title: Adamantyl nitroxide: a spin label for probing membrane surfaces.
abstract: In an effort to understand more about the perturbing properties of adamantane-like molecules on biological membranes, the spin probe adamantyl nitroxide (2,2'-dimethyl-5-adamantyl oxazolidine-N-oxyl) was synthesized, purified and characterized. Electron paramagnetic resonance (EPR) spectra were then obtained from 1:50 and 1:200 mixtures of adamantyl nitroxide with dipalmitoyl and dipalmityl phosphatidylcholine multibilayers. Above the phase transition temperature of these lipids (41 degrees C for dipalmitoyl phosphatidylcholine and 43 degrees C for dipalmityl phosphatidylcholine) the spectra of adamantyl nitroxide are similar to control spectra obtained in liquid oleic acid. Below the phase transition temperatures, however, spectral differences were observed depending on: (1) the concentration of the spin probe in the lipid; (2) the linkage between the polar head group and the hydrocarbon tails of the phospholipid; (3) the temperature of the sample. Partitioning of adamantyl nitroxide between the aqueous and hydrocarbon phases of the sample is most prominent at probe-to-lipid ratios of 1:200 and at temperatures below the pre-transition temperature of the lipid (around 33 degrees C). Computer simulations of the above results, as well as additional experiments performed at 35 GHz, show that the results arise from true partitioning and not from asymmetric probe motion. Two conclusive results of these experiments are that spectra of adamantyl nitroxide in phospholipid multibilayers are sensitive to probe concentration and to the physical characteristics of the phospholipid which they probe. The spectral differences which arise when adamantyl nitroxide is used with ether- and ester-linked phospholipids indicate that it is a sensitive probe of membrane surfaces. Employment of this molecule in membrane research should prove to be useful in obtaining additional information about membrane surface events.
mesh_terms: D000218:Adamantane / Q000031:analogs & derivatives* / Q000138:chemical synthesis; D003497:Cyclic N-Oxides / Q000138:chemical synthesis*; D004578:Electron Spin Resonance Spectroscopy; D008081:Liposomes; D009682:Magnetic Resonance Spectroscopy; D011663:Pulmonary Surfactants; D013055:Spectrophotometry, Infrared; D013113:Spin Labels / Q000138:chemical synthesis*
chemical_list: D003497:Cyclic N-Oxides; D008081:Liposomes; D011663:Pulmonary Surfactants; D013113:Spin Labels; C036803:adamantyl nitroxide; D000218:Adamantane