the ratio Ellegård, Gammelgård-Larsen, Sørensen, & Fedosov, 1999; Meisel 3:0.8:3:1 (Park & Allen, 1998). CPPs, like the native protein, are & Olieman, 1998; Mekmene & Gaucheron, 2011; Zong, Peng, reported to exhibit calcium binding properties, which is primarily Zhang, Lin, & Feng, 2012). However, from the most practical per- attributed to the negatively charged region of their primary struc- spectives of the food industry, the study of the interaction between ture, consisting of three phosphoserines followed by two glutamic calcium and casein hydrolysates (CHs) (containing CPPs) seems to acids, also called the ‘acidic motif’ (Ferraretto, Gravaghi, Fiorilli, & be interesting so as to determine the calcium binding capacity of Tettamanti, 2003). The calcium chelation of CPPs prevents cal- the whole CH product instead of CPPs specifically, since CHs are cium interaction with anions (e.g., phosphates, oxalates, phytates or more accessible than purified CPPs for the food industry. In fact, the palmitates) from food during digestion in the neutral to alkaline interaction of calcium and protein hydrolysates has previously been conditions of intestine, which otherwise leads to precipitation. studied in calcium fortified food based on milk proteins (Canabady- Therefore, calcium solubility under these conditions is promoted by Rochelle, Sanchez, Mellema, & Banon, 2009a) or soy protein the presence of CPPs (FitzGerald, 1998; Skibsted, 2016; Sun et al., hydrolysates (Canabady-Rochelle, Sanchez, Mellema, & Banon, 2016). This makes this compound interesting from a nutritional 2009b). A high calcium binding capacity was observed in these point of view, since the combination of CPPs with calcium might be studies, suggesting such proteins as promising carriers for calcium fortification of food. However, casein hydrolysate interaction with calcium clearly needs further study. For characterisation of the CaeCH interaction, it seems neces- * Corresponding author. Tel.: þ34 678 056 437. sary to determine the stoichiometry of the species involved in the E-mail address: [email protected] (R.T. Recio). complex formed. It will also be useful for the determination of the https://doi.org/10.1016/j.idairyj.2018.08.009 0958-6946/© 2018 Elsevier Ltd. All rights reserved. 26 R.T. Recio et al. / International Dairy Journal 88 (2019) 25e33 binding constant of the complex, since it depends on the number of with only one component at each point were prepared. All solutions ligands involved in calcium chelation. For this purpose, the method were prepared using purified water from Milli-Q Plus (Millipore of Continuous Variations, also known as Job's method, is a simple Corporation, Bedford, MA, USA), and stirring the samples for 1 h at and commonly used procedure (Hill & MacCarthy, 1986; Renny, 25 C before conductivity measurements. The experiment was Tomasevich, Tallmadge, & Collum, 2013; Rossotti & Rossotti, repeated by using Ca(OTf)2 as a source of calcium, instead of CaCl2. 1961). This method is based on the measurement of any physical When the molar fraction of CaCl2 (XM) approaches 0, all the property, such as absorbance or conductivity (Renny et al., 2013), metal is considered to be complexed. On the other hand, when the that correlates linearly with the concentration of the formed metal- molar fraction of the calcium salt approaches 1, all the ligand is ligand complex (M-Ln). presumed to be in the complexed form. Considering that conduc- Therefore, the aim of this work was to characterise the interac- tivity is mainly due to the uncomplexed species, the theoretical tion between calcium and a casein hydrolysate product (containing point of minimum conductivity calculated by extrapolation, as 24% CPP). For this purpose, the stoichiometry of the calcium and CH shown in Fig. 1, is considered the stoichiometric ratio between the complex was determined by the Job's method conductivity-based, metal and the ligand for complex formation. The number of ligands, the calcium binding constant of the CH was determined electro- n, is obtained by the ligand/metal ratio of the complex in the point chemically and the temperature dependence was studied applying of minimum conductivity according to equation (1): van't Hoff equation. Additionally, the calcium and CH complex was structurally characterised by Fourier transform infrared (FT-IR) Cmin ð1 À X Þ spectroscopy, and the thermal stability of the complex was studied n ¼ L ¼ M (1) min X by thermogravimetry-differential scanning calorimetry (TG-DSC). CM M The Job's method used in this work was based on conductivity (k) 2. Materials and methods according to the linear relationship found with the concentration of reagents (Supplementary material Fig. S2). Following the procedure 2.1. Chemicals described by Stapelfeldt, Jun, and Skibsted (1993),theconductivityof the complex was calculated as the difference between the conduc- fl CaCl2.2H2O and Ca(CF3SO3)2 [calcium tri ate; Ca(OTf)2] were tivity of the reacting samples and the conductivity if no reaction had e obtained from Merck (Darmstadt, Germany) and Sigma Aldrich occurred (samples with only one component at the same concen- (Steinheim, Germany), respectively. The commercial product Hyvi- tration as the combined sample) following the equation (2): tal Casein Phosphopeptides, purchased from FrieslandCampina Domo (Amersfoort, The Netherlands) was used as a source of casein k ¼ k À k À k hydrolysates. This product consists of casein hydrolysates at a 17% complex combined sample CaðnonÀreactingÞ CHðnonÀreactingÞ degree of hydrolysis with an average molecular mass of 600 Da, of (2) which 24% are casein phosphopeptides (CPPs). The product contains 91.3% protein, 5.0% moisture and 6.2% ash. 2.3. Conductivity measurement 2.2. Job's method or method of Continuous Variations The conductivity measurements for the Job's method were carried out at 25 C by a 4-pole conductivity cell, model CDC866T (Radiometer, Copenhagen, Denmark), calibrated with a 0.01D KCl Table 1 presents the samples prepared for the Job's method, À standard solution (1408 mScm 1). Each measurement was obtained which consisted of solutions made at 7 different CaCl2 and CH concentrations keeping fixed the total solution concentration at after stirring for 1 min to stabilise the measurement (Garcia, À & 0.020 mol L 1 to obtain 7 different points. Additionally, samples Vavrusova, Skibsted, 2016). Table 1 2.4. Sample preparation for binding constant determination Job's method design.a À1 À1 À1 Point XCa Ca (mol L ) CH (mol L ) Ctotal (mol L ) Samples for binding constant determination were solutions À3 À1 prepared at a constant CaCl2 concentration (1 Â 10 mol L ) and 1 0.00 0.0000 0.0200 0.0200 À À À À increasing CH concentrations (1 Â 10 3,2Â 10 3,5Â 10 3,1Â 10 2, À À À À 0.00 0.0000 0.0167 0.0167 2 Â 10 2,3Â 10 2 and 5 Â 10 2 mol L 1). The same CH concen- 2 0.17 0.0033 0.0167 0.0200 trations were used in the case of Ca(OTf)2, which was used at a 0.17 0.0033 0.0000 0.0033 À À concentration of 1 Â 10 3 mol L 1. All solutions were prepared using 0.00 0.0000 0.0133 0.0133 purified water from Milli-Q Plus (Millipore Corporation). 3 0.33 0.0067 0.0133 0.0200 0.33 0.0067 0.0000 0.0067 2.5. Electrochemical measurement of calcium ion activity 0.00 0.0000 0.0100 0.0100 4 0.50 0.0100 0.0100 0.0200 0.50 0.0100 0.0000 0.0100 For the calcium ion activity determination, samples were stirred for 1 h at 25 C in a climate chamber before the analysis and the 0.00 0.0000 0.0067 0.0067 5 0.67 0.0133 0.0067 0.0200 measurement by a selective electrode ISE25Ca with a reference 0.67 0.0133 0.0000 0.0133 REF251 electrode from Radiometer was performed at 25 C using a water bath with temperature control, following the procedure 0.00 0.0000 0.0033 0.0033 6 0.83 0.0167 0.0033 0.0200 described by Vavrusova and Skibsted (2016).
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