Depth Distribution of Spin-Labeled Liponitroxides within Lipid Bilayers: A Combined EPR and Molecular Dynamics Approach

The distribution in an egg–phosphatidylcholine bilayer of a series of spin-labeled nitroxides, potentially useful as targeted antioxidants, has been investigated using molecular dynamics (MD) simulations. The in silico method has been tested at first for a series of n-doxyl-phosphocholine-doped bilayers, with the doxyl moiety located at different positions (n) of the lipid chain, in analogy to electron paramagnetic resonance (EPR) spin labeling and other MD studies. As a result, a novel calibration curve has been obtained, suitable to determine the absolute membrane penetration depth of any paramagnetic solute from EPR measurements. A second series of MD simulations was then carried out on the newly synthesized series of liponitroxides (NOXs) recently tested as antioxidants against the lipid peroxidation of polyunsaturated fatty acids in membranes: their penetration depths, as determined by EPR in phosphatidylcholine liposomes, were correlated with their antioxidant efficacy. In these NOXs, a glycerol moiety is esterified with a carboxy derivative of a pyrroline nitroxide and one or two oleic acid residues. A very good agreement between the EPR experimental results and those from the current MD simulations indicates that the short distance of the nitroxide moiety from the fatty acid double bonds has been now definitively assessed; moreover, it indicates that our MD methodology could be successfully employed in the absence of nonparamagnetic species.