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R C MacDonald

Publications and source records attributed to R C MacDonald.

65 records · Page 4Linked to original sources
PubMed1977

The noneffect of a large linear hydrocarbon, squalene, on the phosphatidylcholine packing structure.

The interaction of squalene with liposomes and monolayers of dipalmitoyl phosphatidylcholine (DPL) has been studied by differential scanning calorimetry, Raman spectroscopy, and surface potential measurements. Mole ratios of squalene to DPL up to 9 to 1 were studied. In contrast to small, nonpolar molecules, which profoundly influence the structure of lipid bilayers as detected by changes in both their thermodynamic phase transition parameters and membrane fluidity, this large, nonpolar, linear hydrocarbon is devoid of such influences. It is clear from our data that a large nonpolar molecule such as squalene, having no polar group that might anchor it to the aqueous interface, cannot intercalate between the acyl chains either below or above the phase transition of DPL. This behavior is not compatible with models that treat the bilayer interior as a bulk hydrocarbon, and suggests that great caution should be exercised in extrapolating partition coefficients based on bulk hydrocarbon measurements to lipid bilayers.

Liposomes↗
PubMed1976

Ionic influences on the phase transition of dipalmitoylphosphatidylserine.

The ionization and phase behavior of 1,2-dipalmitoyl-sn-glycero-3-phosphoserine have been investigated under a variety of condtions by several different methods. As measured by turbidity changes, the temperature of the crystal-liquid crystal phase transition of this lipid is influenced by pH and mono- and divalent cation concentrations. The pH-transition temperature curve is congruent with the curve relating temperature to the degree of ionization of the carboxyl group of the crystalline form. The transition temperature falls from an upper plateau of 72 degrees C at low pH values, where the carboxyl group is fully protonated, to a lower plateau of 55 degrees C at high pH values, where this group is fully ionized. The apparent pK (pH at 50% ionization) of the crystalline form shifts from 6.0 to 4.6 to 3.7 with an increase of NaCl concentration from 10(-3) to 0.1 to l.0 M, respectively. These observations are in accord with a simple theoretical analysis that utilizes diffuse double layer theory and the influence of surface potential on surface concentration of protons. In qualitative terms, an increase in electrolyte concentration reduces the surface potential, the result of which is a diminution of the surface-bulk pH difference and a lowering of the apparent pK. Assuming an area of 50 A2/molecule, the intrinsic pKa (apparent pK corrected for surface pH) of the carboxyl group is 2.7. A 1000-fold change of NaCl concentration produces a very large change in surface potential without influencing the transition temperature of the ionized form of the lipid.

Calorimetry↗
PubMed1976

Phospholipid exchange between bilayer membrane vesicles.

The turbidity of lipid vesicles, freshly prepared by sonicating purified dimyristoyllecithin (DML) in dilute KCl solutions, was measured as a function of time at various temperatures. A sharp maximum in the rate of increase of turbidity is found just above the crystal:liquid-crystal phase transition temperature (Tm). The initial rate of turbidity increase is first order with respect to DML concentration. Electron and light microscopy reveal large vesicles which are not present before incubation or after incubation at temperatures far from the Tm. When temperature, rather than time, is the independent variable, a sharp drop in turbidity is seen at the Tm. The magnitude of this drop and the temperature at which it occurs were used to measure the rate of lipid transfer between vesicles composed of different lipids. A mixture of DML vesicles and dipalmitoyllecithin (DPL) vesicles exhibits sharp drops in turbidity at 24 and 41 degrees, the corresponding Tm's. With time, the magnitude of the transition at 24 degrees decreases while that which was originally at 41 degrees moves to lower temperatures and increases in magnitude. At equilibrium there is a single transition at 32.5 degrees characteristic of vesicles composed of equimolar DPL and DML. The rate at which equilibrium is approached increases at around 24 degrees and again around 41 degrees. These observations indicate that vesicles are in equilibrium with monomolecular lipid, the concentration of the latter being higher the shorter the lipid acyl group or the smaller the vesicle. DML molecules are therefore lost from small vesicles to large vesicles (DML system) or lost from DML vesicles to DML-DPL vesicles (mixed system). When DML vesicles containing a few percent brain gangliosides were studied, different behavior was observed; the initial rate of increase of turbidity becomes second order in lipid concentration, and the rate constant increases with increasing concentrations of KCl. The kinetic order, coupled with the fact that electrolyte reduces intervesicle electrostatic repulsion, argues that in this situation the mechanism of vesicle growth requires vesicle collision.

Binding Sites↗
PubMed1976

Lipid vesicle-cell interactions. I. Hemagglutination and hemolysis.

The interaction of lipid vesicles (liposomes) of several different compositions with erythrocytes has been investigated. Lecithin liposomes, rendered positively charged with stearylamine, exhibit potent hemagglutination activity in media containing low concentrations of electrolytes. The hemagglutination titer is found to be a linear function of the zeta potential of the lipid vesicles. Hemagglutination is reduced when the surface potential of the cells is made more positive by pH adjustment or enzyme treatment. Similarly, hemagglutination is reduced by increasing concentrations of electrolytes. Hemagglutination is examined theoretically and is shown to be consistent with vesicle-cell interactions that are due to only electrostatic forces. Vesicles containing lysolecithin in addition to lecithin and stearylamine cause lysis of erythrocytes, provided the lipids of the vesicles are above the crystal-liquid crystal phase transition temperature. In addition, hemolysis requires close juxtaposition of the vesicle to the cell membrane; vesicles precoated with antibodies exhibit severely diminished hemolytic activities, only a small fraction of which can be attributed to a reduction in hemagglutination titer. Evidence is presented indicating that a single vesicle is sufficient to lyse one cell. With regard to hemagglutination and hemolysis, lipid vesicles of simple composition mimic paramyxoviruses such as Sendai virus.

Cell Aggregation↗
PubMed1976

Lipid vesicle-cell interactions. II. Induction of cell fusion.

The ability of lipid vesicles of simple composition (lecithin, lysolecithin, and stearylamine) to induce cells of various types to fuse has been investigated. One in every three or four cells in monolayer cultures can be induced to fuse with a vesicle dose of about 100 per cell. At such dosages and for exposures of 15 min to 1 h, vesicles have essentially no effect on cell viability. Under anaerobic conditions, these cells lyse rather than fuse. Avian erythrocytes are readily fused with lipid vesicles in the presence of dextran. Fusion indices increase linearly with the zeta potential of the vesicles (increasing stearylamine content), indicating that contact between vesicle and cell membrane is required. Fusion indices increase sublinearly with increasing lysolecithin content. Divalent cations increase fusion indices at high vesicle doses. The data presented are consistent with the hypothesis that cell fusion occurs via simultaneous fusion of a vesicle with two adhering cell membranes.

Animals↗
PubMed1976

Lipid vesicle-cell interactions. III. Introduction of a new antigenic determinant into erythrocyte membranes.

The introduction of a new antigenic determinant, 2,4-dinitrophenyl-aminocaproyl-phosphatidylethanolamine (DNP-Cap-PE), into the surface membranes of intact human erythrocytes is described. Fresh cells were incubated in the presence of liposomes composed of 10% DNP-Cap-PE, 5% stearylamine, 20% lysolecithin, and 65% lecithin. Such liposome-treated erythrocytes are shown to be susceptible to immune lysis by anti-DNP serum in the presence of complement. Uptake of DNP-Cap-PE by erythrocyte membranes is also demonstrated by immunofluorescence using indirect staining with rabbit anti-DNP serum followed by fluroescein-conjugated goat anti-rabbit IgG and by electron microscopy using ferritin-conjugated antibody. Antigen uptake did not occur at low temperatures or from vesicles lacking lysolecithin and stearylamine. Fluorescence microscopy shows that the antigen-antibody complexes are free to diffuse over the cell surface, eventually coalescing into a single area on the cell membrane. Electron microscopy suggests that a substantial proportion of the lipid antigen is incorporated by fusion of vesicles with the cell membrane. There are indications that vesicle treatment causes a small proportion of cells to invaginate.

Agglutination↗
PubMedDate not supplied

Diquaternary ammonium compounds as transfection agents.

Diquaternary ammonium salts constitute a new class of reagent for mediating transfection of DNA in mammalian cell lines. N,N'-dioleyl-N,N,N',N'-tetramethyl-1,2-ethanediamine (TmedEce), N,N'-dioleyl-N,N,N',N'-tetramethyl-1,3-propanediamine (PropEce), N,N'-dioleyl-N,N,N',N'-tetramethyl-1,6-hexanediamine (HexEce), and their corresponding N,N'-dicetyl saturated analogues (TmedAce, PropAce and HexAce) have all been synthesized and characterized. They were prepared via a bis-Menshutkin reaction of the corresponding tetramethyldiamine with 2.2 M equiv of a long-chain alkyl halide (saturated or unsaturated). The reaction was run in anhydrous acetonitrile for ca. 3 days at 60 degrees C, which produced the diquaternary ammonium halides in good to nearly quantitative yields for most derivatives. DNA transfection comparable to commercially available reagents such as Lipofectin, Lipofectace, Lipofectamine, and O-ethyldioleoylphosphatidylcholinium triflate has been achieved in vitro with these new reagents. There was no need to use a colipid for effective transfection, but serum did significantly inhibit transfection. The saturated and the unsaturated derivatives differed with respect to hydration behavior. The saturated derivatives appeared to retain a lamellar-type crystalline array structure upon hydration, whereas the unsaturated versions formed micelles and/or liposomes, depending on the ionic strength: HexEce was micellar in both water and saline; PropEce was micellar in water but lamellar in saline; and TmedEce was lamellar in both. Despite these different hydration patterns, all of these unsaturated derivatives formed productive transfection complexes with DNA. Varying the distance between the quaternary sites affected transfection efficacy in the order HexAce > TmedAce = PropAce for the saturated derivatives and in the order PropEce = HexEce > TmedEce, with a smaller spread, for the unsaturated derivatives.

Animals↗
PubMedDate not supplied

O-Alkyl dioleoylphosphatidylcholinium compounds: the effect of varying alkyl chain length on their physical properties and in vitro DNA transfection activity.

1,2-Dioleoyl-sn-3-ethylphosphocholine (EDOPC) has been previously shown be a highly effective DNA transfection reagent in vitro. To assess the effect of alkyl chain length on transfection efficiency, the O-methyl, O-propyl, O-hexyl, O-decyl, and O-octadecyl derivatives have been prepared from dioleoylphosphatidylcholine using the corresponding alkyl trifluoromethylsulfonate. The methyl, ethyl, and propyl derivatives formed liposomes which were very large and unilamellar. The ethyl and propyl derivatives were equally efficient at mediating transfection (even in the presence of serum) of BHK cells, but the chemically labile methyl derivative was a much weaker transfection agent. The O-decyl and O-octadecyl compounds, which assume the inverted hexagonal phase in excess water (as determined by X-ray diffraction), were almost inactive after manual agitation in both water and in saline; however, after sonication, these compounds exhibited good transfection activity. The O-hexyl derivative displayed novel behavior, assuming the lamellar phase at low and a cubic phase at high ionic strength. All compounds, whether lamellar or not, formed lamellar structures when complexed with DNA. In water, where the hexyl compound dispersed well, sonication diminished transfection activity, whereas at physiological ionic strength, which led to poor manual dispersion, sonication was essential for good transfection. These results emphasize the importance of optimal dispersion of a cationic lipid: too little, and interaction with DNA is handicapped, too much, and the resultant particle transfects poorly. Lipid dispersibility is thus an important variable in assessing lipid transfection agents, and caution is advised in attributing too much significance to chemical structure until interaction with DNA has been optimized.

Alkylation↗