147 Biochimica et Biophysica Acta, 555 (1979) 147--167 © Elsevier/North-Holland Biomedical Press BBA 78427 PHYSICOCHEMICAL CHARACTERIZATION OF 1,2-DIPHYTANOYL- sn-GLYCERO-3-PHOSPHOCHOLINE IN MODEL MEMBRANE SYSTEMS H. LINDSEY *, N.O. PETERSEN ** and SUNNEY I. CHAN *** A.A. Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125 (U.S.A.) (Received December 18th, 1978) Key words: Diphytanoyl phosphatidylcholine; Phospholipid; Thermal phase transition; Lateral phase separation; (Bilayer membrane, NMR) Summary We report here on a series of studies aimed at characterization of the struc- tural and dynamical properties of the synthetic lipid diphytanoyl phosphatidyl- choline, in multilamellar dispersions and vesicle suspensions. This lipid exhibits no detectable gel to liquid crystalline phase transition over a large temperature range (--120°C to +120°C). Examination of proton nuclear magnetic resonance (NMR) free induction decays obtained from multilayer dispersions of diphytanoyl phosphatidyl- choline provided an estimate of the methylene proton order parameter. The estimated magnitude of 0.21 is comparable to those determined for other phos- pholipids. Sonication of aqueous dispersions of diphytanoyl phosphatidylcholine led to formation of bilayer vesicles as determined by the measurement of the outer/ inner choline methyl proton resonances, vesicle sizes in electron micrographs, and comparison of proton NMR linewidths between multilayer and sonicated dispersions. Ultracentrifugation studies of diphytanoyl phosphatidylcholine vesicles in H20 and 2H20 media yielded a value of 1.013 -+ 0.026 ml/g for the partial specific volume of this lipid. We have measured spin lattice relaxation rates for the methyl and methylene- methyne protons of the hydrocarbon chains of diphytanoyl phosphatidyl- choline in bilayer vesicles over a range of temperatures and at two NMR fre- quencies (100 and 220 MHz). The observed relaxation rates for the methylene protons in this system were approximately twice those previously reported for * Present address: Department of Chemistry, Michigan State University, East Lansing, MI 48824, U.S.A. ** Present address: Department of Chemistry, Cornell University, Ithaca, NY 14850, U,S.A. ** * To whom requests for reprints should be sent.