Role of carboxylate side chains in the cation hofmeister series

Thermodynamic and surface-specific spectroscopic investigations were carried with an elastin-like polypeptide (ELP) containing 16 aspartic acid residues. The goal was to explore the role of the carboxylate moieties in hydrophobic collapse and related Hofmeister effects. Experiments were conducted with a series of monovalent and divalent metal chloride salts. Both phase transition temperature and spectroscopic data demonstrated that the divalent cations showed relatively strong association to the carboxylate sites on the biopolymer with K_d values in the range of 1 to 10 mM. The ordering of the divalent series was: Zn²⁺ > Ca²⁺ > Ba ²⁺ > Sr²⁺ > Mg²⁺. Monovalent cations displayed weaker binding which ranged from 78 mM for NH₄⁺ to 345 mM for Cs⁺. The order for this series was: NH₄ ⁺ > Li⁺ > Na⁺ > NMe₄ ⁺ > K⁺ > Rb⁺ ≥ Cs⁺. These results are in general agreement with the notion that strongly hydrated cations bind more tightly to carboxylate groups than do weakly hydrated cations. Moreover, the data for the monovalent series was partially consistent with the law of matching water affinity, although Li⁺ and NH₄ ⁺ did not follow the model. The series for the divalent cations did not appear to obey the law of matching water affinity at all.