Sreelekha K. Singh, Agnita Kundu, Nand Kishore * Department of Chemistry, Indian Institute of Technology, Bombay, Powai, Mumbai , India

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1 Interactions of some amino acids and glycine peptides with aqueous sodium dodecyl sulfate and cetyltrimethylammonium bromide at T ¼ 98:15 K: a volumetric approach Sreelekha K. Singh, Agnita Kundu, Nand Kishore * Department of Chemistry, Indian Institute of Technology, Bombay, Powai, Mumbai , India Abstract The apparent molar volumes V / of glycine, alanine, valine, leucine, and lysine have been determined in aqueous solutions of 0.05, 0.5, 1.0 mol kg 1 sodium dodecyl sulfate (SDS) and 1.0 mol kg 1 cetyltrimethylammonium bromide (CTAB) by density measurements at T ¼ 98:15 K. The apparent molar volumes have also been determined for diglycine and triglycine in 1 mol kg 1 SDS and CTAB solutions. These data have been used to calculate the infinite dilution apparent molar volumes for the amino acids and peptides in aqueous SDS and CTAB and the standard partial molar volumes of transfer (D tr V;m 0 ) of the amino acids and peptides to these aqueous surfactant solutions. The linear correlation of for a homologous series of amino acids has been utilized to calculate the contribution of the charged end groups (NH þ 3, COO ), CH group and other alkyl chains of the amino acids to The results on the partial molar volumes of transfer from water to aqueous SDS and CTAB have been interpreted in terms of ion ion, ion polar and hydrophobic hydrophobic group interactions. The volume of transfer data suggests that ion ion or ion hydrophilic group interactions of the amino acids and peptides are stronger with SDS compared to those with CTAB. Comparison of the hydration numbers of amino acids calculated in the present studies with those in other solvents from literature shows that these numbers are almost the same at 1 mol kg 1 level of the cosolvent/cosolute. Increasing molality of the cosolvent/cosolute beyond 1 mol kg 1 lowers the hydration number of the amino acids due to increased interactions with the solvent and reduced electrostriction.. Keywords: Partial molar volume; a-amino acids; Peptides; Hydration number; Sodium dodecyl sulfate; Cetyltrimethylammonium bromide 1. Introduction Surfactants are extensively employed in pharmaceutical [1,] and biotechnological processes [3,4]. From a technological perspective, protein surfactant interactions are important because they modulate the functional properties of proteins. Surfactant protein interactions have been studied using conductivity [5,6], chromatography [5], axisymmetric drop shape analysis [7], FTIR [8 10], circular dichroism [11,1], fluorescence [13,14], and direct calorimetry [15 18]. It has been proposed that surfactants may interact with proteins directly by competing for oil water or air water interfaces [19,0] and by binding to them thereby leading to substantial changes in the protein conformation [1,]. Alterations in the molecular characteristics of globular proteins due to their interactions with surfactants may lead to changes in their ability to bind other molecules; i.e. they may selfassociate and be absorbed at interfaces thus altering their functional characteristics. However, several details in the mode of interaction of the surfactants with the proteins remain unanswered. Therefore it is very important to understand the origin and nature of protein surfactant interactions both qualitatively and quantitatively. In

2 8 order to understand the fine details, the interactions of the building blocks of the protein with surfactants must be studied owing to the complex structural organization of the biological macromolecules. To the best of our knowledge, no report is available in the literature on the volumetric properties of the amino acids and peptides in aqueous surfactant solutions. In this paper we report the partial molar volumes of transfer of some amino acids and glycine peptides from water to aqueous sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) solutions and discuss the interactions operating in these systems including their effect on the hydration number of these solutes. Among various physical parameters, partial molar volume has been recognized as a quantity that is sensitive to structural changes occurring in solutions.. Experimental The amino acids and peptides were procured from Sigma Chemical Company, USA. Sodium dodecyl sulfate and cetyltrimethylammonium bromide were of extrapure analytical reagent grade purchased from Sisco Research Laboratories, India, and Spectrochem Ltd., India, respectively. Purity of all the chemicals as reported by the vendors is listed in table 1. All the amino acids and peptides were dried over P O 5 in a vacuum dessicator. The moisture content of the amino acids and peptides was determined using a Karl Fisher Titrator (Systronics, India). Appropriate corrections were applied wherever necessary. Water used for making the solutions of amino acids and peptides was double distilled and deionized by passing it through a Cole Parmer Barnstead mixed-bed ion exchange resin column followed by degassing. Solution densities were measured on a vibrating tube digital densimeter (model DA-10 from Kyoto Electronics, Japan) details of which has been described elsewhere [3]. The temperature of the densimeter cell was maintained by circulating water from a Julabo constant temperature circulation bath. This arrangement gave a temperature stability of 0.01 K. The vibrational period of the densimeter tube containing the solution of interest was measured three times each. The reproducibility in the density measurements on the average was g cm 3. The calibration of the densimeter was performed every day and accuracy checked by measuring the densities of aqueous sodium chloride solutions, the result of which were found to be in excellent agreement with the literature [4] with a maximum difference of 0.03 cm 3 mol 1 in the values of V /. 3. Results and discussion The apparent molar volumes (V / ) were calculated from the measured density data using the following equation: V / ¼ðM=qÞ ðq q 0 Þ10 3 =ðm q q 0 Þ; ð1þ where M is the molar mass of the solute in g mol 1, m is the molality of the solute in mol kg 1 in surfactant water mixtures, q and q 0 are the densities of the ternary system and reference solvent (desired molality of aqueous SDS or CTAB), respectively, in g cm 3. The results of the density measurements are given in table. In the cases where molality dependence of V / was found to be either negligible or having no definite trend, the partial molar volume at infinite dilution, was evaluated by taking an average of all the data points, the standard deviations pertain to the mean value. In all other cases the standard partial molar volumes were obtained by least-squares fitting to the following equation: V / ¼ þ S vm: ðþ Here, S v is the experimental slope, which is sometimes considered to be volumetric pairwise interaction coefficient [5,6]. The standard partial molar volumes along with the S v values are given in table 3. The values of, which are, in all cases, positive in both SDS and CTAB at all the molalities, show an increase with an increase in the molality of SDS. In each solvent system, the values of (table 3) showed a linear variation with the number of carbon atoms in the alkyl TABLE 1 Compounds used in this study with their empirical formula, molar mass (M r ), source (S ¼ Sigma Chemical Company, USA, SRL ¼ Sisco Research Laboratories, India), mass fraction moisture contents (w), and their mole fraction purity (x) as reported by the vendors Compound Formula M r =g mol 1 w Source x Glycine C H 5 NO S >0.99 L-alanine C 3 H 7 NO S >0.99 L-leucine C 6 H 13 N S >0.98 L-valine C 5 H 11 NO S >0.98 L-lysine monohydrochloride C 6 H 14 N O S >0.98 Glycyl glycine C 4 H 8 N O S >0.98 Glycyl glycyl glycine C 6 H 11 N 3 O S >0.99 Sodium dodecyl sulfate (SDS) C 1 H 5 NaO 4 S SRL >0.99 Cetyltrimethylammonium bromide (CTAB) C 19 H 4 BrN Spectrochem >0.98

3 9 TABLE Densities q and apparent molar volumes V / of amino acids and peptides in aqueous surfactants at T ¼ 98:15 K at various molalities m and in the presence of cosolutes m q V / ( (g cm 3 ) (cm 3 mol 1 ) Cosolute: SDS (m ¼ 0:05 Glycine Valine Lysine hydrochloride Alanine Leucine Cosolute: SDS (m ¼ 0:5 Glycine TABLE (continued) m q V / ( (g cm 3 ) (cm 3 mol 1 ) Valine Lysine hydrochloride Alanine Leucine Cosolute: SDS (m ¼ 1:0 Glycine Valine

4 10 TABLE (continued) m q V / ( (g cm 3 ) (cm 3 mol 1 ) Lysine hydrochloride Diglycine Alanine Leucine Triglycine TABLE (continued) m q V / ( (g cm 3 ) (cm 3 mol 1 ) Cosolute: CTAB (m ¼ 1:0 Glycine Valine Lysine hydrochloride Alanine Leucine Diglycine

5 11 TABLE (continued) m q V / ( (g cm 3 ) (cm 3 mol 1 ) chain of the amino acids; the average correlation coefficient is Similar linear correlations have been observed earlier for homologous series of x-amino acids in aqueous potassium thiocyanate solution [7] and a- amino acids [8,9]. This linear variation is represented by ¼ ðnhþ 3 ; COO Þþn c ðch Þ; ð3þ where n c is the number of carbon atoms in the alkyl chain of the a-amino acids, (NHþ 3, COO ) and (CH ) are the zwitterionic end groups and the methylene group contribution to, respectively. The values of (NHþ 3, COO ) and (CH ), calculated by a leastsquare regression analysis are listed in table 4. Since the alkyl chains of homologous series of the a-amino acids studied in this work are CH -(Gly), CH 3 CH-(Ala), CH 3 CH 3 CHCH-(Val) and CH 3 CH 3 CHCH CH-(Leu), the value of (CH ) obtained by this procedure characterize the mean contribution of CH and CH 3 groups to of the a-amino acids. As suggested by Hakin et al. [30,31], the contribution of the other alkyl chain of the a-amino acids reported in table 4. were calculated as follows: ðch 3Þ¼1:5 ðch Þ; ð4þ 0 ðchþ ¼0:5V ðch Þ: ð5þ TABLE 3 Volumetric parameters of amino acids and peptides in aqueous SDS and CTAB solutions at T ¼ 98:15 K a;b Water SDS CTAB (m ¼ 0:05 (m ¼ 0:5 (m ¼ 1:0 (m ¼ 1:0 Amino acids Glycine /(cm3 mol 1 ) 43.14(0.06) c 43.46(0.01) 44.13(0.05) 45.57(0.3) 44.07(0.16) S v /(cm 3 mol kg) 0.86(0.09) 0.69(0.01) 0.70(0.04) R Alanine /(cm3 mol 1 ) 60.43(0.04) c 60.55(0.01) 61.63(0.13) 6.35(0.3) 60.57(0.03) S v /(cm 3 mol kg) 0.73(0.06) 0.68(0.0) 0.96(0.07) R Valine /(cm3 mol 1 ) 90.39(0.14) c 90.99(0.17) 9.44(0.11) 9.55(0.10) 90.70(0.17) Leucine /(cm3 mol 1 ) 107.7(0.4) c 108.7(0.13) (0.09) (0.13) (0.3) Lysine Hydrochloride /(cm3 mol 1 ) 15.90(0.30) d 17.31(0.1) 18.66(0.0) 19.43(0.16) 17.0(0.08) Diglycine /(cm3 mol 1 ) 76.43(0.15) c 79.77(0.11) 77.8(0.07) S v /(cm 3 mol kg) 1.84(0.09) R 0.99 Triglycine /(cm3 mol 1 ) 11.51(0.3) c (0.16) a R, regression coefficient of the linear fit. b Entries in the parentheses are standard deviations. c Ref. [55]. d Ref. [56]. TABLE 4 Contributions of zwitter ionic group (NH þ 3, COO ), CH group and the other alkyl chains to partial molar volume ( ) in aqueous SDS and CTAB solution at T ¼ 98:15 K Group /(cm3 mol 1 ) Water SDS CTAB (m ¼ 0:05 (m ¼ 0:5 (m ¼ 1:0 (m ¼ 1:0 NH þ 3, COO 7.68(1.1) 7.80(0.95) 8.17(1.11) 9.43(1.50) 8.38(0.99) CH 15.91(0.33) 16.00(0.8) 16.3(0.33) 16.18(0.44) 15.83(0.9) CH 3 CH 31.8(0.40) 3.00(0.40) 3.64(0.40) 3.36(0.50) 31.66(0.40) CH 3 CH 3 CHCH 63.64(0.40) 64.00(0.40) 65.8(0.40) 64.7(0.40) 63.3(0.40) CH 3 CH 3 CHCH CH 79.55(0.50) 80.00(0.50) 81.60(0.50) 80.90(0.50) 79.15(0.55) Values in the parentheses are standard deviation values. Average correlation coefficient:

6 1 It is seen in table 4 the (CH ) values for the amino acids are nearly the same in all the studied molality of the SDS, as well as in 1 mol kg 1 CTAB. The average value of (CH ) obtained in this study ( ) cm 3 mol 1 at T ¼ 98:15 K is nearly the same, ( ) cm 3 mol 1 in sodium acetate [9], 15.3 and 15.6 cm 3 mol 1 in aqueous KSCN solution at T ¼ 308:15 K [7]. The (CH ) values observed in both SDS and CTAB are nearly the same as in water indicating the balance of the hydrophobic and hydrophilic interactions in these systems. Larger values of (NHþ 3, COO ) than (CH ) indicate that the interactions of the ions of SDS and CTAB with the zwitterionic groups of the amino acids and peptides are much stronger than those with the hydrophobic groups. The high result can be attributed to the reduction in the electrostriction of the solvent water due to increased amino acid surfactant interactions thereby contributing to the more positive value of the volume. The standard partial molar volumes of the amino acids were used to determine the number of molecules hydrated by these solutes by using the method described below. The values of for the amino acids can be expressed [3] by ¼ ðintþþ ðelectþ; ð6þ where (int) is the intrinsic partial molar volume of the amino acids and (elect) is the electrostriction partial molar volume due to the hydration of the amino acids. The (int) term can be further divided into two terms, one for the van der Waals volume and the other for the volume due to packing effects. Millero et al. [33] have obtained values of (int) for the amino acids from their molar crystal volumes by using the relationship 0 ðintþ ¼ð0:7=0:634ÞV ðcrystþ; ð7þ where 0.7 is the packing density for the molecule in an organic crystal and is the packing density for the random packing spheres. The molar volume of the crystal was calculated from the crystal densities of the amino acids reported by Berlin and Pallansh [34] at T ¼ 98:15 K. The values of (elect) were obtained from the experimentally measured values using equation (6). Further, the decrease in the volume due to electrostriction can be related [33] to the hydration number N w of the amino acids N w ¼ ðelectþ=ð E B Þ; ð8þ where VE 0 is the molar volume of the electrostricted water and VB 0 is the molar volume of the bulk water at T ¼ 98:15 K. This model assumes that for every water molecule taken from the bulk phase to the region near the amino acid, the volume is decreased by (VE 0 V B 0). Using ( E B Þ 3:0 cm3 mol 1 [33] for electrolytes at T ¼ 98:15 K, the hydration numbers have been calculated (see table 5). The calculated N w values of the amino acids in aqueous SDS and CTAB are observed to vary in the following order: N w ðleucineþ > N w ðvalineþ N w ðalanineþ > N w ðglycineþ: This trend is maintained in all molalities of SDS studied. With increase in the molality of SDS, the N w value, in general has a decreasing trend which is more in the case of leucine. This is attributable to stronger interaction of amino acids with the surfactants leading to a reduction in the electrostriction and hence the observed values of hydration numbers. We have made a comprehensive comparison of the hydration number of the amino acids studied in a variety of solvent systems (table 5). From the D tr V;m 0 data available in 1,4-dioxane [39], CaCl [4], NaCl [43], for glycine, alanine, valine and leucine, its seen that the trend in the hydration numbers mentioned above is maintained. The values of N w for glycine in various solvents are, respectively,.9,.1,.6,.5,.8,.8,.4,.6,.9,.4,.3 in water, 1 mol kg 1 SDS, CTAB, glucose [35], glycerol [36], 1,4- dioxane [39], KSCN [40], NH 4 Cl [41], NaCl [43], LiCl [44], and KCl [44]. They are nearly the same with an average value of.9 in water and.5 0. in 1 mol dm 3 cosolvent. For alanine, these values are 3.8 and , respectively. These results in general indicate that 1 mol kg 1 cosolvent affects the hydration number of the amino acids only to a very small extent towards the lower value. However, it is clearly seen in table 5 that increasing molality of the cosolvent beyond 1 mol kg 1 has a tendency to lower the hydration number of the amino acids due to the increased interaction with the cosolvent leading to reduced electrostriction of water Partial molar volumes of transfer from water to aqueous SDS and CTAB The partial molar volumes of transfer (D tr V;m 0 ) for the amino acids and peptides from water to surfactant solutions presented in table 6 were calculated as follows: D tr ;m½water to surfactantðaqþš ¼ 0 ½in surfactantðaqþš V ½in waterš: ð9þ A mole fraction impurity of 0.01 to 0.0 might affect the reported values of V / in table. Though the exact correction requires knowledge of all the impurities within a total of 0.01 to 0.0 mole fraction, calculations suggest that the maximum error attributable to these impurities on the average is 0.0 cm 3 mol 1. However, these corrections will largely cancel out in calculation of D tr The observed D tr ;m ;m. of amino acids and peptides from water to aqueous surfactants have the following order: 0.05 mol kg 1 SDS: lysine > leucine valine > glycine > alanine,

7 13 TABLE 5 Hydration number (N w ) of amino acids in water and different aqueous solvents at T ¼ 98:15 K Cosolvent N w ( a Glycine Alanine Valine Leucine Water SDS SDS SDS CTAB Glucose [35] Glycerol [36] Ethylene glycol [37] DMSO [38] NaCl [38] ,4-Dioxane [39] KSCN [40] NH 4 Cl [41] CaCl [4] NaCl [43] LiCl [44] NaCl [44] KCl [44] a Units refer to molality of the solution in the presence of cosolute. 0.5 mol kg 1 SDS: lysine leucine > valine > glycine alanine, 1 mol kg 1 SDS: lysine leucine > valine > glycine alanine, 1 mol kg 1 CTAB: lysine > leucine valine glycine alanine, 1molkg 1 SDS: triglycine > diglycine > glycine, 1molkg 1 CTAB: diglycine < glycine. The D tr V;m 0 of glycine, alanine, valine and leucine is nearly the same in 0.05 mol kg 1 SDS. However, at higher molalities of SDS, although the D tr V;m 0 values of glycine and alanine are nearly the same, the D tr V;m 0 values of valine and leucine, which are more hydrophobic substances, are higher. Franks et al. [3] explained the partial molar volume of a non-electrolyte as a combination of intrinsic volume of the solute and the volume changes due to its interactions with the solvent. The intrinsic volume is considered to be consisting of two contributions [45]

8 14 TABLE 6 Transfer volumes of amino acids and peptides from water to aqueous surfactants at T ¼ 98:15 K Amino acid D tr V:m 0 /(cm3 mol 1 ) SDS SDS SDS CTAB (m ¼ 0:05 (m ¼ 0:5 (m ¼ 1:0 (m ¼ 1:0 Glycine Alanine Valine Leucine Lysine hydrocloride Diglycine Triglycine V intrinsic ¼ V vw þ V void ; ð10þ where V vw is the volume occupied by the solute due to its van der Waals volume [46] and V void is the volume associated with the voids and empty spaces present therein [47]. The above equation was modified by Shahidi et al. [48] in order to evaluate the contribution of a solute molecule towards its partial molar volume as ;m ¼ V vw þ V void nr s ; ð11þ where r s is the shrinkage in the volume produced by the interaction of hydrogen bonding groups present in the solute with water molecules and n is the potential number of hydrogen bonding sites in the molecule. Thus, V;m 0 of an amino acid or a peptide can be viewed as ;m ¼ V vw þ V void V shrinkage : ð1þ If it is assumed that V vw and V void remain apparently the same in both water and in the aqueous surfactant solutions, the positive volume of transfer for the amino acids and peptides can be rationalized in terms of a decrease in the volume of shrinkage in the presence of the surfactant molecules in aqueous solutions. Since V 0 is, by definition, free of solute-solute interactions, it provides information regarding solute solvent interactions. Surfactant molecules consist of two distinct segments that are opposite in character: the polar part, known as the head group, and the other part that is composed of one or more long hydrophobic tails. Above critical micelle concentration (CMC) these molecules self-aggregate to form micelles. The attractive tail tail hydrophobic interaction provides the driving force for the aggregation of the surfactant molecules, whereas the electrostatic repulsion between the polar head groups limits the size of the micelle. The critical micellar concentrations of SDS [15] and CTAB [49] are reported as (.5 0.) 10 3 mol dm 3 and mol dm 3, respectively. Therefore the surfactants in these experiments are in the form of micelles. The following types of interactions can occur in the ternary system of amino acid (or peptide), surfactant, and water: (a) ion ion interactions between SO 4 of SDS/Br of CTAB and the NH þ 3 group of amino acids/ peptides; (b) between the Na þ of SDS/N þ CH 3 group of CTAB and the COO group of amino acids/peptides; (c) ion peptide group interactions between the ionic head group of the surfactants and the peptide group; (d) hydrophobic hydrophobic interactions between the alkyl chain of the surfactants and the hydrophobic group of the amino acids and peptides. The values of is observed to increase with increasing molality of SDS and the volumes of transfer to 1 mol kg 1 CTAB for all the amino acids and peptides are observed to be very small. Taking the cosphere overlap model [50] as the guideline, (a), (b) and (c) type of interactions would lead to a positive D tr V;m 0 since there is a reduction in the electrostriction effect and overall water structure is enhanced. Interactions of type (d) would lead to a negative D tr V;m 0 because of the reduction of water structure that is formed around those groups as a result of the cosphere overlap. Since we have observed small positive D tr V;m 0 for all the amino acids and peptides, it is concluded that ion ion, ion peptide, and hydrophilic hydrophilic group interactions are overall predominant over the hydrophobic hydrophobic, and hydrophilic hydrophobic group interactions in the ternary solutions. The hydrophobic alkyl chain structures of the surfactants are, CH 3 (CH ) 11 in SDS and CH 3 (CH ) 16 in CTAB. As the alkyl chain length increases, the hydrophobicity of the surfactant also increases. CTAB has five more CH groups than SDS. As the hydrophobicity increases the ionic interaction will decrease leading to a consequent decrease in the transfer volume as observed. The increased D tr V;m 0 of all the amino acids and peptides studied with increase in the molality of SDS indicate enhanced polar/ionic interactions confirming that the interactions of the amino acids with the micellar SDS mainly take place on the surface of the micelles. This is supported by the observation that although valine and leucine are more hydrophobic than glycine and alanine, their D tr V;m 0 values are higher in all the studied molalities of SDS. This suggests that even in valine and leucine, the ion ion interactions between the zwitter ionic amino acids and ionic micellar surface dominate over the other interactions. It has been shown that as the surfactant concentration increases, the shape of the micellar associates changes from spherical to cylindrical [51 53]. The spherical to rod transition for SDS in aqueous micellar solution occurs at 1.16 mol kg 1 [51 53]. Therefore the interactions of the amino acids and peptides with SDS are mainly with the spherical micelles. On the other hand, the D tr V;m 0 values from water to 1 mol kg 1 CTAB are nearly the same in the amino acids and peptides studied except for lysine. This

9 15 indicates a different mode of interaction with the CTAB micelles that maintains an overall balance between ionic hydrophilic and ionic-hydrophobic or hydrophobic hydrophilic group interactions. For CTAB, the spherical to rod transition occurs at molalities m ¼ 0:7 to 0.34 mol kg 1 at K [51 53]. Therefore, in this case, the interactions are mainly with the rod like micelles. Lysine has the highest D tr V;m 0 amongst all the amino acids/peptides studied. This is due to the positively charged character of lysine, which interacts strongly with the ionic groups of SDS and CTAB. Here again the D tr ;m is less in 1 mol kg 1 CTAB than in SDS. Youan Mao et al. [54] used the series piezoelectric quartz crystal technique to probe the interaction process between SDS, CTAB and lysozyme. They showed that with increasing concentration of CTAB, there is the stepwise opening of the four disulfide bridges of lysozyme due to the hydrophobic interaction of the cationic surfactant with the hydrophobic groups of the positively charged protein. In the case of SDS lysozyme interaction they found that, after the injection of a given amount of the anionic surfactant, the anionic ends of some SDS molecules strongly interacted with the charged domains of lysozyme to form a complex that leads to the occurrence of charge neutralization. It was stated that such micelle-like structure possessed very strong hydrophilicity. It has also been observed [15] that at sufficiently low free surfactant concentrations, the entropy of the surfactant mixing dominates the attractive forces between the protein and surfactant molecules so that the surfactant does not bind to the protein. 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