Mechano-chemistry of closed, vesicular membrane systems

Abstract

The definition of a surface pressure in the encapsulating membrane surface of a large vesicle or biological cell cannot be stated in the same manner as for an insoluble monolayer, nor can a surface pressure be measured directly, as in a monolayer system. Our approach is to determine a suitable definition for surface pressure in closed membrane systems of large vesicles and biological cells; this surface pressure will represent an internal equation of state for the membrane that can be compared with the observed monolayer surface pressure versus area behavior. Using mechanical experiments to produce isotropic membrane tension and area expansion, changes in surface pressure can be determined as a function of area change. The relationship between isotropic tension and surface area is called the “mechano-chemical” equation of state. We directly relate the internal equation of state, surface pressure versus area, to the mechano-chemical equation of state, isotropic tension versus area. Using this relationship, the thermodynamic changes produced in a closed, vesicular (or cellular) membrane by isotropic dilation or reduction in surface area are investigated and described in terms of experimentally determinable variables. These variables include the isothermal compressibility modulus, K T , the thermal area expansivity at constant or zero isotropic tension, ( ∂α ∂T ) T-0 , the temperature gradient in the area compressibility modulus, ( dK T dT ) , and the specific heat at constant or zero isotropic tension, C T . In conjunction with known equations of state, the thermoelastic change provides direct assessment of the temperature dependence of the interfacial free-energy density of hydrophobic interaction. In the absence of external forces, the surface pressure is determined by the interfacial free-energy density of hydrophobic interaction.