B(reath)e water my friend: hydration of pulmonary surfactant layers as analyzed by fourier transform-infrared spectroscopy.

Water is essential for sustaining life, as well as the presence of a fully operational pulmonary surfactant (PS) system coating the respiratory surface. In this work, the interconnection between different hydration states and PS membrane structure is explored thanks to the use of ATR-FTIR spectroscopy. Thermotropic phase transitions and hydration level in complex membrane structures formed by two native surfactant systems, derived from animal and human sources, are compared with those found in synthetic mixtures used routinely to mimic PS composition and with re-assembled samples derived from porcine lavages. Native samples exhibited always a faster and higher hydration level than the synthetic ones under comparable conditions, notwithstanding their origin. This hydration degree was found to be crucial in modulating gel to fluid phase transitions. This new insight should be integrated to better comprehend structure-function relationships in PS natural and model membranes, and provides a novel framework to understand permeability and dynamics of alveolar surfactant coatings under physiological conditions.