Thermodynamic parameters of protolytic equilibria of selected dipeptides in aqueous solutions

Abstract

Heat effects of stepwise dissociation of D,L-valyl-D,L-leucine, L-leucyl-L-leucine, D,Lleucylglycine, and glycyl-D,L-leucine (298.15 K; 0.1, 0.5, or 1.0 mol/L KNO 3 ) have been determined by direct calorimetry. Standard thermodynamic parameters of the studied equilibria have been calculated.The influence of the aqueous solution ionic strength on the heat effect of acid-base equilibrium involving dipeptides built of valine, alanine, glycine, serine, and asparagine have been analyzed earlier in [1-10], and standard thermodynamic parameters of the corresponding reactions (pK 0 , Δ r G 0 , Δ r H 0 , and Δ r S 0 ) have been determined.In this work we studied the dipeptides including leucine fragment: D,L-valyl-D,L-leucine, L-leucyl-Lleucine, D,L-leucylglycine, and glycyl-D,L-leucine. The constants of stepwise dissociation of their carboxylic and betaine groups were considerably different, thus allowing independent calorimetric determination of heat effects of reactions (1) and (2).The enthalpy of dissociation of H 2 L + and HL could be found as the difference between the mixing and the dilution enthalpies [Eq. (3)]:where Δ mix H being the enthalpy of mixing of HNO 3 solution with the dipeptide solution in the presence of the background electrolyte; Δ dil H being the enthalpy of dilution of the HNO 3 solution in the background electrolyte solution; α being the degree of protonation of HL and L -. Heat effects of dissociation of the studied dipeptides at 298.15 K in the presence of 0.1, 0.5, and 1.0 mol/L of KNO 3 are collected in Table 1.The values of Δ dis Н found for the set of ionic strengths allowed calculation of the standard thermodynamic parameters of the studied equilibria. In order to extrapolate the heat effects to zero ionic strength we used a one-parametric Eq. (4) [11]:where Δ dis Н and Δ dis Н 0 being the reaction enthalpies at the finite and zero ionic strength, respectively; b being an empirical coefficient; Δz 2 being a difference of charges of the reaction products and the starting reactants; and Ψ(I) being a theoretically derived function of ionic strength [11]. The corresponding thermodynamic parameters of the dipeptides dissociation constants were then calculated using the Vasilev equation (5) [11]: pK 0 = pK + AΔz 2 [I 1/2 /(1+1.6I 1/2 ) -0.05I] + 0.05I, (5) where K 0 and K being thermodynamic and concentration constants of dissociation, respectively, and A being a Debye-Huckel theory constant.Concentration constants of dissociation of D,Lvalyl-D,L-leucine, L-leucyl-L-leucine, D,L-leucylglycine, and glycyl-D,L-leucine have been earlier determined in [12][13][14][15]. Standard thermodynamic parameters of dissociation of the studied dipeptides are collected in Table 2 along with our results of similar calorimetry studies of a series of other dipeptides.The results of this work are in good agreement with the earlier suggestions on the dependence of thermo-