Despite the difficulties determining the Gibbs free energies of arginine binding to the negatively charged amino acids, the interaction enthalpies between all pairs of positively and negatively charged polymers could be determined unambiguously with the accuracy listed in Table 1. The enthalpy of lysine interaction with aspartate was by 1.0 kJ/mol more endothermic than ornithine. Similar comparison for lysine versus ornithine interaction with glutamate yielded 2.2 kJ/mol. Furthermore, the binding constant of pLys interaction with both pAsp and pGlu was approximately 2 times stronger than that of pOrn interaction, giving the differences in the Gibbs free energies ΔΔG = −28.6−(−26.1) = −2.5 kJ/mol and −2.1 kJ/mol, respectively. This difference in the strength of association may be caused by the extra methylene group in lysine as compared with ornithine. To determine the constant pressure heat capacity, ΔCP = (∂ ΔH/∂ T)P, the series of ITC experiments were performed at different constant temperatures in the range of 13−61 °C. The values of ΔCP for all pairs given in the Table 1 were determined from the slopes of ΔH dependencies on temperatur