THE THERMAL-BEHAVIOR OF PHOSPHATIDYLCHOLINE-GLYCOPHORIN MONOLAYERS IN RELATION TO MONOLAYER AND BILAYER INTERNAL-PRESSURE

A thermodynamic relationship which allows calculation of the internal pressure (P(i)) of a monolayer has been derived, viz: P(i) = T(partial derivative-pi/partial derivative T)A-pi. Surface pressure (pi) - area (A) isotherms were determined for dioleoylphosphatidylcholine (DOPC), dipalmitoylphosphatidylcholine (DPPC) and a mixture of the latter with 0.28 mol% glycophorin, an intrinsic membrane protein. The isotherms (10-degrees-C-42-degrees-C) were used to obtain values of (partial derivative-pi/partial derivative T)A. Under conditions in which a monolayer is believed to most closely model a bilayer membrane (i.e. 37-degrees-C and A = 0.6 nm2/molecule) internal pressures were: DOPC = 0.32 N m-1; DPPC = 0.13 N m-1 and glycophorin/DPPC = 0.36 N m-1. The results do not support some measurements on amphiphile solubility in natural membranes which had been interpreted as evidence of a large increase in internal pressure, due to the intrinsic membrane proteins.