title: 1H NMR Shows Slow Phospholipid Flip-Flop in Gel and Fluid Bilayers.
abstract: We measured the transbilayer diffusion of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in large unilamellar vesicles, in both the gel (Lβ') and fluid (Lα) phases. The choline resonance of headgroup-protiated DPPC exchanged into the outer leaflet of headgroup-deuterated DPPC-d13 vesicles was monitored using1H NMR spectroscopy, coupled with the addition of a paramagnetic shift reagent. This allowed us to distinguish between the inner and outer bilayer leaflet of DPPC, to determine the flip-flop rate as a function of temperature. Flip-flop of fluid-phase DPPC exhibited Arrhenius kinetics, from which we determined an activation energy of 122 kJ mol-1. In gel-phase DPPC vesicles, flip-flop was not observed over the course of 250 h. Our findings are in contrast to previous studies of solid-supported bilayers, where the reported DPPC translocation rates are at least several orders of magnitude faster than those in vesicles at corresponding temperatures. We reconcile these differences by proposing a defect-mediated acceleration of lipid translocation in supported bilayers, where long-lived, submicron-sized holes resulting from incomplete surface coverage are the sites of rapid transbilayer movement.
authors.affiliation: Institute of Molecular Biosciences, Biophysics Division, NAWI Graz, University of Graz , Graz 8010, Austria.|BioTechMed-Graz , Graz 8010, Austria.
authors.forename: Drew
authors.identifier: 0000-0001-6848-2497
authors.initials: D
authors.lastname: Marquardt
authors.forename: Frederick A
authors.initials: FA
authors.lastname: Heberle
authors.affiliation: Department of Physics, University of South Florida , Tampa, Florida 33620, United States.
authors.forename: Tatiana
authors.initials: T
authors.lastname: Miti
authors.affiliation: Institute of Molecular Biosciences, Biophysics Division, NAWI Graz, University of Graz , Graz 8010, Austria.|BioTechMed-Graz , Graz 8010, Austria.
authors.forename: Barbara
authors.initials: B
authors.lastname: Eicher
authors.affiliation: Department of Biochemistry and Cell Biology , Stony Brook, New York 11794, United States.
authors.forename: Erwin
authors.initials: E
authors.lastname: London
authors.forename: John
authors.initials: J
authors.lastname: Katsaras
authors.affiliation: Institute of Molecular Biosciences, Biophysics Division, NAWI Graz, University of Graz , Graz 8010, Austria.|BioTechMed-Graz , Graz 8010, Austria.
authors.forename: Georg
authors.identifier: 0000-0003-1967-1536
authors.initials: G
authors.lastname: Pabst