Enthalpies of acid dissociation of L-carnosine in aqueous solution

The enthalpies of L-carnosine interaction with solutions of hydrochloric acid have been measured by calorimetric method in the presence of NaCl at 298.15 K and ionic strength as high as 0.25, 0.50 and 0.75 mol dm⁻³. The enthalpies of acid dissociation have been determined from the obtained data. The effect of a background electrolyte concentration on the dissociation enthalpy of peptide has been considered. Standard thermodynamic quantities (Δ_disH°, Δ_disG°, Δ_disS°) of the acid dissociation of the dipeptide in aqueous solutions have been determined on the basis of the obtained thermochemical results and available data on the acid dissociation constants corrected for the zero-order ionic strength. Ability to acid dissociation of amino acids in a free state is compared with those for amino acid residues involved in the dipeptide linkage. The alteration of acidity has been connected with variation in hydration of reactivity centers.