Effect of fatty acid chain length,fatty acid hydroxylation,and various cations on phase behavior of synthetic cerebroside sulfate

Synthetic species of CBS containing palmitic, stearic, lignoceric, D-2-hydroxy palmitic, or D-2-hydroxy stearic acid were prepared and their phase behavior in the presence of a number of mono- and divalent cations was studied by differential scanning calorimetry and the use of fatty acid spin labels. The results showed that both the non-hydroxy fatty acid (NFA) and hydroxy fatty acid (HFA) forms of cerebroside sulfate (CBS) can occur in two different gel states, a metastable state and a lower entropy stable state. The phase behavior is more sensitive to the type and concentration of cation present than in the case with acidic phospholipids. The sensitivity of the transition temperature (T_m) to cation concentration reflects, in part, increased participation of the lipid in intermolecular hydrogen bonding interactions as the negative charge of the sulfate is shielded. The extra hydroxyl group on the HFA also contributes to the intermolecular hydrogen bonding network causing a significant increase in the T_m.The HFA has an even more significant effect in causing inhibition of formation of the stable state. Formation of the stable state is also inhibited by Li⁺ and divalent cations. A similar mechanism may be involved, i.e.; cross-linking of adjacent lipids or increased intermolecular interactions inhibit the molecular rearrangement necessary to form the stable state. This inhibition is counteracted by an increase in fatty acid chain length. The results suggest that the stable state may be interdigitated as a result of the unequal chain length between the sphingosine base and the fatty acid.     

Peptide antibiotics in action: Investigation of polypeptide chains in insoluble environments by rotational-echo double resonance

Rotational-echo double resonance (REDOR) is a solid-state NMR technique that has the capability of providing intra- and intermolecular distance and orientational restraints in non-crystallizable, poorly soluble heterogeneous molecular systems such as cell membranes and cell walls. In this review, we will present two applications of REDOR: the investigation of a magainin-related antimicrobial peptide in lipid bilayers and the study of a vancomycin-like glycopeptide in the cell walls of Staphylococcus aureus.     

Bacterial lipid composition and the antimicrobial efficacy of cationic steroid compounds (Ceragenins)

Ceragenins are cationic bile salt derivatives having antimicrobial activity. The interactions of several ceragenins with phospholipid bilayers were tested in different systems. The ceragenins are capable of forming specific associations with several phospholipid species that may be involved with their antimicrobial action. Their antimicrobial activity is lower in bacteria that have a high content of phosphatidylethanolamine. Gram negative bacteria with a high content of phosphatidylethanolamine exhibit sensitivity to different ceragenins that corresponds to the extent of interaction of these compounds with phospholipids, including the ability of different ceragenins to induce leakage of aqueous contents from phosphatidylethanolamine-rich liposomes. A second class of bacteria having cell membranes composed largely of anionic lipids and having a low content of phosphatidylethanolamine are very sensitive to the action of the ceragenins but they exhibit similar minimal inhibitory concentrations with most of the ceragenins and for different strains of bacteria. Although Gram negative bacteria generally have a high content of phosphatidylethanolamine, there are a few exceptions. In addition, a mutant strain of Escherichia coli has been made that is essentially devoid of phophatidylethanolamine, although 80% of the lipid of the wild-type strain is phosphatidylethanolamine. Furthermore, certain Gram positive bacteria are also exceptions in that they can have a high content of phosphatidylethanolamine. We find that the antimicrobial action of the ceragenins correlates better with the content of phosphatidylethanolamine in the bacterial membrane than whether or not the bacteria has an outer membrane. Thus, the bacterial lipid composition can be an important factor in determining the sensitivity of bacteria to antimicrobial agents.     

Interaction of biologically active molecules with phospholipid membranes: I. Fluorescence depolarization studies on the effect of polymeric biocide bearing biguanide groups in the main chain

Interaction of poly(hexamethylene biguanide hydrochloride) (PHMB), which is a polymeric biocide bearing biguanide groups in its main chain, with phospholipid bilayers was studied by the fluorescence depolarization method. A strong interaction of PHMB with negatively charged bilayers composed of phosphatidylglycerol(PG) alone or of PG and phosphatidylcholine (PC) was observed, whereas neutral PC bilayers were not affected. On adding PHMB, the fluorescence polarization of diphenylhexatriene embedded in the negatively charged bilayers was reduced to a great extent, especially in the gel phase. This was interpreted in terms of PHMB-induced expansion and fluidization of the bilayer, which enables the probe molecule to undergo less-hindered torsional motion. Similarity between PHMB and polymyxin B in the structure, the mode of action against bacteria and the interaction with lipid membranes is discussed.     

Calorimetric and spectroscopic studies of the effects of cholesterol on the thermotropic phase behavior and organization of a homologous series of linear saturated phosphatidylglycerol bilayer membranes

We have examined the effects of cholesterol (Chol) on the thermotropic phase behavior and organization of aqueous dispersions of a homologous series of linear disaturated phosphatidylglycerols (PGs) by high-sensitivity differential scanning calorimetry and Fourier transform infrared and ³¹P NMR spectroscopy. We find that the incorporation of increasing quantities of Chol alters the temperature and progressively reduces the enthalpy and cooperativity of the gel-to-liquid-crystalline phase transition of the host PG bilayer. With dimyristoyl-PG:Chol mixtures, cooperative chain-melting phase transitions are completely or almost completely abolished at Chol concentrations near 50 mol%, whereas with the dipalmitoyl- and distearoyl-PG:Chol mixtures, cooperative hydrocarbon chain-melting phase transitions are still discernable at Chol concentrations near 50 mol%. We are also unable to detect the presence of significant populations of separate domains of the anhydrous or monohydrate forms of Chol in our binary mixtures, in contrast to previous reports. We ascribe the previously reported large scale formation of Chol crystallites to the fractional crystallization of the Chol and phospholipid phases during the removal of organic solvent from the binary mixture before the hydration of the sample. We further show that the direction and magnitude of the change in the phase transition temperature induced by Chol addition is dependent on the hydrocarbon chain length of the PG studied. This finding agrees with our previous results with phosphatidylcholine bilayers, where we found that Chol increases or decreases the phase transition temperature in a hydrophobic mismatch-dependent manner (Biochemistry 1993, 32:516-522), but is in contrast to our previous results for phosphatidylethanolamine (Biochim. Biophys. Acta 1999, 1416:119-234) and phosphatidylserine (Biophys. J. 2000, 79:2056-2065) bilayers, where no such hydrophobic mismatch-dependent effects were observed. We also show that the addition of Chol facilitates the formation of the lamellar crystalline phase in PG bilayers, as it does in phosphatidylethanolamine and phosphatidylserine bilayers, whereas the formation of such phases in phosphatidylcholine bilayers is inhibited by the presence of Chol. Moreover, the formation of the lamellar crystalline phase in PG bilayers at lower temperatures excludes Chol, resulting in an apparent Chol immiscibility in gel-state PG bilayers. We suggest that the magnitude of the effect of Chol on the thermotropic phase behavior of the host phospholipid bilayer, and its miscibility in phospholipids dispersions generally, depend on the strength of the attractive interactions between the polar headgroups and the hydrocarbon chains of the phospholipid molecule, and not on the charge of the polar headgroups per se.     

A comparative study on the interactions of SMAP-29 with lipid monolayers

This work investigates the discrimination of lipid monolayers by the ovine antimicrobial peptide SMAP-29 and compares it to that of the human LL-37 peptide. Fluid phospholipid monolayers were formed in a Langmuir trough and subsequently studied with the X-ray scattering techniques of X-ray reflectivity and grazing incidence X-ray diffraction. Any changes in the phospholipid structure after injection of peptide under the monolayer were considered to be due to interactions between the peptides and lipids. The data show that SMAP-29 discriminates against negatively charged phospholipids in a similar way to LL-37. However, it is even more interesting to note that despite a higher concentration of SMAP-29 near the monolayer, ensured by its greater charge as compared to LL-37, the amount of SMAP-29 needed to observe monolayer disruption was around three and a half times the number of molecules of LL-37 used to see similar changes with the same system. This result suggests that the structure, amino acid sequence or size of the peptide may well be as important as electrical charge and therefore gives many implications for the further study of antimicrobial peptides with regards to novel drug design and development.     

Thermodynamics of lipid interactions in complex bilayers

The mutual interactions between lipids in bilayers are reviewed, including mixtures of phospholipids, and mixtures of phospholipids and cholesterol (Chol). Binary mixtures and ternary mixtures are considered, with special emphasis on membranes containing Chol, an ordered phospholipid, and a disordered phospholipid. Typically the ordered phospholipid is a sphingomyelin (SM) or a long-chain saturated phosphatidylcholine (PC), both of which have high phase transitions temperatures; the disordered phospholipid is 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) or dioleoylphosphatidylcholine (DOPC). The unlike nearest-neighbor interaction free energies (ω_AB) between lipids (including Chol), obtained by an variety of unrelated methods, are typically in the range of 0-400 cal/mol in absolute value. Most are positive, meaning that the interaction is unfavorable, but some are negative, meaning it is favorable. It is of special interest that favorable interactions occur mainly between ordered phospholipids and Chol. The interpretation of domain formation in complex mixtures of Chol and phospholipids in terms of phase separation or condensed complexes is discussed in the light of the values of lipid mutual interactions.     

Raman spectroscopic study of an interdigitated lipid bilayer. Dipalmitoylphosphatidylcholine dispersed in glycerol

Dipalmitoylphosphatidylcholine (DPPC) dispersed in perdeuterated glycerol was investigated in order to determine the effects on the Raman spectra of hydrocarbon chain interdigitation in gel-phase lipid bilayers. Interdigitated DPPC bilayers formed from glycerol dispersions in the gel phase showed a decrease in the peak height intensity $\text{I}{}_{2850}\text{/I}{}_{2880}$ ratio, for the symmetric and asymmetric methylene CH stretching modes, respectively, as compared to non-interdigitated DPPC/water gel-phase dispersions. The decrease in this spectral ratio is interpreted as an increase in chain-chain lateral interactions. Spectra recorded in the 700–740 cm^?1 CN stretching mode region, the 1000–1200 cm^?1 CC stretching mode region and the 1700–1800 cm^? CO stretching mode region were identical for both the interdigitated and non-interdigitated hydrocarbon chain systems. At low temperatures the Raman peak height intensity ratios $\text{I}{}_{2935}\text{/I}{}_{2880}$ were identical for the DPPC/glycerol and DPPC/water dispersions, indicating that this specific index for monitoring bilayer behavior is insensitive to acyl chain interdigitation. The increase, however, in the change of this index at the gel-liquid crystalline phase transition temperature for the DPPC/glycerol dispersions implies a larger entropy of transition in comparison to the non-interdigitated DPPC/water bilayer system.     

Thermal phase transitions in the polar lipids of plant membranes. Their induction by disaturated phospholipids and their possible relation to chilling injury

The phase behaviour of leaf polar lipids from three plants, varying in their sensitivity to chilling, was investigated by differential scanning calorimetry. For the lipids from mung bean (Vigna radiata L. var. Berken), a chilling-sensitive plant, a transition exotherm was detected beginning at $\text{10 ± 2°}\text{C}$. No exotherm was evident above 0°C with polar lipids from wheat (Triticum aestivum cv. Falcon) or pea (Pisum sativum cv. Massey Gem), plants which are insensitive to chilling. The enthalpy for the transition in the mung bean polar lipids indicated that only about 7% w/w of the lipid was in the gel phase at ?8°C. The thermal transition of the mung bean lipids was mimicked by wheat and pea polar lipids after the addition of 1 to 2% w/w of a relatively high melting-point lipid such as dipalmitoylphosphatidylcholine, dipalmitoylphosphatidylglycerol or dimyristoylphosphatidylcholine. Analysis of the polar lipids from the three plants showed that a dipalmitoylphosphatidylglycerol was present in mung bean (1.7% w/w) and pea (0.3% w/w) but undetected in wheat, indicating that the transition exotherm temperature of 10°C in mung bean, 0°C in pea and about ?3°C in wheat correlates with the proportion of the high melting-point disaturated component in the polar lipids. The results indicate that the transition exotherm, observed at temperatures above 0°C in the membranes of chilling-sensitive plants, could be induced by small amounts of high melting-point lipids and involves only a small proportion of the membrane polar lipids.