Structure and phase behaviour of dimyristoylphosphatidic acid/poly(L-lysine) systems

Effect of Escherichia coli on phospholipid monolayers: surface tensiometry and Brewster angle microscopy measurements

The effect of Escherichia coli (E. coli) cells on two phospholipids [dipalmitoyl phosphatidylcholine (DPPC) and dimyristoyl phosphatidylcholine (DMPC)] monolayers at the surface of a 1.5 wt% NaCl salt solution has been investigated using surface tension measurement and Brewster angle microscopy. The results showed that a DPPC monolayer that has an elastic structure was changed in morphology by interaction with E. coli cells, whereas a DMPC monolayer that has an expandable structure did not change in morphology. In particular, the morphology changed significantly around the liquid-expanded (LE)-liquid-condensed (LC) phase transition point for the DPPC monolayer. It was found that the LE-LC phase transition range in a DPPC monolayer was sensitive to influence from the outside of the monolayer such as the action of E. coli cells. Such a monolayer has the potential for application as a membrane sensor for detecting a small amount of bacteria in a short time.

Binding of cationic pentapeptides with modified side chain lengths to negatively charged lipid membranes: Complex interplay of electrostatic and hydrophobic interactions

Basic amino acids play a key role in the binding of membrane associated proteins to negatively charged membranes. However, side chains of basic amino acids like lysine do not only provide a positive charge, but also a flexible hydrocarbon spacer that enables hydrophobic interactions. We studied the influence of hydrophobic contributions to the binding by varying the side chain length of pentapeptides with ammonium groups starting with lysine to lysine analogs with shorter side chains, namely omithine (Orn), alpha, gamma-diaminobutyric acid (Dab) and alpha, beta-diaminopropionic acid (Dap). The binding to negatively charged phosphatidylglycerol (PG) membranes was investigated by calorimetry, FT-infrared spectroscopy (FT-IR) and monolayer techniques. The binding was influenced by counteracting and sometimes compensating contributions. The influence of the bound peptides on the lipid phase behavior depends on the length of the peptide side chains. Isothermal titration calorimetry (ITC) experiments showed exothermic and endothermic effects compensating to a different extent as a function of side chain length. The increase in lipid phase transition temperature was more significant for peptides with shorter side chains. FTIR-spectroscopy revealed changes in hydration of the lipid bilayer interface after peptide binding. Using monolayer techniques, the contributions of electrostatic and hydrophobic effects could clearly be observed. Peptides with short side chains induced a pronounced decrease in surface pressure of PG monolayers whereas peptides with additional hydrophobic interactions decreased the surface pressure much less or even lead to an increase, indicating insertion of the hydrophobic part of the side chain into the lipid monolayer.

Polyelectrolyte-coated liposomes: stabilization of the interfacial complexes

Anionic liposomes, composed of egg lecithin (EL) or dipalmitoylphosphatidylcholine (DPPC) with 20 mol% of cardiolipin (CL(2-)), were mixed with cationic polymers, poly(4-vinylpyridine) fully quaternized with ethyl bromide (P2) or poly-L-lysine (PL). Polymer/liposome binding studies were carried out using electrophoretic mobility (EPM), fluorescence, and conductometry as the main analytical tools. Binding was also examined in the presence of added salt and polyacrylic acid (PAA). The following generalizations arose from the experiments: (a) Binding of P2 and PL to small EL/CL(2-) liposomes (60-80 nm in diameter) is electrostatic in nature and completely reversed by addition of salt or PAA. (b) Binding can be enhanced by hydrophobization of the polymer with cetyl groups. (c) Binding can also be enhanced by changing the phase state of the lipid bilayer from liquid to solid (i.e. going from EL to DPPC) or by increasing the size of the liposomes (i.e. going from 60-80 to 300 nm). By far the most promising systems, from the point of view of constructing polyelectrolyte multilayers on liposome cores without disruption of liposome integrity, involve small, liquid, anionic liposomes coated initially with polycations carrying pendant alkyl groups.

Contrasting behavior of zwitterionic and cationic polymers bound to anionic liposomes

Zwitterionic polymers were prepared by quaternizing polyvinylpyridine (DP = 1100) with bromoacids (Br(CH2)nCOOH, where n = 1, 2, 3, and 5). The resulting polymers were then added to unilamellar liposomes composed of egg lecithin or dipalmitoylphosphatidylcholine admixed with 20 mol % of cardiolipin (a phospholipid with two negative charges). These systems were compared (along with polyethylvinylpyridinium chloride, a polycation) by light scattering, electrophoretic mobility, fluorescence, and high-sensitivity differential scanning calorimetry. The external zwitterionic polymers induce no flip-flop of cardiolipin from the inner leaflet to the outer leaflet as does the polycation. Aside from this similarity, the four zwitterionic polymers all behave differently from each other toward the anionic liposomes: (a) For n = 1, there is no detectable interaction between the polymer and the liposomes. (b) For n = 2, electrostatic attraction induces polymer-liposome association (reversed by the addition of NaCl) that maintains the original negative charge on the liposome. Aggregation of the liposomes accompanies polymer adsorption. (c) For n = 3, electrostatic binding also occurs along with aggregation. However, the binding is so strong that NaCl is unable to induce polymer/liposome dissociation. (d) For n = 5, there is polymer binding and NaCl-promoted dissociation but no substantial aggregation. These differences among the closely related polymers are discussed and analyzed in molecular terms.

Interfacial interactions of hydrophobic peptides with lipid bilayers

Four hydrophobic laminin-related peptides and their corresponding parent peptides were synthesized to use them to target liposomes to tumoral cells. The peptide sequence was YIGSR((NH(2))), and hydrophobic residues linked to the alpha-amino terminal end were decanoyl, myristoyl, stearoyl, and cholesteryl-succinoyl. Before use in biological systems, a physicochemical study was carried out in order to determine their interaction with DPPC bilayers that could compromise both the toxicity and the stability of liposomal preparations. The experiments were based on DSC, fluorescence polarization, outer-membrane destabilization, and vesicle leakage. These peptides showed in general a low interaction with the vesicles, promoting in all cases the rigidification of bilayers. This lack of strong disturbances in the ordered state of phospholipid molecules seems more likely due to the similarity of peptide acyl chains with those of lipids than to the absence of interactions. The bulkiness of cholesteryl derivative as well as its tendency toward aggregation resulted in weak interaction levels except in thermograms. The binding of peptides to the surface of liposomes loaded with doxorubicin resulted in preparations with good entrapment yields and small size, required for long circulating vesicles (especially for the myristoyl derivative). The alternative method based on the reaction of parent peptide to the surface of liposomes through an amide linkage was slightly more efficient when the peptide was linked to the carboxy-terminal end of the DSPE-PEG-COOH-containing liposomes. Nevertheless, the final decision must be made with the simplicity of the procedure and reduction in losses during all the steps of the processes taken into consideration.

Interaction of poly(l-lysines) with negatively charged membranes: an FT-IR and DSC study

The influence of the binding of poly(L-lysine) (PLL) to negatively charged membranes containing phosphatidylglycerols (PG) was studied by DSC and FT-IR spectroscopy. We found a general increase in the main transition temperature as well as increase in hydrophobic order of the membrane upon PLL binding. Furthermore we observed stronger binding of hydration water to the lipid head groups after PLL binding. The secondary structure of the PLL after binding was studied by FT-IR spectroscopy. We found that PLL binds in an alpha-helical conformation to negatively charged DPPG membranes or membranes with DPPG-rich domains. Moreover we proved that PLL binding induces domain formation in the gel state of mixed DPPC/DPPG or DMPC/DPPG membranes as well as lipid remixing in the liquid-crystalline state. We studied these effects as a function of PLL chain length and found a significant dependence of the secondary structure, phase transition temperature and domain formation capacity on PLL chain length and also a correlation between the peptide secondary structure and the phase transition temperature of the membrane. We present a system in which the membrane phase transition triggers a highly cooperative secondary structure transition of the membrane-bound peptide from alpha-helix to random coil.

Influence of poly(l-lysine) on the structure of dipalmitoylphosphatidylglycerol/water dispersions studied by X-ray scattering

The interaction between the negatively charged phospholipid DPPG and positively charged poly(L: -lysine) (PLL) of different lengths was studied by X-ray scattering in the SAXS and WAXS region. As a reference pure DPPG (Na salt) was investigated over a wide temperature range (-30 to 70 degrees C). The phase behavior of DPPG in aqueous and in buffer/salt dispersions showed a metastable subgel phase at low temperatures and a recrystallization upon heating before reaching the liquid-crystalline phase. The presence of additional salt stabilizes the bilayer structure and decreases the recrystallization temperature. Large changes in the SAXS region are not connected with changes in chain packing. In DPPG/PLL samples, the PLL is inserted between adjacent headgroup layers and liberates counterions which give rise to a freezing point depression. In the complex with DPPG PLL form an alpha-helical secondary structure at pH 7 and temperatures below the gel to liquid-crystalline phase transition. This prevents DPPG from recrystallization and strongly increases the stacking order. The lamellar repeat distance is decreased and fixed by the helix conformation of PLL in the gel phase. PLL with n = 14 is too short to form helices and is squeezed out reversibly from the interbilayer space upon cooling by freezing of trapped water. In dispersions with longer PLLs (n > 400) at -20 degrees C a 1D crystallization of PLL alpha-helices in the aqueous layer between the headgroups takes place. A structural model is presented for the lateral periodic complex, which is similar to the known cationic lipid/DNA complex.

Action mechanism of water soluble ethanol on phospholipid monolayers using a quartz crystal oscillator

Interaction between phospholipid monolayers (dihexadecyl phosphate: DHP, dipalmitoyl phosphatidyl choline: DPPC) and water soluble ethanol has been studied using quartz crystal microbalance (QCM) method and quartz crystal impedance (QCI) method. The quartz crystal oscillator was attached horizontally on the DHP and DPPC monolayers that were formed on the water surface. At low concentration, increased ethanol concentration decreased the frequency for QCM and increased the resistance for QCI. Both frequency and resistance approached asymptotically to a saturation value. A further increase in ethanol concentration induced a sudden and discontinuous linear change (a decrease in frequency and an increase in resistance). Based on these results, we propose the following action mechanism of ethanol on phospholipid monolayers: at low concentration, the ethanol hydrates adsorb into the monolayer/water interface and saturate on the interface. The monolayer viscosity also increases with the adsorption of hydrates. A further increase in concentration causes multilayer formation of hydrates and/or penetration of hydrates into the monolayer core. The viscosity of the interfacial layer (monolayer and interfacial structured water) changes dramatically according to the action of ethanol hydrates.

Reversibility of structural rearrangements in the negative vesicular membrane upon electrostatic adsorption/desorption of the polycation

Interaction of small unilamellar vesicles (SUVs), composed of negative diphosphatidylglycerol (cardiolipin, CL(2-)) and neutral dipalmitoylphosphatidylcholine (DPPC), with poly(N-ethyl-4-vinylpyridinium bromide) (PEVP) was studied in water solution above and below the vesicular membrane melting point by means of differential scanning calorimetry, photon correlation spectroscopy, microelectrophoresis, conductometry, and fluorescence techniques. It has been found that CL(2-) species are homogeneously distributed within DPPC-CL(2-) SUV membrane leaflets and between them. Interaction of PEVP with DPPC-CL(2-) SUVs led to drastic structural rearrangements in the membrane if it was in the fluid state (liquid SUVs). Negative CL(2-) molecules migrated from the inner to the outer membrane leaflet and segregated in the vicinity of adsorbed PEVP chains. In addition, PEVP adsorption terminated completely the exchange of lipid molecules between the SUVs. At the same time, the integrity of liquid SUVs contacting PEVP remained unchanged. Since the interaction of PEVP with liquid SUVs was predominantly electrostatic in nature, the polycation could be completely removed from the vesicular membrane by addition of an excess of polyacrylic acid (PAA) polyanions forming a more stable electrostatic complex with PEVP. Removal of PEVP resulted in complete resumption of the original distribution of lipids in lateral and transmembrane directions as well as intervesicular lipid exchange. In contrast, PEVP interacting with DPPC-CL(2-) SUVs formed defects in the vesicular membrane if it was in the gel state (solid SUVs). Such interaction was contributed not only by electrostatic but most likely by hydrophobic interactions involving the defected membrane sites. PEVP kept contacting solid SUVs in the presence of an abundant amount of PAA. The established phenomena may be important for understanding the biological effects of polycations.

Polylysine-induced 2H NMR-observable domains in phosphatidylserine/phosphatidylcholine lipid bilayers

The interaction of three polylysines, Lys(5) (N = 5), Lys(30) (N = 30), and Lys(100) (N = 100), where N is the number of lysine residues per chain, with phosphatidylserine-containing lipid bilayer membranes was investigated using 2H NMR spectroscopy. Lys(30) and Lys(100) added to multilamellar vesicles composed of (70:30) (mol:mol) mixtures of choline-deuterated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) + 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS) produced two resolvable 2H NMR spectral components under conditions of low ionic strength and for cases where the global anionic lipid charge was in excess over the global cationic polypeptide charge. The intensities and quadrupolar splittings of the two spectral components were consistent with the existence of polylysine-bound domains enriched in POPS, in coexistence with polylysine-free domains depleted in POPS. Lys(5), however, yielded no 2H NMR resolvable domains. Increasing ionic strength caused domains to become diffuse and eventually dissipate entirely. At physiological salt concentrations, only Lys(100) yielded 2H NMR-resolvable domains. Therefore, under physiological conditions of ionic strength, pH, and anionic lipid bilayer content, and in the absence of other, e.g., hydrophobic, contributions to the binding free energy, the minimum number of lysine residues sufficient to produce spectroscopically resolvable POPS-enriched domains on the 2H NMR millisecond timescale may be fewer than 100, but is certainly greater than 30.

Effect of phospholipid bilayers on solution conformation of branched polymeric polypeptides and peptide-polymer conjugates

This report provides a detailed analysis on the influence of phosholipid bilayers on the conformation of poly[Lys(X(i)-DL-Ala(m))] (XAK, where X = Ser, Orn, Glu, or AcGlu) type branched polypeptides and their peptide conjugates. CD spectra of polycationic (SAK, OAK), amphoteric (EAK), or polyanionic (Ac-EAK) polylysine derivatives were recorded in 0.25M acetate buffer at pH 7.4 as well as in the presence of DPPC or DPPC/PG (95/5, 80/20 mol/mol) liposomes. Based on these data, two groups of polypeptides are described. Group one contains polypeptides with significantly ordered conformation even in buffer solution (SAK, AcEAK), which is essentially not altered by phospholipids. Group two, branched polypeptides (OAK, EAK), with only partially ordered conformation in aqueous solution in the presence of phospholipid bilayers with high PG content, could adopt more (EAK) or less (OAK) ordered alpha-helical structure depending on their charge properties. In addition, we report on the synthesis of two new sets of oligopeptide-branched polypeptide conjugates. Studies with selected conjugates suggest that these compounds are highly ordered in buffer solution almost regardless from the helix-forming ability of the carrier (AK, SAK, EAK) and from the hydrophilic/hydrophobic character of peptides attached (AVKDEL vs FWRGDLVFDFQV). Addition of phospholipid bilayers with different composition essentially had no modifying effect on conformation of conjugates. From this we can conclude that the covalently coupled oligopeptides has a predominant effect of the conformational properties of conjugates.