Thermodynamic parameters for the binding of inorganic and organic anions by biogenic polyammonium cations

Thermodynamic parameters for the interaction of protonated biogenic polyamines with inorganic or organic polyanions were studied potentiometrically (H+-glass electrode) and calorimetrically, at 25°C. No background salt was used in the measurements to avoid interferences, and the formation constants and formation enthalpies were extrapolated to zero ionic strength. Species formed are ALHr [L=Cl−, SO42−, HPO42−, P2O74− and P3O105−; tartrate, malate, citrate, glutamate, 1,2,3-propanetricarboxylate, 1,2,3,4-butanetetracarboxylate], with r=1, 2 … (n+m−2) and r=1, 2 … (n+m−1) for inorganic and organic ligands, respectively (n, m=maximum degree of protonation of amine and ligand, respectively). The stability of the various species formed is a function of charges involved in the formation reaction. ΔH0 values are generally positive, and therefore these complexes are entropically stabilized. Results are discussed in connection with several previously reported data on similar systems. ΔG0 and TΔS0 follow a linear trend as a function of polyammonium cation and inorganic or carboxylic anion charges. ΔG0 and TΔS0 charge relationships are reported. In particular, mean values of ΔG0 and TΔS0 for single interaction were calculated: ΔG0=7.0 kJ mol−1 ṉ−1, TΔS0=9.1 kJ mol−1 ṉ−1 and ΔG0=5.7 kJ mol−1 ṉ−1 and TΔS0=8.7 kJ mol−1 n−1, for the species of inorganic and organic polyanions, respectively (ṉ=number of possible salt bridges). A linear relationship was also found for TΔS0 versus ΔG0, whose equation is TΔS0=−7–1.39 ΔG0 (with r=0.9409; r, correlation coefficient). The body of correlations found for these thermodynamic parameters shows quite good predictive value.