Temperature Dependence of the Dissociation Constants of Several Amino Acids

The apparent dissociation constants of the amino acids lysine, histidine, arginine, glutamic acid, tyrosine, phenylalanine, tryptophan, and threonine were determined at 283.1 K, 298.1 K, 313.1 K, and 333.1 K at various ionic strengths by potentiometric titration. The Davies equation was used to extrapolate the dissociation constants to zero ionic strength. The pK values of the carboxylic acid attached to the α-carbon and the pK of the carboxylic acid on the side chain of glutamic acid were almost independent of temperature. Conversely, the pK values of the amino groups attached to the α-carbon and those on the side chain of the basic amino acids varied substantially with temperature. van't Hoff type plots of the pK values showed linear relationships indicating that the standard enthalpy changes of reaction are constant over this range of temperatures.