Catalase monolayers at the air/water interface I. Reversibilities and irreversibilities

/A-isotherms of catalase monolayers established at the air/water-interface are discussed quantitatively on the basis of molecular data: A relationship between a critical value of the surface pressure, the corresponding molecular area, and the molecular dimension of the molecules at the interface is proposed. It is shown that the unfolding of molecules at the water surface is pH-dependent. For each pH-value there is a distinct degree of unfolding; the molecules keep their globular state at neutral pH. Establishment at the surface of bulk solutions corresponding to globular and partly unfolded states, respectively, catalase molecules keep their original configuration on changing the pH-value of bulk-phase. The monolayers are confirmed to show reversibility with regard to lateral changes of state as well as irreversibility with respect to desorption of molecules.A model is proposed to explain the nature of the critical/A-value occurring in the/A-isotherms: on compression beyond_c, molecular segments are transferred from the surface into the bulkphase via a subsurface layer. From the experiments it is concluded that the surface pressure is determined, not only by the surface itself, but also by this subsurface layer.