Thermodynamics of proton dissociations from aqueous threonine and isoleucine at temperatures from (278.15 to 393.15) K, molalities from (0.01 to 1.0) mol · kg−1, and at the pressure 0.35 MPa: Apparent molar heat capacities and apparent molar volumes of zwitterionic, protonated cationic, and deprotonated anionic forms

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Abstract

We have measured the densities of aqueous solutions of isoleucine, threonine, and equimolal solutions of these two amino acids with HCl and with NaOH at temperatures 278.15  T/K  368.15, at molalities 0.01  m/mol · kg−1  1.0, and at the pressure 0.35 MPa using a vibrating tube densimeter. We have also measured the heat capacities of these solutions at 278.15  T/K  393.15 and at the same m and p using a twin fixed-cell differential temperature-scanning calorimeter. We used the densities to calculate apparent molar volumes Vϕ and the heat capacities to calculate apparent molar heat capacities Cp,ϕ for these solutions. We used our results and values from the literature for Vϕ(T, m) and Cp,ϕ(T, m) for HCl(aq), NaOH(aq), and NaCl(aq) and the molar heat capacity change ΔrCp,m(T, m) for ionization of water to calculate parameters for ΔrCp,m(T, m) for the two proton dissociations from each of the protonated aqueous cationic amino acids. We used Young’s Rule and integrated these results iteratively to account for the effects of equilibrium speciation and chemical relaxation on Vϕ(T, m) and Cp,ϕ(T, m). This procedure gave parameters for Vϕ(T, m) and Cp,ϕ(T, m) for threoninium and isoleucinium chloride and for sodium threoninate and isoleucinate which modeled our observed results within experimental uncertainties. We report values for ΔrCp,m, ΔrHm, pQa, ΔrSm, and ΔrVm for the first and second proton dissociations from protonated aqueous threonine and isoleucine as functions of T and m.

Introduction

Although there is an abundance of work reported in the literature on the properties of aqueous amino acids, the great majority of this work focuses on the unionized zwitterionic forms of the amino acids. The works that include information for the charged species often neglect or approximate the effects of incomplete ionization and equilibrium concentrations, introducing significant uncertainties in the values of their thermodynamic properties. In our continuing efforts to enlarge the database of accurate thermodynamic properties of aqueous solutions of l-2-amino acids in their protonated, zwitterionic, and deprotonated forms, taking into account the actual species concentrations, we have studied aqueous threonine and isoleucine. Both are essential amino acids, meaning that they are not produced by humans and must be included in the diet. Threonine is required for the formation of collagen and elastin and is concentrated in the central nervous system. Isoleucine is found particularly in muscle tissue and is necessary for formation of hemoglobin. It contributes significantly to the tertiary structure of proteins.

We recently reported our results for the apparent molar volumes Vϕ and apparent molar heat capacities Cp,ϕ of aqueous proline [1], valine [2], l-2-aminobutanoic acid [2], serine [3], alanine [4], and others. In this paper we report our measured densities and heat capacities of aqueous solutions of threonine and isoleucine in their zwitterionic forms HThr±(aq) and HIle±(aq) and with the addition of equimolal HCl {HThr±(aq) + HCl(aq)} and {HIle±(aq) + HCl(aq)}, and equimolal NaOH {HThr±(aq) + NaOH(aq)} and {HIle±(aq) + NaOH(aq)}. Our analysis applies Young’s Rule and a relaxation heat capacity term to account for the equilibrium molalities of the species H2Thr+Cl(aq), H2Ile+Cl(aq), HThr±(aq), HIle±(aq), Na+Thr(aq), and Na+Ile(aq) present in the solutions containing HCl(aq) and NaOH(aq). Our resulting values of Vϕ(T, m) and Cp,ϕ(T, m) for H2Thr+Cl(aq), H2Ile+Cl(aq), Na+Thr(aq), and Na+Ile(aq) allow calculation of the thermodynamic quantities ΔrCp,m, ΔrHm, pQa, ΔrSm, and ΔrVm for the first and second proton dissociations from protonated aqueous threonine and isoleucine. We have compared all of our results to those found in the literature [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45].

Section snippets

Experimental

Crystalline l-threonine {HThr±(c), 2-amino-3-hydroxybutanoic acid, CH3CH(OH)CH(NH2)COOH, molar mass M2 = 119.1197 g · mol−1, density ρc = 1.5 g · cm3; Fluka 89179, lot 1132757, 0.995+ mass fraction} and l-isoleucine {HIle±(c), 2-amino-3-methylpentanoic acid, C2H5CH(CH3)CH(NH2)COOH, M2 = 131.1736 g · mol−1, ρc = 1.2 g · cm3; Fluka 58879, lot 413187/1, 0.995+ mass fraction} were used as received. The purity of each solute was confirmed by elemental analysis by MHW Laboratories. We prepared aqueous stock solutions of

Results and discussion

Values of Vϕ,obs(T, m) for HThr±(aq), HIle±(aq), {HThr±(aq) + HCl(aq)}, {HIle±(aq) + HCl(aq)}, {HThr±(aq) + NaOH(aq)}, and {HIle±(aq) + NaOH(aq)} at 0.01  m/mol · kg−1⩽ 1.0 and 278.15  T/K  368.15 are given in TABLE 1A, TABLE 1B, TABLE 2A, TABLE 2B, TABLE 3A, TABLE 3B equation (3) was fit by regression to these results:Vϕ(T,m)=w3/2·AV·(m)1/2+ν0+ν1·m+ν2·(m)2+ν3·m·T+ν4·(m)2·T+ν5·ln(T)+ν6·T+ν7·(T)2.In equation (3), νi are the parameters of the regression, T = (T/T) with T = 1 K, m = (m/m) with m = 1 mol · kg

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