Cooperative effects in models of steady-state transport across membranes. II. Oscillating phase transition.

A simple model for steady-state transport of a neutral molecule across a membrane is investigated in a preliminary way. In this model, there is a possible conformation change in each membrane unit which alters the access of the binding site for the transported molecule from one bath to the other. Thus, transport cannot be accomplished without a conformation change. Furthermore, we assume a cooperative interaction between nearest-neighbor membrane units in the same conformation. Then, with suitable rate constants and bath concentrations, and if the interaction energy is large enough, the membrane will oscillate back and forth between the two conformational phases, producing a surge of flux in each cycle. The period of the cycle depends on the times necessary to nucleate the two phase transitions.