The monolayer technique: a potent tool for studying the interfacial properties of antimicrobial and membrane-lytic peptides and their interactions with lipid membranes.
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Erudites of the antiquity already knew the calming effect of oil films on the sea waves. But one had to wait until 1774 to read the first scientific report on oil films from B. Franklin and again 1878 to learn the thermodynamic analysis on adsorption developed by J. Gibbs. Then, in 1891, Agnes Pockels described a technique to manipulate oil films by using barriers. Finally, in 1917, I. Langmuir introduced the experimental and theoretical modern concepts on insoluble monolayers. Since that time, and because it has been found to provide invaluable information at the molecular scale, the monolayer technique has been more and more extensively used, and, during the past decade, an explosive increase in the number of publications has occurred. Over the same period, considerable and ever-increasing interest in the antimicrobial peptides of various plants, bacteria, insects, amphibians and mammals has grown. Because many of these antimicrobial peptides act at the cell membrane level, the monolayer technique is entirely suitable for studying their physicochemical and biological properties. This review describes monolayer experiments performed with some of these antimicrobial peptides, especially gramicidin A, melittin, cardiotoxins and defensin A. After giving a few basic notions of surface chemistry, the surface-active properties of these peptides and their behavior when they are arranged in monomolecular films are reported and discussed in relation to their tridimensional structure and their amphipathic character. The penetration of these antimicrobial peptides into phospholipid monolayer model membranes, as well as their interactions with lipids in mixed films, are also emphasized. (+info)
Differential scanning calorimetry and X-ray diffraction studies of the specificity of the interaction of antimicrobial peptides with membrane-mimetic systems.
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Interest in biophysical studies on the interaction of antimicrobial peptides and lipids has strongly increased because of the rapid emergence of antibiotic-resistant bacterial strains. An understanding of the molecular mechanism(s) of membrane perturbation by these peptides will allow a design of novel peptide antibiotics as an alternative to conventional antibiotics. Differential scanning calorimetry and X-ray diffraction studies have yielded a wealth of quantitative information on the effects of antimicrobial peptides on membrane structure as well as on peptide location. These studies clearly demonstrated that antimicrobial peptides show preferential interaction with specific phospholipid classes. Furthermore, they revealed that in addition to charge-charge interactions, membrane curvature strain and hydrophobic mismatch between peptides and lipids are important parameters in determining the mechanism of membrane perturbation. Hence, depending on the molecular properties of both lipid and peptide, creation of bilayer defects such as phase separation or membrane thinning, pore formation, promotion of nonlamellar lipid structures or bilayer disruption by the carpet model or detergent-like action, may occur. Moreover, these studies suggest that these different processes may represent gradual steps of membrane perturbation. A better understanding of the mutual dependence of these parameters will help to elucidate the molecular mechanism of membrane damage by antimicrobial peptides and their target membrane specificity, keys for the rationale design of novel types of peptide antibiotics. (+info)
Headgroup conformation and lipid--cholesterol association in phosphatidylcholine vesicles: a 31P(1H) nuclear Overhauser effect study.
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The nuclear Overhauser effect has been observed in the nuclear magnetic resonance spectra of 31P. The information content of the nuclear Overhauser effect has been applied to the structure and dynamic properties of phosphatidylcholine vesicles. In the vesicles only 1/3 of the theoretical maximum nuclear Overhauser effect enhancement is observed. This result is accounted for by dipolar interactions between the N-methyl protons and the phosphate of phosphatidylcholine, and a correlation time for internal motion of 1.4 X 10(-9) sec. Addition of up to 30% cholesterol does not change the nuclear Overhauser effect enhancement or spin-lattice relaxation time of the vesicles. It is argued that the OH group of cholesterol is hydrogen bonded to the ester carbonyl oxygen of the phosphatidylcholine molecules. (+info)
Effect of membrane composition and structure on solute removal and biocompatibility in hemodialysis.
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Effect of membrane composition and structure on solute removal and biocompatibility in hemodialysis. Significant changes in extracorporeal membranes have occurred over the past five decades in which hemodialysis (HD) has been available as a therapy for both acute renal failure (ARF) and end-stage renal disease (ESRD). For cellulosic membranes, these changes have included a reduction in thickness, hydroxyl group substitution, and an increase in pore size. These modifications have resulted in enhanced efficiency of small solute removal, a broader spectrum of overall solute removal, and an attenuation of complement activation in comparison to the thick, unsubstituted cellulosic membranes of low permeability used in the early days of HD therapy. Synthetic membranes, originally developed specifically for use in high-flux HD and hemofiltration, have also evolved during this same time period. In fact, the initially clear distinction between low-flux regenerated cellulosic and high-flux synthetic membranes has become blurred, as membrane formulators have developed products designed to appeal to enthusiasts for both membrane formats. The purpose of this review is to characterize both the solute removal and biocompatibility characteristics of dialysis membranes according to their composition (that is, polymeric makeup) and structure. In this regard, the manner in which membrane biocompatibility interacts with flux is highlighted. (+info)
A theory of the chain melting phase transition of aqueous phospholipid dispersions.
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A model for the chain melting phase transition in dilute aqueous phospholipid bilayer dispersions is presented. This model includes interactions between head groups, between hydrocarbon chains, and within the chains. The head groups are modeled as hard disks which are constrained to lie on a two-dimensional surface separating the aqueous and hydrocarbon regions. The chain statistics problem is treated in an approximate manner using an approach motivated by scaled particle theory to describe the inter-chain steric repulsions in a mathematically tractable way. In this approach the whole system interacts with any given chain through an average lateral pressure which is proportional to the hard disk pressure. Following Nagle, we assume that the steric repulsions between chains and between head groups and the trans-gauche rotation energies are the dominant interactions in determining the transition and we describe the effect of the other interactions with a mean field approximation. Using the known transition temperature of a series of 1,2-diacyl phosphatidyl cholines to adjust two parameters in the theory, the model gives enthalpy and area changes that are in quite reasonable agreement with experiment. Moreover, the curvature observed in the plot of the transition temperature against acyl chain length is reproduced. (+info)
SP-B refining of pulmonary surfactant phospholipid films.
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Pulmonary surfactant stabilizes the alveoli by lining the air-fluid interface with films that reduce surface tension to near 0 mN/m (gamma(min)). Surfactant protein B (SP-B) enhances the surface activity of surfactant phospholipids. A captive bubble tensiometer (CBT) was used to study the properties of adsorbed films of dipalmitoylphosphatidylcholine (DPPC) with acidic 1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (POPG) or neutral 1-palmitoyl-2-oleoyl-sn-glycerol-3-phosphocholine with (7:3) and without 1% dimeric SP-B. SP-B enhanced the adsorption rate of DPPC-containing neutral or acidic lipid suspensions (1 mg/ml) to a similar extent. Quasi-static cycling of these films revealed that SP-B significantly decreased the film area reduction required to reach gamma(min) for the acidic but not for the neutral system. The results obtained with DPPC-phosphatidylglycerol (PG)-SP-B were consistent with selective DPPC adsorption into the surface monolayer during film formation. Film area reduction required to reach gamma(min) with this system (with and without calcium) approached that of pure DPPC, suggesting selective DPPC insertion and PG squeeze-out. Dynamic cycling of such films showed that larger film area reductions were required to reach gamma(min) for the neutral than for acidic system, even after 20 cycles. Fluorescence microscopy of solvent-spread DPPC-POPG-SP-B planar films revealed highly condensed structures at approximately 25 mN/m, although no specific PG phase-segregated structures could be identified. The study suggests that specific interactions of SP-B with acidic phospholipids of surfactant may be involved in the generation and maintenance of DPPC-rich films in the alveoli. (+info)
Fusion of dipalmitoylphosphatidylcholine vesicle membranes induced by concanavalin A.
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The temperature dependence of fatty acid spin label resonance spectra and freeze fracture micrographs of sonicated dipalmitoylphosphatidylcholine vesicles in the absence and presence of concanavalin A demonstrate a strong interaction of concanavalin A with these lipid membranes, which results in fusion of the vesicles. The rate of this reaction as followed with use of magnetic resonance exhibits a pronounced maximum at 36 degrees, the midpoint of the phase transition range of dipalmitoylphosphatidylcholine vesicles. This maximum is discussed in terms of structural fluctuations, which are maximal in the phase transition range of the membranes. (+info)
Intradialytic removal of protein-bound uraemic toxins: role of solute characteristics and of dialyser membrane.
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BACKGROUND: The efficiency of dialysis membranes is generally evaluated by assessing their capacity to remove small, water-soluble and non-protein-bound reference markers such as urea or creatinine. However, recent data suggest that protein-bound and/or lipophilic substances might be responsible for biochemical alterations characterizing the uraemic syndrome. METHODS: In the present study, the total concentrations of four uraemic retention compounds (indoxyl sulphate, hippuric acid, 3-carboxy-4-methyl-5-propyl-2-furanpropionic acid (CMPF) and p-cresol) and of tryptophan, the only protein-bound amino acid and a precursor of indoxyl sulphate, were compared with those of urea and creatinine in pre- and post-dialysis serum and in dialysate of 10 patients; two high-flux (HF) membranes (cellulose triacetate (CTA) and polysulphone (PS)) and a low-flux polysulphone (LFPS) membrane were compared in a crossover design, using HPLC. RESULTS: Except for hippuric acid (67.3+/-17.5% decrease), major differences were found in the percentage removal of the classical uraemic markers on one hand (creatinine 66.6+/-7.0% and urea 75.5+/-5.8% decrease) and the studied protein-bound and/or lipophilic substances on the other (indoxyl sulphate, 35.4+/-15.3% and p-cresol 29.0+/-14.2% decrease; tryptophan, 27.5+/-40.3%, and CMPF, 22.4+/-17.5% increase; P<0.01 vs urea and creatinine in all cases). Hippuric acid removal was more pronounced than that of the remaining protein-bound compounds (P<0. 01). After correction for haemoconcentration, per cent increase of tryptophan and CMPF was less substantial, while per cent negative changes for the remaining compounds became more important. There was a correlation between creatinine and urea per cent removal at min 240 (r=0.51, P<0.01), but all the other compounds showed no significant correlation with either of these two. The three membranes were similar regarding the changes of total solute concentrations from the start to the end of dialysis. CONCLUSIONS: Urea and creatinine are far more efficiently removed than the other compounds under study, except for hippuric acid. There are no striking differences between the HF membranes. Moreover, compared with the LF membrane these HF membranes do not appear to be superior in removing the studied compounds. (+info)