A thermodynamic analysis of monovalent cation permeation through a K(+)-selective ion channel

Ionic selectivity characteristics of the K+ channel from the sarcoplasmic reticulum were studied as a function of temperature in order to decompose the profile of Gibbs free energy along the conduction pore into its enthalpic and entropic components. For Li+, Na+, K+, and Rb+, the enthalpy of binding to the channel is close to zero. Activation enthalpies for transferring ions from bulk aqueous solution to the channel's selectivity region are 20-25 kJ/mol for Na+, K+, and Rb+ and substantially higher for Li+ and Cs+. Transfer of Li+ and Na+ to the selectivity region involved large favorable entropies. The results argue that the group IA cations shed about half their waters of hydration in permeating the selectivity region of this channel.