Application of the transition state theory to water transport across cell membranes

Abstract

We have applied the transition state theory of Eyring et al. (The Theory of Rate Processes, McGraw-Hill, 1941) to water transport across cell membranes. We have then evaluated free energy (ΔF≠), enthalpy (ΔH≠) and entropy (ΔS≠) of activation for water permeation across membranes, such as Arbacia eggs, Xenopus oocytes with or without aquaporin water channels, mammalian erythrocytes, aquaporin proteoliposomes, liposomes and collodion membrane. ΔH≠ was found to be correlated with ΔS≠. This is so-called ΔH≠ and ΔS≠ compensation over the ranges of ΔH≠ and ΔS≠ from 2 to 22 kcal/mol and from −26 to 45 e.u., respectively, indicating that low ΔH≠ values correspond to negative ΔS≠. Large positive ΔS≠ and high ΔH≠ values might be accompanied by reversible breakage of secondary bonds in the membrane, presumably in membrane lipid bilayer. Largely negative ΔS≠ and low ΔH≠ values for aquaporin water channels, aquaporin proteoliposomes and porous collodion membrane could be explained by the immobilization of permeating water molecules in the membrane, i.e., the partial loss of rotational and/or translational freedoms of water molecules in water channels.