We determined the influence of the two hydrophobic proteins, SP-B and SP-C, on the thermodynamic barriers that limit adsorption of pulmonary surfactant 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 phospholipids (N&PL) without the proteins. Three stages generally characterized the various adsorption isotherms: an initial delay during which surface tension remained constant, a fall in surface tension at decreasing rates, and, for experiments that reached ∼40 mN/m, a late acceleration of the fall in surface tension to ∼25 mN/m. For the initial change in surface tension, the surfactant proteins accelerated adsorption for CLSE relative to N&PL by more than ten-fold, reducing the Gibbs free energy of transition (ΔG0‡) from 119 to 112 kJ/mole. For the lipids alone in N&PL, the enthalpy of transition (ΔH0‡, 54 kJ/mole) and entropy (-T · ΔS0‡, 65 kJ/mole at 37°C) made roughly equal contributions to ΔG0‡. The proteins in CLSE had little effect on -T · ΔS0‡, (68 kJ/mole), but lowered ΔG0‡ for CLSE by reducing ΔH0‡ (44 kJ/mole). Models of the detailed mechanisms by which the proteins facilitate adsorption must meet these thermodynamic constraints.
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