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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 (ΔG o/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.

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In the context of the rapid increase of antibiotic-resistant infections, in particular of pneumonia, antimicrobial photodynamic therapy (aPDT), the microbiological application of photodynamic therapy (PDT), comes in as a promising treatment alternative since the induced damage and resultant death are not dependent on a specific biomolecule or cellular pathway. The applicability of aPDT using the photosensitizer indocyanine green with infrared light has been successfully demonstrated for different bacterial agents in vitro, and the combination of pulmonary delivery using nebulization and external light activation has been shown to be feasible. However, there has been little progress in obtaining sufficient in vivo efficacy results. This study reports the lung surfactant as a significant suppressor of aPDT in the lungs. In vitro, the clinical surfactant Survanta® reduced the aPDT effect of indocyanine green, Photodithazine®, bacteriochlorin-trizma, and protoporphyrin IX against Streptococcus pneumoniae . The absorbance and fluorescence spectra, as well as the photobleaching profile, suggested that the decrease in efficacy is not a result of singlet oxygen quenching, while a molecular dynamics simulation showed an affinity for the polar head groups of the surfactant phospholipids that likely impacts uptake of the photosensitizers by the bacteria. Methylene blue is the exception, likely because its high water solubility confers a higher mobility when interacting with the surfactant layer. We propose that the interaction between lung surfactant and photosensitizer must be taken into account when developing pulmonary aPDT protocols.

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