COUPLING BETWEEN TRANSPORT PHENOMENA AND CONFORMATIONAL TRANSITIONS - ISOTHERMAL WATER TRANSPORT AND ASYMMETRIC OSMOTIC-PRESSURE ACROSS A MEMBRANE PHASE PROMOTED BY A HELIX-COIL TRANSITION

A membrane system is described consisting of an aqueous concentrated solution of poly-L-lysine bounded by two rigid conventional membranes permeable to water and to small ions but impervious to the polymer. When a large and steady pH difference is maintained between two external isoosmotic solutions α and β, a vertical volume flow may be observed from the acid solution to the basic solution across the membrane. The flow may give rise to a pressure difference between the basic and the acid solution corresponding to a non-equilibrium steady state of the system. These effects, first reported by Liquori et al. [1] are extensively analyzed from a thermodynamic point of view. It is shown that the pH gradient across the membrane maintains a conformational gradient of poly-L-lysine which is known to undergo a pH regulated helix-coil transition. The latter gradient in turn determines a gradient of chemical potential of water within the membrane phase acting as the main driving force of the volume flow. Using the flux equations of irreversible thermodynamics in connection with a model of helix-coil transition proposed by the author [3,4] compact equations are derived for the isobaric volume flow across the membrane phase and the steady state osmotic pressure in agreement with the experimental results. Possible implications of this study in connection with active transport phenomena across biological membranes are discussed. © 1978.