Thermodynamics for proton dissociations from aqueous l -histidine at temperatures from 278.15 K to 393.15 K and at the pressure 0.35 MPa: apparent molar volumes and apparent molar heat capacities of the protonated cationic, neutral zwitterionic, and deprotonated anionic forms

Abstract

Apparent molar volumes V φ and apparent molar heat capacities C p,φ were determined for individual solutions of aqueous L-histidine, of aqueous L-histidine with equimolal HCl, and of aqueous L-histidine with equimolal NaOH at molalities m = (0.015 to 0.66) mol · kg −1 , at temperatures T = (278.15 to 393.15) K, and at the pressure p = 0.35 MPa. Apparent molar volumes were generated from density measurements obtained with a vibrating-tube densimeter. Apparent molar heat capacities were generated from heat capacity measurements with a twin fixed-cell, power-compensation, differential-output, temperature-scanning calorimeter. These results were then fitted by regression to empirical equations to describe the (V φ ,m,T ) and (C p,φ , m, T ) surfaces for each of the three systems. These regression equations were then used to calculate the changes in partial molar volume r V m and partial molar heat capacity r C p,m as functions of m and T for both the first and second proton dissociation reactions for protonated aqueous L-histidine. The changes in enthalpy r H m and entropy r S m and the acid dissociation molality quotient Q a were then obtained as functions of m and T for each proton dissociation reaction by integration, using our ( r C p,m , m, T ) results and literature values for r H m and Q a . Our results illustrate the unique thermodynamic properties of the cationic, neutral zwitterionic, and anionic forms of L-histidine in aqueous solution.

Thermodynamics of proton dissociations from aqueous l-methionine at temperatures from (278.15 to 393.15)K, molalities from (0.0125 to 1.0)mol·kg−1, and at the pressure 0.35MPa: Apparent molar heat capacities and apparent molar volumes of l-methionine, methioninium chloride, and sodium methioninate

“…We have studied the most simple amino acid glycine [1], the aliphatic amino acids valine [2], L-2-aminobutanoic acid [2], proline [3], alanine [4], and isoleucine [5], as well as the heteroatomic threonine [5], serine [6], and histidine [7,8]. Our current work focuses on aqueous L-methionine.…”

Thermodynamics of the first and second proton dissociations from aqueous l-aspartic acid and l-glutamic acid at temperatures from (278.15 to 393.15)K and at the pressure 0.35MPa: Apparent molar heat capacities and apparent molar volumes of zwitterionic, protonated cationic, and deprotonated anionic forms at molalities from (0.002 to 1.0)mol·kg−1

“…We used the procedure employed in our recent work on the other aqueous amino acids [2][3][4][5][6][7][8][9] to analyze our results with equations (9) to (13). Reference values used for integration constants in this report for L-aspartic acid at T * = 298.15 are pK a = (1.95 3 ± 0.09 1 ) [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] and [31][32][33][34][35][36] for reaction (5), and pK a = (3.7 6 ± 0.1 0 ) [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][37][38][39]…”

“…Solution heat capacities c p,s were determined with a Calorimetry Sciences Corp. (Lindon, UT, USA) model 6100 Nano-DSC twin fixed-cell, differential-output, powercompensation, temperature-scanning calorimeter at 278. 15 6 T/K 6 393.15 as described previously [2][3][4][5][6][7][8][9]60]. We used values of the heat capacity of water c p,w [3] and our c p,s to calculate C p,/ (T, m) with the following equation: C p;/ ¼ ðM 2 Á c p;s Þ þ ðc p;s À c p;w Þ=m: ð2Þ…”

“…where q w [3] is the density of water as described previously [2][3][4][5][6][7][8][9]60]. Solution heat capacities c p,s were determined with a Calorimetry Sciences Corp. (Lindon, UT, USA) model 6100 Nano-DSC twin fixed-cell, differential-output, powercompensation, temperature-scanning calorimeter at 278.…”

“…The monosodium salt of the anionic form of glutamic acid (monosodium glutamate or MSG) is used as a food additive and is responsible for the flavor known by the Japanese word ''umami.' ' The body of work from our laboratory on amino acids includes V / and C p,/ of aqueous histidine [2], proline [3], valine [4], L-2-aminobutanoic acid [4], glycine [5], serine [6], alanine [7], threonine [8], isoleucine [8], and methionine [9]. In this final paper of the series we report our measured densities and heat capacities of aqueous solutions of L-aspartic acid and L-glutamic acid in their zwitterionic forms H 2 Asp ± (aq) and H 2 Glu ± (aq), with the addition of equimolal HCl {H 2 Asp ± (aq) + HCl(aq)} and {H 2 Glu ± (aq) + HCl(aq)}, and in equimolal NaOH {H 2 Asp ± (aq) + NaO-H(aq)} and {H 2 Glu ± (aq) + NaOH(aq)}.…”