Oxygen profiles in membranes.

Transmembrane profiles of molecular oxygen in lipid bilayers are not only significant for membrane physiology and pathology, but also are essential to the determination of membrane protein structure by site-directed spin labeling. Oxygen profiles obtained with spin-labeled lipid chains have a Boltzmann sigmoidal dependence on the depth into each lipid leaflet, which represents a two-compartment distribution between outer and inner regions of the membrane, with a transfer free energy that depends linearly on distance from the dividing planes. Transmembrane profiles for intramembrane polarity, and for water penetration into the membrane, have an identical form, but are of the reverse sign. Comparison with recently published oxygen profiles from a site-specifically spin-labeled alpha-helical transmembrane peptide validates the use of spin-labeled lipids for all these profiles and provides the necessary bridge to generate the full bilayer from a single lipid leaflet.