Thermodynamic measurements of actin polymerization with various cation species

We measured the critical concentration of actin polymerized with different polymerization ions and bound divalent cations at low temperatures and estimated thermodynamic parameters. The entropy and enthalpy changes of actin polymerization were 36-55 (cal/mol K) and 2-8 (kcal/mol), respectively, with some exceptions. Both the entropy and enthalpy changes of the polymerization of Ca²⁺ -actin were sensitive to the polymerization ion (K⁺ or Na⁺ ): ΔS = 39 or 36 (cal/mol K), ΔΗ= 3.9 or 2.7 (kcal/mol). The entropy and enthalpy changes (cal/mol K, kcal/mol) of Mg²⁺ -actin were also sensitive to the polymerization ion in the following order: Mg²⁺ (55, 7.6) > K⁺ (46, 5.3) > Na⁺ (38, 2.4). Those values largely decreased and became even negative in the presence of a high concentration (0.1 M) of K⁺ , which was likely caused by the charge screening effect of that ion.