Structure and Thermotropic Behavior of Bovine- and Porcine-Derived Exogenous Lung Surfactants.

Two commercial exogenous pulmonary surfactants, Curosurf and Survanta, are investigated. Their thermotropic behavior and associated structural changes for the samples in bulk are characterized and described. For Survanta, the obtained results of differential scanning calorimetry showed a thermogram with three peaks on heating and only a single peak on cooling. Curosurf on the other hand, presents calorimetric thermograms with only one peak in both the heating and cooling scans. This distinct thermotropic behavior between the two pulmonary surfactants, a consequence of their particular compositions, is associated with structural changes that were evaluated by simultaneous small- and wide-angle X-ray scattering experiments with in situ temperature variation. Interestingly, for temperatures below ∼35 °C for Curosurf and ∼53 °C for Survanta, the scattering data indicated the coexistence of two lamellar phases with different carbon chain organizations. For temperatures above these limits, the coexistence of phases disappears, giving rise to a fluid phase in both pulmonary surfactants, with multilamelar vesicles for Curosurf and unilamellar vesicles for Survanta. This process is quasi-reversible under cooling, and advanced data analysis for the scattering data indicated differences in the structural and elastic properties of the pulmonary surfactants. The detailed and systematic investigation shown in this work expands on the knowledge of the structure and thermodynamic behavior of Curosurf and Survanta, being relevant from both physiological and biophysical perspectives and also providing a basis for further studies on other types of pulmonary surfactants.

Lipophilicity of Coarse-Grained Cholesterol Models.

The lipophilicity of cholesterol was investigated by using coarse-grained molecular dynamics and umbrella sampling. The previous coarse-grained cholesterol models in the literature are more hydrophobic than our model. The Gibbs free energy of transferring cholesterol from the octanol phase to water phase (ΔGo/w) was 11.88 ± 0.08 kcal mol-1, and the octanol-water partitioning coefficient (logP) was estimated to be 8.72 ± 0.06. The latter is in agreement with the logP values found by bioinformatics, which are standard methods to predict the lipophilicity, giving excellent octanol/water partitioning coefficients compared with experimental ones for different molecules. We also performed the first experimentally direct measurement of this important property for cholesterol. The experimental octanol/water partitioning coefficient of cholesterol was measured to be 8.86 ± 0.79, which is in excellent agreement with our calculated logP value from our parametrized coarse-grained cholesterol model. This shows the significance of systematic optimization of the lipophilicity for developing coarse-grain models of important biomolecules with complicated molecular structures and hydrophobic character like cholesterol.