Methanol Accelerates DMPC Flip-Flop and Transfer: A SANS Study on Lipid Dynamics

© 2019 Biophysical Society Methanol is a common solubilizing agent used to study transmembrane proteins/peptides in biological and synthetic membranes. Using small angle neutron scattering and a strategic contrast-matching scheme, we show that methanol has a major impact on lipid dynamics. Under increasing methanol concentrations, isotopically distinct 1,2-dimyristoyl-sn-glycero-3-phosphocholine large unilamellar vesicle populations exhibit increased mixing. Specifically, 1,2-dimyristoyl-sn-glycero-3-phosphocholine transfer and flip-flop kinetics display linear and exponential rate enhancements, respectively. Ultimately, methanol is capable of influencing the structure-function relationship associated with bilayer composition (e.g., lipid asymmetry). The use of methanol as a carrier solvent, despite better simulating some biological conditions (e.g., antimicrobial attack), can help misconstrue lipid scrambling as the action of proteins or peptides, when in actuality it is a combination of solvent and biological agent. As bilayer compositional stability is crucial to cell survival and protein reconstitution, these results highlight the importance of methanol, and solvents in general, in biomembrane and proteolipid studies

Peptide-Induced Lipid Flip-Flop in Asymmetric Liposomes Measured by Small Angle Neutron Scattering

© 2019 American Chemical Society. Despite the prevalence of lipid transbilayer asymmetry in natural plasma membranes, most biomimetic model membranes studied are symmetric. Recent advances have helped to overcome the difficulties in preparing asymmetric liposomes in vitro, allowing for the examination of a larger set of relevant biophysical questions. Here, we investigate the stability of asymmetric bilayers by measuring lipid flip-flop with time-resolved small-angle neutron scattering (SANS). Asymmetric large unilamellar vesicles with inner bilayer leaflets containing predominantly 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and outer leaflets composed mainly of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) displayed slow spontaneous flip-flop at 37 -C (half-time, t1/2 = 140 h). However, inclusion of peptides, namely, gramicidin, alamethicin, melittin, or pHLIP (i.e., pH-low insertion peptide), accelerated lipid flip-flop. For three of these peptides (i.e., pHLIP, alamethicin, and melittin), each of which was added externally to preformed asymmetric vesicles, we observed a completely scrambled bilayer in less than 2 h. Gramicidin, on the other hand, was preincorporated during the formation of the asymmetric liposomes and showed a time resolvable 8-fold increase in the rate of lipid asymmetry loss. These results point to a membrane surface-related (e.g., adsorption/insertion) event as the primary driver of lipid scrambling in the asymmetric model membranes of this study. We discuss the implications of membrane peptide binding, conformation, and insertion on lipid asymmetry

Structural systematics of the isomeric di-μ-chlorobis(N, N-dialkylbenzylamine-2,C, N)dipalladium(II) complexes. The crystal structures of the ortho-, meta- and para-methoxy substituted complexes

The crystal structures of three isomeric methoxy substituted bis-chloro bridged (N, N-dimethylbenzylamine)palladium(II) dimers have been determined by X-ray diffraction. Both the ortho-isomer (1) and the meta-isomer (2) are monoclinic, spacegroup P2/c with Z = 2 in cells of dimensions a 5.7391(7), b 17.794(4), c 10.907(2) Å, β 97.76(1)° (1), and a 9.897(1), b 11.178(2), c 10.660(2) Å, β 105.76(1)° (2). The four molecules of the para-isomer (3) crystallize with two molecules of toluene of solvation in a cell of dimensions a 8.403(1), b 20.056(3), c 15.521(2) Å, β = 90.54(1)°, space group P2/n. All three complexes have the expected trans-related dimeric structures about square planar palladium(II) centres, although 3 does not have the centrosymmetry of 1 and 2. The PdC and PdN bond lengths show no significant variation across the series (range 1.967-1.986 Å; mean, 1.977 Å) and (range 2.068-2.075 Å; mean, 2.071 Å), respectively. The PdCl bridge bonds are asymmetric, consistent with the trans effect of the PdC bond (range, 2.466-2.488 Å; mean 2.476 Å and 2.334-2.341 Å; mean, 2.337 Å)