Thermodynamic effects of the hydrophobic surfactant proteins on the early adsorption of pulmonary surfactant

We determined the influence of the two hydrophobic proteins, SP-B and SP-C, on the thermodynamic barriers that limit adsorption of pulmonary surfactan t to the air-water interface. We compared the temperature and concentration dependence of adsorption, measured by monitoring surface tension, between calf lung surfactant extract (CLSE) and the complete set of neutral and pho spholipids (N&PL) without the proteins. Three stages generally characterize d the various adsorption isotherms: an initial delay during which surface t ension remained constant, a fall in surface tension at decreasing rates, an d, for experiments that reached similar to 40 mN/m, a late acceleration of the fall in surface tension to similar to 25 mN/m. For the initial change i n surface tension, the surfactant proteins accelerated adsorption for CLSE relative to N&PL by more than ten-fold, reducing the Gibbs free energy of t ransition (DeltaG(0)(double dagger):) from 119 to 112 kJ/mole. For the lipi ds alone in N&PL, the enthalpy of transition (DeltaH(0)(double dagger), 54 kJ/mole) and entropy (- T.DeltaS(0)(double dagger), 65 kJ/mole at 37 degree sC) made roughly equal contributions to DeltaG(0)(double dagger). The prote ins in CLSE had little effect on -T.DeltaS(0)(double dagger) (68 kJ/mole), but lowered AW for CLSE by reducing DeltaH(0)(double dagger) (44 kJ/mole). Models of the detailed mechanisms by which the proteins facilitate adsorpti on must meet these thermodynamic constraints.