A calorimetric study of binary mixtures of saturated and monounsaturated mixed-chain phosphatidylethanolamines

Evidence for patchy lipid layers on gold nanoparticle surfaces

Gold nanoparticles bearing multiple surface ligands are becoming favored candidates as multifunctional targeting, imaging, and therapeutic vehicles for biomedicine. The question of spatial location of different ligands on nanoparticle surfaces, especially with those of diameters less than 100 nm, is an important one that is difficult to quantitatively address. Here we functionalize the surface of 20, 50, and 90 nm gold nanoparticles with two different lipids, both single and mixed, using two different surface chemical procedures. Mass spectrometry supports the presence of both lipids in the mixed-lipid systems on nanoparticles, while electron microscopy evidence shows domain sizes for one lipid apparently a quarter to a half the projected diameter for 50 and 90 nm particles; but for 20 nm particles, there is no evidence for the existence of patches of the two lipids. Larger gold nanoparticles (90 nm) can be decorated with an array of 12 nm gold nanoparticles by use of a third lipid and antibody-antigen connectors; the display of the 12 nm particles about the 90 nm particles can be controlled to some extent by the initial surface chemistry and is quantified via a new angle analysis procedure.

An index of lipid phase diagrams

There is a growing awareness of the utility of lipid phase behavior data in studies of membrane-related phenomena. Such miscibility information is commonly reported in the form of temperature-composition (T-C) phase diagrams. The current index is a conduit to the relevant literature. It lists lipid phase diagrams, their components and conditions of measurement, and complete bibliographic information. The main focus of the index is on lipids of membrane origin where water is the dispersing medium. However, it also includes records on acylglycerols, fatty acids, cationic lipids, and detergent-containing systems. The miscibility of synthetic and natural lipids with other lipids, with water, and with biomolecules (proteins, nucleic acids, carbohydrates, etc.) and non-biological materials (drugs, anesthetics, organic solvents, etc.) is within the purview of the index. There are 2188 phase diagram records in the index, the bulk (81%) of which refers to binary (two-component) T-C phase diagrams. The remainder is made up of more complex (ternary, quaternary) systems, pressure-T phase diagrams, and other more exotic miscibility studies. The index covers the period from 1965 through to July, 2001.

Phase diagram of 1,2-dioleoylphosphatidylethanolamine (DOPE):water system at subzero temperatures and at low water contents

The phase behavior of partially hydrated 1, 2-dioleoylphosphatidylethanolamine (DOPE) has been studied using differential scanning calorimetry and X-ray diffraction methods together with water sorption isotherms. DOPE liposomes were dehydrated in the H(II) phase at 29 degrees C and in the L(alpha) phase at 0 degrees C by vapor phase equilibration over saturated salt solutions. Other samples were prepared by hydration of dried DOPE by vapor phase equilibration at 29 degrees C and 0 degrees C. Five lipid phases (lamellar liquid crystalline, L(alpha); lamellar gel, L(beta); inverted hexagonal, H(II); inverted ribbon, P(delta); and lamellar crystalline, L(c)) and the ice phase were observed depending on the water content and temperature. The ice phase did not form in DOPE suspensions containing <9 wt% water. The L(c) phase was observed in samples with a water content of 2-6 wt% that were annealed at 0 degrees C for 2 or more days. The L(c) phase melted at 5-20 degrees C producing the H(II) phase. The P(delta) phase was observed at water contents of <0.5 wt%. The phase diagram, which includes five lipid phases and two water phases (ice and liquid water), has been constructed. The freeze-induced dehydration of DOPE has been described with the aid of the phase diagram.

Partitioning of a fluorescent phospholipid between fluid bilayers: dependence on host lipid acyl chains

The partition coefficient Kp was measured for a headgroup-labeled phospholipid (12:0,12:0)-N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-PE (12-NBD-PE), equilibrated between LUV of a series of phosphatidylcholines (PC). Fluorescence resonance energy transfer between the 12-NBD-PE and a headgroup-rhodamine-labeled PE was used to find the equilibrium concentration of the 12-NBD-PE in the different LUV. Reliable equilibrium concentrations were obtained by monitoring the approach to equilibrium starting from a concentration below and from a concentration above the ultimate values. Using (16:0,18:1delta9)-PC as the reference lipid, Kp ranged from a high value of 1.65 favoring (16:0,18:1delta9)-PC over (16:1delta9,16:1delta9)-PC, to a low value of 0.90, favoring (22:1delta13,22:1delta13)-PC over (16:0,18:1delta9)-PC. The Kp values enabled calculation of the acyl chain contribution to the excess free energy of mixing for (12:0,12:0) acyl chains at infinite dilution in the L alpha phase of PC having acyl chains of (16:0,18:1delta9), (16:1delta9,16:1delta9), (18:1delta9,18:1delta9), (18:1delta6,18:1delta6), (20:1delta11,20:1delta11), and (22:1delta13,22:1delta13). (14:1delta9,14:1delta9)-PC was found to transfer so rapidly between LUV as to preclude reliable Kp measurement.