Thermodynamic Effects of the Hydrophobic Surfactant Proteins on the Early Adsorption of Pulmonary Surfactant

Abstract

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 (ΔG 0 ‡) from 119 to 112 kJ/mole. For the lipids alone in N&PL, the enthalpy of transition (ΔH 0 ‡, 54 kJ/mole) and entropy (− T · ΔS 0 ‡, 65 kJ/mole at 37°C) made roughly equal contributions to ΔG 0 ‡. The proteins in CLSE had little effect on − T · ΔS 0 ‡ (68 kJ/mole), but lowered ΔG 0 ‡ for CLSE by reducing ΔH 0 ‡ (44 kJ/mole). Models of the detailed mechanisms by which the proteins facilitate adsorption must meet these thermodynamic constraints.