Thermophysical properties and taste behavior of L

Journal of Molecular Liquids 241 (2017) 237–245

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Thermophysical properties and taste behavior of L-serine/L-valine in aqueous glucose, sucrose and lactose solutions at different temperatures Ashwani Kumar, Ruby Rani, Balwinder Saini 1, Rajinder K. Bamezai ⁎ Department of Chemistry, University of Jammu, Jammu 180006, J & K, India

a r t i c l e

i n f o

Article history: Received 8 March 2017 Received in revised form 22 May 2017 Accepted 1 June 2017 Available online 02 June 2017 Keywords: Density Speed of sound Saccharides Apparent molar volume Hydration number Taste behavior

a b s t r a c t The volumetric and compressibility properties of L-serine and L-valine in the concentration range (0.0 to 0.2) mol kg−1 in water and various saccharides (0.1 glucose, 0.1 sucrose and 0.1 lactose) mol dm−3 have been studied over a temperature range of (293.15 to 313.15) K. The experimental data is used to obtain the apparent molar volume (Vϕ), limiting apparent molar volume (Voϕ), limiting apparent molar transfer volume (ΔtrVoϕ), as well as apparent molar compressibility (Ks,ϕ), limiting apparent molar compressibility (Kos,ϕ), limiting apparent molar transfer compressibility (ΔtrKos,ϕ), limiting partial molar expansibility (ϕoE), coefficient of thermal expansion (α), hydration number (nH) and taste behavior of amino acids. The results have been interpreted in terms of solute-solute and solute-solvent interactions. As the taste behavior is divided into four basic taste qualities in different ranges, so an effort has also been made to compare the taste behavior of the two amino acids in water and in aqueous solution of saccharides. © 2017 Elsevier B.V. All rights reserved.

1. Introduction The interactions of saccharides with proteins have been an intense field of research as it finds applications in number of biochemical processes like immunology, biosynthesis, medicine, pharmacology, etc. Amino acids, being the basic structural units of proteins, make up the major content of human body in playing a vital role. So, the thermodynamic study of these compounds in aqueous and mixed aqueous saccharide solutions can provide valuable information on solute-solute and solute-solvent interactions [1–4]. The physiological importance of amino acids can also be gauzed from the fact that they are delicate to the gustatory system, therefore, every amino acid makes a contribution of varying degree towards the taste of foods [5,6]. Both L-Serine and Lvaline have common α-amino and carboxyl groups. Apart from it, L-serine has a side chain hydroxyl group, classifying it as a polar and non-essential amino acid, where as L-valine has a side chain isopropyl variable group, making it a non-polar and essential amino acid. The later cannot be synthesized in human body and thus must be obtained from the diet. Saccharide solutions are often used in various food related industries to control the water activity, pH and to monitor the growth of microorganisms that are contaminated [7,8]. The addition of cosolute (saccharide) to the system (amino acid + water) can drastically affect the

⁎ Corresponding author. E-mail address: [email protected] (R.K. Bamezai). 1 School of Physical Sciences, Lovely Professional University, Phagwara (Punjab) 144402, India.

http://dx.doi.org/10.1016/j.molliq.2017.06.004 0167-7322/© 2017 Elsevier B.V. All rights reserved.

interactions between amino acids and water molecules and, accordingly, the structural order of the water molecule may increase or decrease. Further, a better understanding of the mechanism of taste chemoreception can be monitored by studying the parameters on solute-water interactions and particularly, the changes involved in the hydration layer of the solute, through a number of studies on the molar volumes of savor molecules [9–16]. Therefore, physicochemical data determining the hydration behavior of saccharides is required for a better understanding of protein stabilization, taste chemoreception and antidesiccation mechanisms [17]. Although, a detailed study of thermophysical properties of amino acids are known [18–22], however, volumetric and compressibility studies along with taste behavior of amino acids (L-serine and L-valine) with some saccharides (glucose, sucrose and lactose) in the aqueous medium seem to be scarce. This led us to study the thermodynamic behavior of L-serine and L-valine in the concentration range (0.0 to 0.2) mol kg−1 in water and in various aqueous solution of saccharides (0.1 glucose, 0.1 sucrose and 0.1 lactose) mol dm− 3 at T = (293.15, 298.15, 303.15, 308.15 and 313.15) K, i.e., as a function of concentration and temperature, from density and speed of sound data. The derived parameters like apparent molar volume, limiting apparent molar volume, transfer volume, as well as apparent molar compressibility, limiting apparent molar compressibility, transfer compressibility, partial molar expansibilities, thermal expansion coefficient and hydration number are obtained from the experimental results. These evaluated parameters are found to be helpful to understand the interactions between aqueous saccharides and the two amino acids. Additionally, an effort is also made to compare their taste behavior.

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A. Kumar et al. / Journal of Molecular Liquids 241 (2017) 237–245

a fixed frequency of 2 MHz. The uncertainty in speed measurement was found to be within ±0.5 m s−1. The temperature of solutions was maintained to an accuracy of ± 0.02 K using an electronic controlled thermostatic water bath (Model: TIC-4000N, Thermotech, India).

2. Experimental 2.1. Source and purity of chemicals The chemicals along with their mass fraction purity used in the present work are: L-serine, L-valine, α-lactose monohydrate (all N99.8%), D(+)-glucose, sucrose (both N99.5%). All these chemicals are obtained from Sigma Aldrich, India and used after keeping them over anhydrous calcium chloride in a vacuum desiccator overnight at room temperature, except α-lactose monohydrate which was used as such. All the solutions are prepared in freshly prepared triple distilled water on molality basis. Specific conductance of the triple distilled water used for the preparation of aqueous solutions was found to be b 1 × 10−6 S cm−1. 2.2. Apparatus and procedure The stock solutions of glucose, sucrose and lactose (each being 0.1 mol dm−3) were used as solvent to prepare L-serine and L-valine solutions of nine different molal concentrations (ranging from 0.0 to 0.2) mol kg− 1. An electronic single pan five digit analytical balance (Model: Mettler AE-240) with a precision of ± 0.01 mg was used to measure the mass. All the solutions were prepared with precaution and stored in special airtight bottles to avoid their exposure to air and evaporation. A vibrating-tube density meter (Model: DMA 5000 M, Anton Paar, Austria) with an uncertainty of ± 5 × 10−2 kg m− 3 was used to measure the density of solutions. A density calibration was performed first with triply distilled water followed by dry air at 293.15 K at atmospheric pressure before each series of measurement. The density, being extremely sensitive to temperature, was controlled to ± 1 × 10−3 K by built-in Peltier system. The speed of sound of solutions was measured using a single-crystal variable-path multi-frequency ultrasonic interferometer (Model: M-82S, Mittal Enterprises, India) having stainless steel sample cell (with a digital micrometer) operating at

3. Results and discussion 3.1. Volumetric studies The experimental values of densities and speeds of sound of amino acids (L-serine and L-valine) in the concentration range (0.000, 0.025, 0.050, 0.075, 0.100, 0.125, 0.150, 0.175 and 0.200) mol kg−1 in water and aqueous solution of saccharides (0.1 glucose, 0.1 sucrose and 0.1 lactose) mol dm−3 at T = (293.15, 298.15, 303.15, 308.15 and 313.15) K are listed in supplementary Table S1 as supporting information. The densities of amino acids in solutions (representative 3-D plot, shown in Fig. 1, of density versus molality of L-serine and L-valine in aqueous solution of 0.1 mol dm−3 lactose) increase with the increase in the concentration of respective amino acids but decreases with the rise of temperature. The apparent molar volume (Vϕ) has been obtained from the density values using Eq. (1). Vϕ ¼

1000ðρo –ρÞ M þ mρρo ρ

ð1Þ

where M and m are the molar mass and molality of solute (amino acids), ρ and ρo are the densities of the solution (amino acid + saccharide + water) and solvent (saccharide + water), respectively. The data presented in Table 1 reveals that apparent molar volume increases as the temperature and concentration of L-serine/L-valine increase. However, a marginal increase in this value is noticed in moving from aqueous solution of sucrose to lactose monohydrate at all temperatures and concentrations. This may probably be

330 340

320 320

310

T/ K

T/ K

300

290

280 260 240 1006

300

0.20-1

1008

kg 0.15 ol m / 0.10 m 1010

1012

240 260 280 300 320 340

ol /m 0.10 m 0.15

270 1008

0.05

1014

1016 ρ/ k 1018 g m -3

0.20-1

280

1010

(a)

0.05

1012

ρ/ k g m -3

0.00

kg

1014 1016

0.00

270 280 290 300 310 320 330

Fig. 1. Plot of density (ρ) against molality (m) for (a) L-serine, (b) L-valine in aqueous solution of lactose at (293.15, 298.15, 303.15, 308.15 and 313.15) K.

(b)


239

Table 1 Apparent molar volume (Vϕ) and apparent molar compressibility (Ks,ϕ) of L-serine and L-valine of different molalities in water and aqueous saccharides solutions at different temperatures. m (mol kg−1)

T (K) 293.15 298.15 106 ×Vϕ (m3 mol−1)

303.15

308.15

313.15

293.15 298.15 1014 ×Ks,ϕ (Pa−1 m3 mol−1)

303.15

308.15

313.15

60.05 60.14 60.25 60.36 60.47 60.58 60.70 60.82

60.39 60.48 60.59 60.70 60.78 60.88 60.98 61.07

60.69 60.79 60.87 60.97 61.05 61.13 61.22 61.31

60.88 60.95 61.04 61.13 61.20 61.28 61.36 61.44

61.15 61.23 61.29 61.36 61.44 61.52 61.59 61.67

−3.25 −2.99 −2.73 −2.47 −2.21 −1.95 −1.73 −1.53

−2.90 −2.65 −2.39 −2.20 −1.99 −1.72 −1.50 −1.26

−2.58 −2.33 −2.09 −1.90 −1.69 −1.47 −1.25 −1.04

−2.29 −2.05 −1.81 −1.63 −1.42 −1.21 −1.02 −0.85

−2.02 −1.79 −1.55 −1.31 −1.08 −0.84 −0.64 −0.48

−3 L-Serine + 0.1 mol dm aqueous-glucose 0.025 61.69 61.95 0.050 61.78 62.06 0.075 61.91 62.17 0.100 62.02 62.28 0.125 62.14 62.39 0.150 62.24 62.47 0.175 62.36 62.58 0.200 62.48 62.69

62.17 62.27 62.36 62.46 62.56 62.66 62.76 62.84

62.28 62.38 62.47 62.55 62.64 62.73 62.83 62.91

62.48 62.57 62.66 62.73 62.81 62.89 62.98 63.07

−2.28 −2.03 −1.85 −1.58 −1.36 −1.06 −0.80 −0.60

−2.20 −1.95 −1.71 −1.46 −1.21 −0.97 −0.76 −0.57

−1.91 −1.67 −1.43 −1.18 −0.94 −0.71 −0.50 −0.34

−1.82 −1.58 −1.34 −1.10 −0.87 −0.67 −0.46 −0.29

−1.82 −1.59 −1.36 −1.13 −0.94 −0.74 −0.53 −0.37

−3 L-Serine + 0.1 mol dm aqueous-sucrose 0.025 63.02 63.24 0.050 63.15 63.35 0.075 63.28 63.46 0.100 63.40 63.58 0.125 63.53 63.70 0.150 63.66 63.82 0.175 63.77 63.93 0.200 63.89 64.05

63.39 63.50 63.61 63.71 63.82 63.93 64.05 64.16

63.51 63.60 63.71 63.82 63.90 64.01 64.11 64.22

63.67 63.76 63.84 63.94 64.04 64.14 64.23 64.32

−1.85 −1.60 −1.35 −1.04 −0.76 −0.48 −0.29 −0.14

−1.79 −1.54 −1.30 −1.00 −0.77 −0.50 −0.30 −0.15

−1.51 −1.28 −1.04 −0.75 −0.52 −0.29 −0.12 0.01

−1.47 −1.24 −1.01 −0.78 −0.50 −0.28 −0.11 0.01

−1.44 −1.21 −0.99 −0.76 −0.53 −0.30 −0.14 −0.01

−3 L-Serine + 0.1 mol dm aqueous-lactose 0.025 63.16 63.35 0.050 63.29 63.48 0.075 63.43 63.61 0.100 63.55 63.73 0.125 63.68 63.85 0.150 63.81 63.96 0.175 63.95 64.07 0.200 64.09 64.19

63.50 63.61 63.72 63.83 63.95 64.06 64.16 64.27

63.58 63.69 63.80 63.90 64.01 64.12 64.21 64.30

63.74 63.83 63.92 64.03 64.12 64.22 64.32 64.43

−1.59 −1.22 −0.94 −0.61 −0.31 −0.11 0.04 0.15

−1.54 −1.18 −0.90 −0.64 −0.34 −0.13 0.01 0.13

−1.27 −1.03 −0.79 −0.45 −0.19 −0.01 0.12 0.22

−1.24 −1.01 −0.78 −0.54 −0.31 −0.12 0.02 0.13

−1.21 −0.99 −0.76 −0.53 −0.35 −0.15 −0.01 0.11

90.23 90.34 90.42 90.51 90.60 90.69 90.77 90.86

90.65 90.73 90.81 90.87 90.95 91.04 91.10 91.18

90.99 91.05 91.12 91.20 91.25 91.31 91.38 91.45

91.27 91.33 91.40 91.45 91.51 91.56 91.61 91.66

−3.79 −3.52 −3.26 −3.01 −2.75 −2.49 −2.31 −2.08

−3.41 −3.15 −2.90 −2.71 −2.45 −2.27 −2.01 −1.78

−3.05 −2.81 −2.64 −2.44 −2.22 −1.99 −1.76 −1.50

−2.74 −2.50 −2.26 −2.08 −1.87 −1.66 −1.48 −1.25

−2.46 −2.22 −1.98 −1.81 −1.61 −1.40 −1.22 −1.02

−3 L-Valine + 0.1 mol dm aqueous-glucose 0.025 91.27 91.54 0.050 91.37 91.64 0.075 91.48 91.75 0.100 91.59 91.83 0.125 91.68 91.94 0.150 91.81 92.03 0.175 91.91 92.13 0.200 92.03 92.23

91.88 91.96 92.06 92.13 92.23 92.31 92.40 92.49

92.15 92.23 92.31 92.39 92.47 92.54 92.62 92.72

92.39 92.47 92.56 92.62 92.70 92.78 92.85 92.92

−2.81 −2.56 −2.30 −1.99 −1.80 −1.55 −1.30 −1.05

−2.70 −2.45 −2.29 −2.02 −1.77 −1.52 −1.27 −1.05

−2.60 −2.36 −2.12 −1.94 −1.69 −1.45 −1.21 −1.00

−2.30 −2.07 −1.83 −1.60 −1.37 −1.17 −0.94 −0.76

−2.24 −2.00 −1.77 −1.55 −1.32 −1.09 −0.93 −0.69

−3 L-Valine + 0.1 mol dm aqueous-sucrose 0.025 92.47 92.75 0.050 92.59 92.85 0.075 92.71 92.96 0.100 92.81 93.08 0.125 92.95 93.16 0.150 93.07 93.27 0.175 93.18 93.39 0.200 93.29 93.50

92.97 93.07 93.18 93.26 93.36 93.47 93.57 93.67

93.16 93.25 93.33 93.44 93.53 93.62 93.72 93.82

93.29 93.40 93.49 93.58 93.67 93.75 93.85 93.94

−2.36 −2.11 −1.86 −1.62 −1.32 −1.12 −0.85 −0.61

−2.27 −2.02 −1.78 −1.54 −1.35 −1.06 −0.79 −0.56

−2.19 −1.92 −1.70 −1.47 −1.24 −1.01 −0.77 −0.51

−2.12 −1.89 −1.66 −1.43 −1.24 −0.97 −0.74 −0.54

−2.07 −1.85 −1.62 −1.39 −1.17 −0.94 −0.75 −0.55

−3 L-Valine + 0.1 mol dm aqueous-lactose 0.025 92.52 92.80 0.050 92.64 92.90

93.02 93.12

93.17 93.28

93.30 93.41

−2.11 −1.86

−2.02 −1.78

−1.95 −1.71

−1.89 −1.66

−1.85 −1.62

L-Serine

+ water

0.025 0.050 0.075 0.100 0.125 0.150 0.175 0.200

L-Valine

0.025 0.050 0.075 0.100 0.125 0.150 0.175 0.200

+ water 89.88 89.98 90.08 90.17 90.27 90.38 90.50 90.60

(continued on next page)

240


Table 1 (continued) m (mol kg−1)

0.075 0.100 0.125 0.150 0.175 0.200

T (K) 293.15 298.15 106 ×Vϕ (m3 mol−1)

303.15

308.15

313.15

293.15 298.15 1014 ×Ks,ϕ (Pa−1 m3 mol−1)

303.15

308.15

313.15

92.76 92.90 93.00 93.12 93.25 93.37

93.21 93.32 93.43 93.52 93.62 93.73

93.37 93.48 93.56 93.65 93.76 93.84

93.50 93.59 93.68 93.77 93.86 93.96

−1.61 −1.36 −1.07 −0.88 −0.63 −0.30

−1.48 −1.24 −1.01 −0.74 −0.51 −0.31

−1.43 −1.15 −0.97 −0.74 −0.52 −0.29

−1.39 −1.17 −0.94 −0.72 −0.49 −0.32

93.01 93.13 93.23 93.34 93.45 93.56

attributed to small difference in their respective molar masses in comparison to glucose. A plot of Vϕ against molality of L-serine/L-valine in water as well as aqueous solution of saccharides at different temperatures has been found to be linear (Fig. 2 is for L-serine/L-valine in one of the saccharides, namely, 0.1 mol dm− 3 glucose). The limiting apparent molar volume of solute, also known as partial molar volume (Voϕ), is then obtained using the least square fitting of Vϕ values through the following relation: V ϕ ¼ V o ϕ þ Sv m

ð2Þ

−1.54 −1.30 −1.06 −0.82 −0.55 −0.29

where Sv, the experimental slope, is characteristic of interactions between solute molecules. The values of Voϕ and Sv along with the standard deviation, σ, for Lserine and L-valine in water as well as aqueous solution of saccharides at different temperatures are presented in Table 2. Our values of Voϕ in water are found to be in good agreement with those reported in literature [23,24]. An introspection of this Table shows that Voϕ values of these amino acids in water and cosolute (aqueous solution of saccharides) are positive at all temperatures which are an indicative of strong solute–solvent interactions. The greater Voϕ values for L-serine and L-valine in aqueous glucose/sucrose/lactose solutions in comparison to aqueous solution may be attributed to its hydration behavior. The values also increase with the increase in temperature from aqueous to aqueous saccharides. This may be ascribed on the basis of the size of primary and secondary solvation layers around the zwitterions of L-serine and L-valine in aqueous saccharides. An increase in temperature allows the solvent molecule from the secondary solvation layer of L-serine and L-valine zwitterions to get released into the bulk of the solvent rather than binding to the charged end groups which results in the expansion of the solution [25]. Further, as is evident from this table, the Voϕ values of L-valine are greater than L-serine which may be a result of an increase in its molar mass. The limiting apparent molar transfer volume (ΔtrV°ϕ, Table 2) of Lserine and L-valine from water to aqueous saccharides was calculated using the following relation:

Δtr V

°

ϕ

¼V

°

° ϕ;aq−saccharides −V ϕ;water

ð3Þ

where V°ϕ,water is the limiting apparent molar volume of the two amino acids in water. The ΔtrV°ϕ values are positive which increases with increase in the complexity of saccharides, i.e., in the order lactose N sucrose N glucose but decreases with increase in temperature. The limiting apparent molar volume of L-serine and L-valine can also be expressed in the following way [26,27]: V

Fig. 2. Plot of apparent molar volume (Vϕ) against molality (m) for (a) L-serine, (b) Lvaline in aqueous solution of glucose at different temperatures.

°

ϕ

¼ VvdW þ V void −Vs

ð4Þ

where VvdW is the van der Waals volume [28] occupied by the solute, Vvoid is the volume linked with the void and vacant spaces present therein and Vs is shrinkage in the volume due to hydrogen bonding between the solute and water molecules. It has been well documented that the contribution due to VvdW and Vvoid experimentally remain same in both water as well as aqueous glucose/sucrose/lactose solutions [29]. Hence, positive ΔtrV°ϕ values for the studied amino acids can be ascribed due to decrease in the shrinkage volume in presence of aqueous saccharide solutions. The interactions between solute and cosolute in aqueous solution can be explained using a co-sphere overlap model [30]. When two molecules come closer to each other, their hydration co-spheres overlap. This results in the displacement of co-sphere material accompanied by change in thermodynamic properties of solutions which may be used as one of the tools to explain ΔtrV° ϕ values. As per this model, the main interactions between two amino acids (solute) and three saccharides (cosolute) may be classified as: (a)


241

Table 2 Limiting apparent molar volume (V°ϕ), standard deviation (σ), slope (Sv), transfer volume (V°ϕ,tr), limiting apparent molar compressibility (K°s,ϕ), standard deviation (σ), slope (Sk) and transfer compressibility, (K°s,ϕ,tr) of L-serine and L-valine in water and aqueous saccharides solutions at different temperatures. Property

T (K) 293.15

298.15

303.15

308.15

313.15

σ 106 × Sv (m3 mol−2 kg) 1014 × K°s,ϕ (Pa−1 m3 mol−1) σ 1014 × Sk (Pa−1 m3 mol−2 kg)

59.92(±0.01) (60.06)23 0.01 4.42(±0.05) −3.48(±0.02) 0.03 9.97(±0.17)

60.29(± 0.01) (60.56)23 0.01 3.92(±0.04) −3.12(±0.02) 0.02 9.26(±0.12)

60.61(± 0.01) (60.96)23 0.01 3.51(±0.02) −2.77(±0.01) 0.02 8.69(±0.10)

60.80(±0.01) (61.34)23 0.01 3.19(±0.04) −2.46(±0.02) 0.03 8.22(±0.16)

61.07(±0.01) (61.63)23 0.01 2.94(±0.04) −2.22(±0.02) 0.03 8.98(±0.19)

−3 L-Serine + 0.1 mol dm aqueous glucose 106 × V°ϕ (m3 mol−1) σ 106 ×Sv (m3 mol−2 kg) 106 × ΔtrV°ϕ (m3 mol−1) 1014 × K°s,ϕ (Pa−1 m3 mol−1) σ 1014 × Sk (Pa−1 m3 mol−2 kg) 1014 × ΔtrK°s,ϕ (Pa−1 m3 mol−1)

61.56(±0.01) 0.01 4.56(±0.04) 1.64 −2.54(±0.02) 0.03 9.77(±0.19) 0.94

61.85(± 0.01) 0.01 4.21(±0.06) 1.56 −2.42(±0.02) 0.02 9.46(±0.15) 0.70

62.07(± 0.01) 0.01 3.88(±0.04) 1.46 −2.12(±0.02) 0.03 9.17(±0.21) 0.65

62.20(±0.01) 0.01 3.58(±0.03) 1.40 −2.01(±0.03) 0.03 8.84(±0.20) 0.45

62.40(±0.01) 0.01 3.32(±0.04) 1.33 −2.00(±0.02) 0.03 8.33(±0.17) 0.22

−3 L-Serine + 0.1 mol dm aqueous sucrose 106 × V°ϕ (m3 mol−1) σ 106 × Sv (m3 mol−2 kg) 106 × ΔtrV°ϕ (m3 mol−1) 1014 × K°s,ϕ (Pa−1 m3 mol−1) σ 1014 × Sk (Pa−1 m3 mol−2 kg) 1014 × ΔtrK°s,ϕ (Pa−1 m3 mol−1)

62.90(±0.01) 0.01 5.01(±0.05) 2.98 −2.09(± 0.04) 0.06 10.21(±0.35) 1.39

63.12(± 0.01) 0.01 4.67(±0.03) 2.83 −2.01(±0.04) 0.05 9.70(±0.29) 1.11

63.28(± 0.01) 0.01 4.36(±0.03) 2.67 −1.71(±0.05) 0.07 9.25(±0.41) 1.06

63.40(±0.01) 0.01 4.07(±0.03) 2.60 −1.66(±0.04) 0.05 8.80(±0.21) 0.80

63.57(±0.01) 0.01 3.79(±0.04) 2.50 −1.62(±0.03) 0.04 8.41(±0.27) 0.60

−3 L-Serine + 0.1 mol dm aqueous lactose 106 × V°ϕ (m3 mol−1) σ 106 × Sv (m3 mol−2 kg) 106 × ΔtrV°ϕ (m3 mol−1) 1014 × K°s,ϕ (Pa−1 m3 mol−1) σ 1014 × Sk (Pa−1 m3 mol−2 kg) 1014 × ΔtrK°s,ϕ (Pa−1 m3 mol−1)

63.03(±0.01) 0.01 5.26(±0.04) 3.11 −1.71(±0.09) 0.12 10.11(±0.70) 1.77

63.24(± 0.01) 0.01 4.78(±0.07) 2.95 −1.66(±0.08) 0.10 9.64(±0.62) 1.46

63.39(± 0.01) 0.01 4.41(±0.03) 2.78 −1.43(±0.08) 0.10 8.94(±0.60) 1.34

63.48(±0.01) 0.01 4.13(±0.07) 2.68 −1.39(±0.05) 0.06 8.09(±0.37) 1.07

63.63(±0.01) 0.01 3.93(±0.03) 2.56 −1.35(±0.04) 0.05 7.70(±0.32) 0.87

σ 106 × Sv (m3 mol−2 kg) 1014 × K°s,ϕ (Pa−1 m3 mol−1) σ 1014 × Sk (Pa−1 m3 mol−2 kg)

89.77(±0.01) (90.54)24 0.01 4.12(±0.05) −4.00(±0.02) 0.03 9.80(±0.19)

90.15(± 0.01) (90.87)24 0.01 3.54(±0.03) −3.62(±0.02) 0.02 9.17(±0.13)

90.57(± 0.01) (90.22)24 0.01 3.03(±0.03) −3.28(±0.02) 0.03 8.70(±0.18)

90.93(±0.01) (91.48)24 0.01 2.59(±0.04) −2.92(±0.02) 0.02 8.35(±0.13)

91.23(±0.01) – 0.01 2.20(±0.04) −2.63(±0.02) 0.02 8.10(±0.15)

−3 L-Valine + 0.1 mol dm aqueous glucose 106 × V°ϕ (m3 mol−1) σ 106 × Sv (m3 mol−2 kg) 106 × ΔtrV°ϕ (m3 mol−1) 1014 × K°s,ϕ (Pa−1 m3 mol−1) σ 1014 × Sk (Pa−1 m3 mol−2 kg) 1014 × ΔtrK°s,ϕ (Pa−1 m3 mol−1)

91.15(±0.01) 0.01 4.34(±0.04) 1.38 −3.05(±0.02) 0.02 10.03(±0.15) 0.95

91.45(± 0.01) 0.01 3.89(±0.03) 1.30 −2.96(±0.02) 0.03 9.53(±0.16) 0.66

91.79(± 0.01) 0.01 3.52(±0.04) 1.22 −2.83(±0.01) 0.02 9.16(±0.10) 0.45

92.07(±0.01) 0.01 3.23(±0.05) 1.14 −2.51(±0.02) 0.02 8.90(±0.12) 0.41

92.32(±0.01) 0.01 3.01(±0.04) 1.09 −2.44(±0.02) 0.02 8.80(±0.13) 0.19

−3 L-Valine + 0.1 mol dm aqueous sucrose 106 ×V°ϕ (m3 mol−1) σ 106 × Sv (m3 mol−2 kg) 106 × ΔtrV°ϕ (m3 mol−1) 1014 × K°s,ϕ (Pa−1 m3 mol−1) σ 1014 × Sk (Pa−1 m3 mol−2 kg) 1014 × ΔtrK°s,ϕ (Pa−1 m3 mol−1)

92.35(±0.01) 0.01 4.72(±0.06) 2.58 −2.61(±0.01) 0.02 10.06(±0.11) 1.39

92.64(± 0.01) 0.01 4.28(±0.06) 2.49 −2.52(±0.02) 0.02 9.75(±0.14) 1.10

92.87(± 0.01) 0.01 3.98(±0.03) 2.30 −2.41(±0.01) 0.01 9.42(±0.08) 0.87

93.06(±0.01) 0.01 3.78(±0.04) 2.13 −2.34(±0.01) 0.02 9.07(±0.10) 0.58

93.21(±0.01) 0.01 3.65(±0.03) 1.98 −2.28(±0.01) 0.02 8.78(±0.10) 0.35

−3 L-Valine + 0.1 mol dm aqueous lactose 106 × V°ϕ (m3 mol−1) σ 106 × Sv (m3 mol−2 kg) 106 × ΔtrV°ϕ (m3 mol−1)

92.40(±0.01) 0.01 4.85(±0.04) 2.63

92.69(± 0.01) 0.01 4.37(±0.03) 2.54

92.92(± 0.01) 0.01 4.03(±0.03) 2.35

93.09(±0.01) 0.01 3.81(±0.04) 2.16

93.22(±0.01) 0.01 3.68(±0.03) 1.99

L-Serine

+ water 106 × V°ϕ (m3 mol−1)

L-Valine

+ water 106 × V°ϕ (m3 mol−1)

(continued on next page)

242


Table 2 (continued) Property

T (K) 293.15

298.15

303.15

308.15

313.15

1014 × K°s,ϕ (Pa−1 m3 mol−1) σ 1014 × Sk (Pa−1 m3 mol−2 kg) 1014 × ΔtrK°s,ϕ (Pa−1 m3 mol−1)

−2.37(±0.02) 0.03 10.16(±0.18) 1.63

−2.28(±0.01) 0.01 9.84(±0.09) 1.34

−2.19(±0.01) 0.02 9.49(±0.10) 1.09

−2.11(±0.02) 0.02 9.13(±0.12) 0.81

−2.06(±0.01) 0.02 8.84(±0.11) 0.57

hydrophilic-ionic interactions between OH groups of saccharides and zwitterions of amino acids, (b) hydrophilic-hydrophobic interactions between the OH groups of saccharides and non-polar (− CH2) in side chain of amino acids and (c) hydrophobic-hydrophobic interactions between hydrophobic groups of solute and cosolute [31]. The V°ϕ values increase due to reduction in the electrostriction at terminals by positive contribution from the interactions of type (a) leading to positive Δtr V° ϕ values. On the contrary, V° ϕ values may decrease due to disruption of side group hydration by that of the charged end by negative contribution from the interactions of type (b) and (c) leading to negative Δ tr V° ϕ values. Our positive Δ tr V° ϕ results justify dominance of interactions of type (a). The ΔtrV°ϕ values of both the solutes in aqueous cosolute at all temperatures follow the order: lactose N sucrose N glucose. Glucose molecule is a monosaccharide, whereas both sucrose and lactose has two monosaccharide units with the difference that the lactose molecule has galactose and glucose subunits whereas the sucrose molecule consists of glucose and fructose subunits. The above trend is due to the strong hydrophilic-ionic interactions of solute in aqueous lactose and sucrose solutions due to the presence of higher number of OH groups than in glucose one. In order to evaluate the limiting apparent molar expansibility (ϕoE), one makes use of variation of V°ϕ with T using the following general polynomial equation: V

°

ϕ

¼ a þ bT þ cT 2

ð5Þ

temperature derivative of apparent molar expansibility at infinite dilution, which gives pivotal judgment regarding the solvation of a solute and its structure making or structure breaking ability in a solvent, can be determined using the following relation: ∂ϕo E =∂T

p

¼ ∂2 V

°

ϕ =∂T

2

p

¼ 2c

ð7Þ

For structure making and structure breaking solutes, (∂2V°ϕ/∂T2)p values should be positive and negative, respectively [32]. Our observation (Table S3) suggests that L-serine and L-valine act as structural breaker both in water and aqueous saccharides. The thermal expansion coefficient, α, has been calculated using the following relation: α ¼ 1=V

°

ϕ

∂V

°

ϕ =∂T

ð8Þ

p

A general observation which arises from Table 3 is that α values decrease with increasing temperature. This trend shows that the structure plays an important part in deciding the magnitude of α, which further supports the chaotropic behavior of the two amino acids in aqueous solution of various saccharides. In general, it has observed that thermal expansion coefficient in conjunction with limiting apparent volume may be used as a tool to explain the solute-solvent interactions [33]. 3.2. Compressibility studies

where a, b and c are coefficients whose values are listed in Table S3. The limiting apparent molar expansibility, reported in Table 3, can now be evaluated as:

The isentropic compressibility (κs) is given by the following equation:

ϕo E ¼ ∂V

κ s ¼ u2 ρ

°

ϕ =∂T

p

¼ b þ 2cT

ð6Þ

It is evident that ϕoE values are positive and decrease with temperature, which is also shown by negative (∂2V°ϕ/∂T2)p values. This means that molecular motions get swifter and the disagreement in water structure becomes little between hydration shell and bulk water resulting into the feeble overlap of hydration shell. Another parameter, the

−1

ð9Þ

The isentropic compressibility of L-serine and L-valine in water and aqueous saccharides shows a decrease with increase in molality of the two amino acids as well as temperature (see supplementary Table S2). The decreasing κs values indicate that the water molecules around the solute (L-serine and L-valine) associated with ionic groups of zwitterion/hydrophilic groups are less compressible than water molecules in

Table 3 Limiting apparent molar expansibility (ϕoE) and coefficient of thermal expansion (α) of L-serine and L-valine in water and aqueous saccharides solutions at different temperatures. m (mol kg−1)

T (K) 293.15 ϕoE × 3

298.15 6

α × 10 10 (m mol1 K−1) (K−1)

L-Serine

1.252

L-Serine

3

o

303.15 6

ϕ E × 10 α × 10 (m3 mol1 K−1) (K−1)

3

o

308.15 6

ϕ E × 10 α × 10 (m3 mol1 K−1) (K−1)

3

o

313.15 6

ϕ E × 10 α × 10 (m3 mol1 K−1) (K−1)

3

ϕoE × 106 α × 103 (m3 mol1 K−1) (K−1)

0.075 + water + 0.1 mol dm−3 aqueous glucose 0.056 −3 L-Serine +0.1 mol dm aqueous sucrose 0.040

0.066

1.095

0.056

0.924

0.047

0.773

0.037

0.910

0.048

0.776

0.041

0.661

0.033

0.531

0.025

0.401

0.636

0.036

0.570

0.032

0.506

0.028

0.442

0.024

0.378

L-Serine

+ 0.1 mol dm−3 aqueous lactose

0.039

0.619

0.034

0.538

0.029

0.457

0.024

0.378

0.019

0.299

L-Valine

+ water

0.087

0.969

0.080

0.887

0.074

0.817

0.068

0.748

0.0610

0.669

+ 0.1 mol dm−3 aqueous glucose 0.068 −3 L-Valine + 0.1 mol dm aqueous sucrose 0.061

0.746

0.064

0.700

0.059

0.643

0.055

0.597

0.050

0.542

0.661

0.052

0.561

0.043

0.463

0.034

0.365

0.025

0.268

+0.1 mol dm−3 aqueous lactose

0.649

0.049

0.529

0.038

0.409

0.027

0.290

0.016

0.172

L-Valine

L-Valine

0.060

0.606


the bulk solution [34], which is imperative due to strong attractive interactions. The observed κs values in water and 0.1 mol dm−3 aqueous solutions each of lactose, sucrose and glucose are 4.47187, 4.37862, 4.38680 and 4.42884 (10− 10 m2 N−1), respectively at 298.15 K. The trend lactose b sucrose b glucose is obeyed at other temperatures as well. Apparent molar compressibility (Ks,ϕ) is evaluated from density and speed of sound values using Eq. (10). K s;ϕ ¼

1000 κ s ρo –κ mρρo

°

sρ

þ

M ρ

ð10Þ

where κs and κos are isentropic compressibilities of solution and solvent (aqueous saccharides), respectively. The values of apparent molar compressibility of L-serine and L-valine are presented in Table 1. Its variation with molality of two amino acids in water as well as aqueous saccharides at different temperatures has been found to be linear (Fig. 3 is for L-serine/L-valine in one of the saccharides, namely, 0.1 mol dm−3 glucose). The variation of apparent molar compressibility (Ks,ϕ) with the molality can be represented using the following relation. K s;ϕ ¼ K

°

s;ϕ

þ Sk m

ð11Þ

243

where K°s,ϕ is the limiting apparent molar compressibility or partial molar compressibility of solute and Sk is the experimental slope. Both these values, obtained using linear regression of Kos,ϕ against m, along with the standard deviation, σ, are given in Table 2. An examination of this table suggests that K°s,ϕ and Sk values, at all temperatures, are negative and positive, respectively. The negative values of K°s,ϕ reaffirm that the water molecules surrounding the ionic charged groups of L-serine and L-valine provide great resistance to compression than water molecules present in the bulk of the solution, thereby, advocating the existence of strong solute-solvent interactions than weak solute-solvent interactions. The more negative values at lower temperature may be due to the strong attractive interactions between solute and solvent molecules. As the temperature is raised, K°s,ϕ values becomes less negative which indicates that electrostriction is reduced and some water molecules from the secondary solvation layer of zwitterions from the two amino acids are released into the bulk, thus, making the solutions more compressible. The limiting apparent molar transfer compressibility (ΔtrK°s,ϕ) of Lserine and L-valine from water to aqueous saccharides is calculated using the following expression: Δtr K

°

s;ϕ

¼K

°

° s;ϕ;aq:saccharides −K s;ϕ;water

ð12Þ

where K°s,ϕ,water is the limiting apparent molar compressibility of the two amino acids in water. The ΔtrK°s,ϕ values of the two amino acids from water to aqueous saccharides, shown in Table 2, are positive which decreases with increase in temperature but increase as we move from glucose to lactose. The positive ΔtrK°s,ϕ values signify the importance of the charged end groups of amino acids. The interactions among zwitterionic centres of amino acids and saccharides increase with the increase in the complexity of saccharides. With the increasing complexity of saccharides, electrostriction increases and structure making tendency of ions increase, making electrostricted water much less compressible than bulk water. The decrease in ΔtrK°s,ϕ values with increase in temperature indicate that release of water molecules from the secondary solvation layer of zwitterions into the bulk becomes difficult as compared to aqueous medium. The observed trends in K°s,ϕ and ΔtrK°s,ϕ support the conclusions drawn from V°ϕ and ΔtrV°ϕ. 3.3. Hydration number The determination of number of water molecules hydrated to L-serine and L-valine depends upon the type of model used, i.e., electrostriction partial molar volume and electrostriction partial molar compressibility [35]. The former term is obtained using the following expression: V

°

ϕ ðelect:Þ

¼V

°

° ϕ −V ϕ ð

int:Þ

ð13Þ

where V°ϕ(elect.) is the electrostriction partial molar volume due to the hydration of L-serine and L-valine, while V°ϕ(int.) is its intrinsic partial molar volume. The term V°ϕ(int.) is evaluated from Eq. (14). V

°

ϕð

int:Þ ¼ ð0:7=0:634ÞV

°

ϕ ðcryst:Þ

ð14Þ

where 0.7 and 0.634 represents packing density of molecules in organic crystals and for random packing spheres, respectively. The molar volume of crystal, i.e., V°ϕ(cryst.) is obtained from the crystal densities of the two amino acids using the following relation [36]: V Fig. 3. Plot of apparent molar compressibility (Ks,ϕ) against molality (m) for (a) L-serine, (b) L-valine in aqueous solution of glucose at different temperatures.

°

ϕ ðcryst:Þ

¼ Ms =ρðcryst:Þ

ð15Þ

Here, Ms and ρ(cryst.) are the molar mass and crystal density of the two amino acids, respectively. The hydration number (nH) may now

244


be represented as: nH ¼ V

°

ϕ ðelect:Þ=

°

V

° ϕ;e −V ϕ;b

ð16Þ

where V°ϕ,e is the molar volume of electrostricted water, V°ϕ,b is the molar volume of bulk water and (V°ϕ,e - V°ϕ,b) = − 2.8, − 3.3, − 3.7, − 4.0 and − 4.2 cm3 mol−1 at T = 293.15 K, 298.15 K, 303.15 K, 308.15 K and 313.15 K, respectively. Hence, the hydration number is obtained by substituting the values of (V°ϕ,e − V°ϕ,b) in Eq. (16). Another model helpful in the determination of hydration number may also be derived from electrostriction partial molar compressibility, K°s,ϕ(elect.), using the following relation: nH ¼ −K

°

V

s;ϕ ðelect:Þ=

°

° ϕ;b K s;ϕ;b

ð17Þ

where K°s,ϕ,b is the isothermal compressibility of bulk water. The numeric value in denominator is estimated to be 0.81 × 10−14 Pa−1 m3mol− 1, while numerator term is evaluated using the relation: ¼ K ° s;ϕ −K ° s;ϕ ð int:Þ Here, K°s,ϕ(int.) is K°s,ϕ(isomer) for both L-serine and L-valine (3 × 10−15 Pa−1 m3 mol−1). Since this value is b 5 × 10−15 Pa−1 m3 mol−1, therefore, for ionic crystals and many organic solutes in water K°s,ϕ(int.) is assumed to be zero [35]. One, thus, obtains:

K

°

s;ϕ ðelect:Þ

nH ¼ −K

°

−1 0:81 10−14 Pa−1 m3 mol

s;ϕ =

ð18Þ

The nH values, thus, obtained for L-serine and L-valine using volumetric and compressibility models, given in supplementary Table S4, shows a decreasing trend as the complexity of the aqueous saccharides is increased which follows the order: glucose N sucrose N lactose. The reason for the lower value in lactose than sucrose may be due to the non fitment of its structure in water as compared to sucrose, resulting in strong interaction with the solute and accordingly the hydration number is less [37,38]. It may be pointed out here that the evaluation of hydration number is dependent on the model one uses, and hence is bound to have different values.

esters and unesterified monosaccharides in water [39] shows that none of the acetate esters of monosaccharides were sweet in taste; rather the D-glucose, L-glucose and D-mannose pentaacetate molecules were bitter in taste. L-Serine in water and in aqueous saccharides may have opposed hydrophilic and hydrophobic sides which may be believed to have its association with its sweet behavior [40]. Contrary to this, L-valine in aqueous solutions of saccharides may be bitter due to its hydrophobic character. It has been observed by Sheridan et al. [41] that the intensity of bitterness is related to the region of the molecules in which hydrophobicity resides. In substances with large apolar surfaces, it is thought that hydrophobic hydration is the main form of solute-solvent interactions. This is structurally enhanced water, also called ‘stiffened’ water, formed from the rearrangement of the molecules in an organised fashion with strong hydrogen bonds between them and is less compressible that bulk water. 4. Conclusions The densities and speeds of sound of solutions of L-serine and L-valine in water and in aqueous saccharides (0.1 mol dm−3 glucose, 0.1 mol dm−3 sucrose and 0.1 mol dm−3 lactose) are measured at different temperatures. From the experimental density and speed of sound results, several thermodynamic parameters are calculated. The degree of interactions increases with an increase in the molar mass of amino acids and an increase in the complexity of saccharides. The positive values of ΔtrV°ϕ and ΔtrK°s,ϕ show that solute-solvent interactions are prominent over solute-solute interactions and this overpowering effect decreases with the increase in temperature for all the systems under study. The obtained result of hydration number predicts the solute-cosolute interactions. The apparent specific volume result shows that L-serine and L-valine have opposite taste qualities in aqueous saccharides, the former lying in the sweet taste range and the later in the bitter taste range. Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.molliq.2017.06.004.

3.4. Taste behavior References The mechanism of taste chemoreception has been advanced by microscopic and macroscopic studies of solute-solvent interactions. In order to account for the taste behavior of L-serine and L-valine, a volumetric property namely apparent specific volume (ASV) has been used in aqueous and mixed aqueous solutions. This parameter represents hydrostatic packing of sweet molecules among water molecules which specifically relates to the characteristic volume in solution. ASV is known to be more appropriate parameter in defining taste quality at different concentrations and to distinguish sweeteners on the basis of taste, quality and distinction of salty, sour, sweet and bitter molecules. The four main taste ranges known are [15,16]: salt [ASV = (b 0.33 × 10−3 m3 kg−1)], sour [ASV = (0.33–0.52 × 10−3 m3 kg−1)], sweet [ASV = (0.52–0.71 × 10−3 m3 kg− 1)] and bitter [ASV = (0.71–0.93 × 10−3 m3 kg−1)]. The ASV value is calculated from volumetric property by the following equation: ASV ¼ V ϕ =M

ð19Þ

A perusal of Table S2 shows that ASV values of L-serine in water and aqueous saccharides, for all the concentration range (0.025– 0.2 mol kg−1), lies in the clean sweet taste range. On the other hand, ASV values of L-valine in water and aqueous saccharides over the same concentrations lie in the clean bitter taste range. This taste is ideal over all the studied concentration and temperature as a slight increase in the sweetness/bitterness of solute is noticed upon increasing its concentration as well as temperature [9]. A report of the taste quality of

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