Structure and water permeability of fully hydrated diphytanoylPC

Diphytanoylphosphatidylcholine (DPhyPC) is a branched chain lipid often used for model membrane studies, including peptide/lipid interactions, ion channels and lipid rafts. This work reports results of volume measurements, water permeability measurements Pf, X-ray scattering from oriented samples, and X-ray and neutron scattering from unilamellar vesicles at T=30.C. We measured the volume/lipid VL = 1426±1Å3. The area/lipid was found to be 80.5±1.5Å2 when both X-ray and neutron data were combined with the SDP model analysis (Ku.cerka, N., Nagle, J.F., Sachs, J.N., Feller, S.E., Pencer, J., Jackson, A., Katsaras, J., 2008. Lipid bilayer structure determined by the simultaneous analysis of neutron and X-ray scattering data. Biophys. J. 95, 2356-2367); this is substantially larger than the area of DOPC which has the largest area of the common linear chain lipids. Pf was measured to be (7.0±1.0)×10-3 cm/s; this is considerably smaller than predicted by the recently proposed 3-slab model (Nagle, J.F., Mathai, J.C., Zeidel, M.L., Tristram-Nagle, S., 2008. Theory of passive permeability through lipid bilayers. J. Gen. Physiol. 131, 77-85). This disagreement can be understood if there is a diminished diffusion coefficient in the hydrocarbon core of DPhyPC and that is supported by previous molecular dynamics simulations (Shinoda, W., Mikami, M., Baba, T., Hato, M., 2004. Molecular dynamics study on the effects of chain branching on the physical properties of lipid bilayers. 2. Permeability. J. Phys. Chem. B 108, 9346-9356). While the DPhyPC head-head thickness (DHH = 36.4 Å), and Hamaker parameter (H= 4.5×10-21 J) were similar to the linear chain lipid DOPC, the bending modulus (KC = 5.2±0.5×10-21 J) was 30% smaller. Our results suggest that, from the biophysical perspective, DPhyPC belongs to a different family of lipids than phosphatidylcholines that have linear chain hydrocarbon chains.