Large Headgroup Research Articles

Interaction of local anaesthetic articaine enantiomers with brain lipids: A Langmuir monolayer study

The interactions of the racemic mixture of articaine as well as pure (R)-articaine and pure (S)-articaine with monolayers of glycerophospholipids and brain lipids have been studied using the Langmuir monolayer technique. Articaine was added to the glycerophospholipids dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylserine (DPPS), 1-palmitoyl-2-oleoylphosphatidylserine (POPS) and total lipid extract from pig brain (TLPB). The amount of articaine in the monolayers was 30mol%. The intercalation of each of the two enantiomers of articaine into a glycerophospholipid/brain lipid monolayers composed of chiral phospholipids will be diastereoisomeric in nature, hence different intercalation pattern for the two enantiomers can be expected. All the articaine species are found to intercalate into the DPPC monolayer and to increase the monolayer stability, this is most pronounced for the (R)-enantiomer. Intercalation of the articaine species into the DPPS monolayer increases the MMA and hardly affects the stability of the DPPS monolayer. In this monolayer, the articaine species intercalates into the head group region of the small and negatively charged serine head group, this is pronounced for the (R)-enantiomer. Our results indicate that by introducing an unsaturated acyl chain in the monolayer as in POPS, the monolayer discriminates between the articaine species. The (R)-enantiomer is located deep in the acyl chain region, whereas the (S)-enantiomer is found at or close to the head group. The data also might indicate that the (R)-enantiomer in the racemic mixture forms dimers in the POPS monolayer. Both articaine species as well as the racemic mixture intercalate into the monolayer of TLPB. Intercalation into this monolayer did not show any distinct difference in intercalation mode of the articaine species, probably due to camouflaging effect of large head groups like gangliosides and/or formation of lipid rafts in the monolayer. However, the (R)-enantiomer appears to intercalate better into the TLPB monolayer than the (S)-enantiomer. With proper standardization the Langmuir monolayer technique is a powerful method to discriminate between (R)- and (S)-enantiomer articaine interaction with model membranes.

Molecular Convergence of Bacterial and Eukaryotic Surface Order

The conservation of fluidity is a theme common to all cell membranes. In this study, an analysis of lipid packing was conducted via C-laurdan spectroscopy of cell surface membranes prepared from representative species of Bacteria and Eukarya. We found that despite their radical differences in composition (namely the presence and absence of membrane-rigidifying sterol) the membrane order of all taxa converges on a remarkably similar level. To understand how this similarity is constructed, we reconstituted membranes with either bacterial or eukaryotic components. We found that transmembrane segments of proteins have an important role in buffering lipid-mediated packing. This buffering ensures that sterol-free and sterol-containing membranes exhibit similar barrier properties.

Tribological properties of fluid phase phospholipid bilayers

Our recent studies showed that solid phase phospholipid bilayers play an important role in joint lubrication mechanism. We investigate the tribological properties of fluid phase bilayers depending on various experimental parameters. Bilayers containing a fluorescent lipid with a large head-group and surfaces covered with only one bilayer poorly lubricate meaning that the slip plane during shear is located in the thin aqueous layer between the bilayers. Lubrication and resistance to normal and tangential stresses are greatly improved in buffered saline solution. Bilayers prepared by vesicle fusion method improve the tribological properties of even very rough substrates by smoothing their topographical defects.

Atomic Force Microscopy Studies of a Floating-Bilayer Lipid Membrane on a Au(111) Surface Modified with a Hydrophilic Monolayer

The surface of a gold electrode was functionalized with a hydrophilic monolayer of 1-thio-β-D-glucose formed by spontaneous self-assembly. The Langmuir-Blodgett/Langmuir-Schaefer (LB/LS) method was then used to assemble a bilayer onto the modified Au(111) surface. The bilayer lipid membrane (BLM) was separated from the Au(111) electrode surface by incorporating the monosialoganglioside GM1 into the inner leaflet of a bilayer composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and cholesterol. To make the inner leaflet, monolayers of GM1/DMPC/cholesterol with mole ratios of 1:6:3, 2:5:3, and 3:4:3 were used. The outer leaflet was composed of a 7:3 mole ratio of DMPC/cholesterol. Because of the amphiphilic properties of GM1, the hydrophobic acyl chains were incorporated into the BLM, whereas the large hydrophilic carbohydrate headgroups were physically adsorbed to the Au(111) electrode surface, creating a "floating" BLM (fBLM). This model contained a water-rich reservoir between the BLM and the gold surface. In addition, because of the bilayer being physically adsorbed onto the support, the fluidity of the BLM was maintained. The compression isotherms were measured at the air/water interface to determine the phase behavior and optimal transfer conditions. The images acquired using atomic force microscopy (AFM) and the force-distance measurements showed that the structure of the fBLM evolved with increasing GM1 content from 10 to 30 mol %, undergoing a transition from a corrugated to a homogeneous phase. This change was associated with a significant increase in bilayer thickness (from ∼5.3 to 7.3 nm). The highest-quality fBLM was produced with 30 mol % GM1.

Solution Self-Assembly of the Sophorolipid Biosurfactant and Its Mixture with Anionic Surfactant Sodium Dodecyl Benzene Sulfonate

The self-assembly in aqueous solution of the acidic (AS) and lactonic (LS) forms of the sophorolipid biosurfactant, their mixtures, and their mixtures with anionic surfactant sodium dodecyl benzene sulfonate, LAS, has been studied using predominantly small-angle neutron scattering, SANS, at relatively low surfactant concentrations of <30 mM. The more hydrophobic lactonic sophorolipid forms small unilamellar vesicles at low surfactant concentrations, in the concentration range of 0.2 to 3 mM, and transforms via a larger unilamellar vesicle structure at 7 mM to a disordered dilute phase of tubules at higher concentrations, 10 to 30 mM. In marked contrast, the acidic sophorolipid is predominantly in the form of small globular micelles in the concentration range of 0.5 to 30 mM, with a lower concentration of larger, more planar aggregates (lamellar or vesicular) in coexistence. In mixtures of AS and LS, over the same concentration range, the micellar structure associated with the AS sophorolipid dominates the mixed-phase behavior. In mixtures of anionic surfactant LAS with the AS sophorolipid, the globular micellar structure dominates over the entire composition and concentration range studied. In contrast, mixtures of LAS with the LS sophorolipid exhibit a rich evolution in phase behavior with solution composition and concentration. At low surfactant concentrations, the small unilamellar vesicle structure present for LS-rich solution compositions evolves into a globular micelle structure as the solution becomes richer in LAS. At higher surfactant concentrations, the disordered lamellar structure present for LS-rich compositions transforms to small vesicle/lamellar coexistence, to lamellar/micellar coexistence, to micellar/lamellar coexistence, and ultimately to a pure micellar phase as the solution becomes richer in LAS. The AS sophorolipid surfactant exhibits self-assembly properties similar to those of most other weakly ionic or nonionic surfactants that have relatively large headgroups. However, the more hydrophobic nature of the lactonic sophorolipid results in a more complex and unusual evolution in phase behavior with concentration and with concentration and composition when mixed with anionic surfactant LAS.

How Cells Cope with Obesity

​Currently,Currently, more than 2.7 million American adults are diagnosed as obese each year. Many of these people suffer from a disorder known as metabolic syndrome, which includes symptoms like hypertension and elevated blood cholesterol. They are also at risk for developing additional diseases such as heart disease and diabetes mellitus. In fact, obesity may be a major cause of all these problems … the question is, why? That's the question Kirsi H. Pietilainen, Antonio Vidal-Puig, Matej Oresic, and colleagues set out to address in their paper published in this month's PLoS Biology. Remodeling of adipose tissue membrane lipids as adaptation to acquired obesity: benefits and costs. One prominent theory explaining how obesity could drive advanced disease states is the “adipose tissue expandability” hypothesis. This theory is based on the idea that as excess fat is added to the body, its fat compartment (adipose tissue) expands as more fat-storing cells are made and existing ones increase in size. However, expandability of the fat compartment is finite; like a water balloon, the fat compartment can only expand so much before leaks develop. For the body, “leaks” in the fat compartment manifest as depositions of fat in tissues where its presence can be disruptive to normal function—as happens in morbidly obese individuals. But, lower levels of expansion are tolerated—possibly due to compensatory mechanisms—so that tissues and systems can function normally. So what is going on in the body when the fat compartments are expanded but not yet full? To explore this question in humans, the authors decided to compare adipose tissue and health parameters amongst several sets of monozygotic twins. In each twin pair, one twin was obese (but still healthy, i.e., not morbidly obese), while the other twin exhibited a normal body mass index. Because monozygotic twins share the same DNA and early upbringing, the impact of these factors on adult body mass phenotypes is controlled for, leaving other factors such as adult diet and lifestyle choices as the major remaining variables. When the authors compared dietary intake within twin sets, they found that obese twins had lower amounts of polyunsaturated fatty acids in their diets than did their non-obese counterparts. The kinds of fats a person eats can affect what types of lipids are present in the body, so the authors next compared the lipid content of obese versus non-obese twins' adipose tissue. In fact, they found the obese people had higher amounts of certain types of phospholipids in their adipose tissues than did their non-obese co-twins. This finding is interesting because cell membranes are primarily composed of lipids, and different lipids can alter membranes' physical properties. For example, lipids with larger head groups, or longer or branched side chains are found to pack less tightly in membranes, making the membranes that host them more fluid. When the authors used computers to model the effect these different lipids have on adipose cell membranes, they found that the new lipids observed in obese twins' adipose cells balance each other in such a way that membrane fluidity overall is unaffected. The authors concluded that lipid content changes in obese individuals might actually be an adaptation that serves to preserve membrane function as the cells expand. However, additional analyses suggested that this adaptation can only go so far, and breaks down in the morbidly obese. To find out how this adaptive response is achieved, the authors conducted a statistical network analysis (based on clinical and genetic variables) to try to identify the regulatory mechanisms that underpin these changes. This analysis showed that the gene encoding the fatty acid elongase Elovl6 might be involved in fatty acid remodeling in obese people. Indeed, when the researchers reduced Elovl6 expression in an adipocyte cell line, they found the cells could no longer maintain the right level of the adaptive lipids observed in obese twins. Collectively, the authors' data point to some of the mechanisms the body may use to adapt to excess fat. These results may also help explain why obese people are at risk for developing inflammatory disorders like diabetes mellitus: the kinds of lipids that accumulate in obese people's adipocytes are precursors for compounds that are known to aggravate the immune system. Of course, these findings need to be validated by more studies, including ones using transgenic animals. Nonetheless, they represent a valuable angle from which to approach the problem. Pietilainen KH, Rog T, Seppanen-Laakso T, Virtue S, Gopalacharyulu P, et al. (2011) Association of Lipidome Remodeling in the Adipocyte Membrane with Acquired Obesity in Humans. doi:10.1371/journal.pbio.1000623

Crowe Type I and II Ddh Managed by Large Diameter Metal-on-Metal Total Hip Arthroplasty

Large bearing metal-on-metal (MOM) total hip arthroplasty (THA) may offer advantages relating to stability and range of motion in patients with Crowe Type I and II developmental dysplasia of the hip (DDH). The purpose of this study was to provide an analysis of the clinical and radiological results of MOM THA in this context and compare the results with a cohort of patients treated with metal-on-polyethylene (MOP) bearing surfaces. 75 hips in 65 patients were treated with cementless MOM THA using large femoral heads (36-56 mm). The mean age of the patients was 47.4 years (29 to 59) and 54 were female. A group of 47 hips (41 patients) treated with conventional THA (screwed cup-polyethylene insert-28 mm head) was used for comparison. The study group was followed up for a mean of 62.1 months (32 to 76). No difference was found between the two groups in relation to improvement in Harris hip score (HHS) (43.1 to 90.3 points in the study group, 42.6 to 89.5 points in the control group, p>0.05). Although the preoperative range of motion in all planes were similar in both groups, the large head group demonstrated greater motion in all planes postoperatively, which was significant (all p=0.001). Additionally, there was a significant difference between groups in relation to the necessity for acetabular structural graft (8% and 31.9%, respectively; p=0.001). No major complications or adverse reactions to metal debris (ARMD) were observed in the study group. The results of large head MOM THA in young and active patients with DDH are similar to conventional THA at early follow-up, but the former offers the advantages of secure acetabular fixation without screws, greater range of motion, and lower risk of dislocation.

Serum Metal Ion Concentration: Comparison Between Small and Large Head Metal-on-Metal Total Hip Arthroplasty

The purpose of this study was to compare serum metal ion concentrations (chromium and cobalt) in 3 groups, 2 with metal-on-metal articulations and a control metal-on-polyethylene group. Forty-six patients with good to well-functioning hips were recruited for the study. Serum ion levels of all patients were drawn, and Harris Hip Score, University of California, Los Angeles activity score, and radiographs were performed. Serum chromium concentrations were significantly lower in the large head group compared with the small head group ( P = .013). There was no difference in the cobalt concentrations between the 2 groups ( P = .087). There was a significant difference between both metal-on-metal groups when compared with controls for both chromium and cobalt levels ( P = .0005 and P = .0004, control vs small; P = .001 and P = .0001, control vs large, respectively).

Effect of Sphingomyelin Headgroup Size on Molecular Properties and Interactions with Cholesterol

Sphingomyelins (SMs) and sterols are important constituents of the plasma membrane and have also been identified as major lipid components in membrane rafts. Using SM analogs with decreasing headgroup methylation, we systemically analyzed the effect of headgroup size on membrane properties and interactions with cholesterol. An increase in headgroup size resulted in a decrease in the main phase transition. Atom-scale molecular-dynamics simulations were in agreement with the fluorescence anisotropy experiments, showing that molecular areas increased and acyl chain order decreased with increasing headgroup size. Furthermore, the transition temperatures were constantly higher for SM headgroup analogs compared to corresponding phosphatidylcholine headgroup analogs. The sterol affinity for phospholipid bilayers was assessed using a sterol-partitioning assay and an increased headgroup size increased sterol affinity for the bilayer, with a higher sterol affinity for SM analogs as compared to phosphatidylcholine analogs. Moreover, the size of the headgroup affected the formation and composition of cholesterol-containing ordered domains. Palmitoyl-SM (the largest headgroup) seemed to attract more cholesterol into ordered domains than the other SM analogs with smaller headgroups. The ordering and condensing effect of cholesterol on membrane lipids was also largest for palmitoyl-SM as compared to the smaller SM analogs. The results show that the size of the SM headgroup is crucially important for SM-SM and SM-sterol interactions. Our results further emphasize that interfacial electrostatic interactions are important for stabilizing cholesterol interactions with SMs.

Adsorption of Bile Salts and Pancreatic Colipase and Lipase onto Digalactosyldiacylglycerol and Dipalmitoylphosphatidylcholine Monolayers

It is increasingly recognized that changes in the composition of the oil-water interface can markedly affect pancreatic lipase adsorption and function. To understand interfacial mechanisms determining lipase activity, we investigated the adsorption behavior of bile salts and pancreatic colipase and lipase onto digalactosyldiacylglycerol (DGDG) and dipalmitoylphosphatidylcholine (DPPC) monolayers at the air-water interface. The results from Langmuir trough and pendant drop experiments showed that a DGDG interface was more resistant to the adsorption of bile salts, colipase, and lipase compared to that of DPPC. Atomic force microscopy (AFM) images showed that the adsorption of bile salts into a DPPC monolayer decreased the size of the liquid condensed (LC) domains while there was no visible topographical change for DGDG systems. The results also showed that colipase and lipase adsorbed exclusively onto the mixed DPPC-bile salt regions and not the DPPC condensed phase. When the colipase and lipase were in excess, they fully covered the mixed DPPC-bile salt regions. However, the colipase and lipase coverage on the mixed DGDG-bile salt monolayer was incomplete and discontinuous. It was postulated that bile salts adsorbed into the DPPC monolayers filling the gaps between the lipid headgroups and spacing out the lipid molecules, making the lipid hydrocarbon tails more exposed to the surface. This created hydrophobic patches suitable for the binding of colipase and lipase. In contrast, bile salts adsorbed less easily into the DGDG monolayer because DGDG has a larger headgroup, which has strong intermolecular interactions and the ability to adopt different orientations at the interface. Thus, there are fewer hydrophobic patches that are of sufficient size to accommodate the colipase on the mixed DGDG-bile salt monolayer compared to the mixed DPPC-bile salt regions. The results from this work have reinforced the hypothesis that the interfacial molecular packing of lipids at the oil-water interface influences the adsorption of bile salts, colipase, and lipase, which in turn impacts the rate of lipolysis.

Effect of Headgroup Size, Charge, and Solvent Structure on Polymer−Micelle Interactions, Studied by Molecular Dynamics Simulations

We performed atomistic molecular dynamics simulations of anionic and cationic micelles in the presence of poly(ethylene oxide) (PEO) to understand why nonionic water-soluble polymers such as PEO interact strongly with anionic micelles but only weakly with cationic micelles. Our micelles include sodium n-dodecyl sulfate (SDS), n-dodecyl trimethylammonium chloride (DTAC), n-dodecyl ammonium chloride (DAC), and micelles in which we artificially reverse the sign of partial charges in SDS and DTAC. We observe that the polymer interacts hydrophobically with anionic SDS but only weakly with cationic DTAC and DAC, in agreement with experiment. However, the polymer also interacts with the artificial anionic DTAC but fails to interact hydrophobically with the artificial cationic SDS, illustrating that large headgroup size does not explain the weak polymer interaction with cationic micelles. In addition, we observe through simulation that this preference for interaction with anionic micelles still exists in a dipolar "dumbbell" solvent, indicating that water structure and hydrogen bonding alone cannot explain this preferential interaction. Our simulations suggest that direct electrostatic interactions between the micelle and polymer explain the preference for interaction with anionic micelles, even though the polymer overall carries no net charge. This is possible given the asymmetric distribution of negative charges on smaller atoms and positive charges on larger units in the polymer chain.

Modulating Pancreatic Lipase Activity with Galactolipids: Effects of Emulsion Interfacial Composition

It is widely known that the interfacial quality of lipid emulsion droplets influences the rate and extent of lipolysis. The aim of this work was to investigate the effect of two galactolipids, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), adsorbed at the interface on in vitro digestibility of olive oil by porcine pancreatic lipase. The experiments were performed under simulated duodenal conditions in the presence of phosphatidylcholine (lecithin) and bile salts. It was found that emulsions prepared with DGDG had a longer lag phase prior to lipase activation with a decrease in lipolysis rate. In contrast, no inhibitory effect on lipase kinetics was observed in emulsions prepared with MGDG. We postulated that the larger headgroup and more tightly packed molecular organization of DGDG at the interface gave rise to steric hindrance that retarded colipase and lipase adsorption onto the substrate surfaces and hence delayed and reduced lipolysis. It was noted that the lag phase and lipolysis rate strongly depended on the DGDG/lecithin molar ratio in the systems: the higher the molar ratio, the longer the lag phase followed by a reduced lipolysis rate. The ability of DGDG to inhibit bile salt adsorption/displacement was also investigated. The results showed that bile salts did not completely displace DGDG from the interface, explaining the reason why DGDG still possessed inhibitory activity even in the presence of bile salts at a physiological relevant concentration. The results provide interesting insights into the influence of the galactolipid headgroup and lecithin on the emulsion interfacial quality which in turn regulates the lipolysis. The findings potentially could lead to the production of generic foods and drugs designed for regulating dietary fat absorption in the prevention and treatment of obesity and related disorders.

Hydrophobically Self-Assembled Nanoparticles as Molecular Receptors in Water

Biomolecular and artificial receptors are typically designed to exploit the hydrophobic effect in order to enhance the stability of receptor-ligand complexes in water. For example, artificial receptors are often built around hydrophobic cavities. These receptors exploit the hydrophobic effect toward ligand recognition, but the structure of the binding site requires a rigid framework to overcome the hydrophobic effect-driven tendency to collapse. Here we present an artificial receptor that exploits the hydrophobic effect to define its structure in water. The receptor is based on amphiphilic building blocks that assemble into micelle-like aggregates of a very high stability, attributed to the unusual shape of the amphiphile: a relatively rigid molecule composed of a large hydrophobic segment, based on the cholesterol molecule, and a very large headgroup build around a Zn-metalloporphyrin moiety. The assemblies, persistent down to the nanomolar range, are better described as self-assembled nanoparticles. Within the nanoparticle-water interface, Zn-metalloporphyrin moieties form multiple binding sites that specifically bind ligands bearing basic nitrogen atoms. The nanoparticles show enhanced binding affinity relative to a model receptor that does not self-assemble. Structurally related ligands show a correlation between the enhancement of binding and the octanol/water partition coefficient, log P, suggesting that the desolvation of binding sites is the main driving force for the enhancement of binding affinity at the nanoparticle-water interface. In addition, the highest affinity observed for the ditopic ligands relative to the monotopic ligands is evidence of a multivalent effect operating within this type of receptors. The nanoparticle readily deassembles upon addition of water-miscible organic solvents, such as methanol, or in the presence of detergents. This approach toward self-assembled receptors can be easily adapted to the development of differential receptors by the simple expedient of mixing slightly different amphiphiles (i.e., different metals in the porphyrin ring for the amphiphiles described here) in variable proportions.

Templating Route for Mesostructured Calcium Phosphates with Carboxylic Acid- and Amine-Type Surfactants

Mesostructured calcium phosphates constructed by ionic frameworks were synthesized using carboxylic acid- and amine-type surfactants in mixed solvent systems of ethanol and water. A lamellar mesostructured calcium phosphate was prepared using palmitic acid as an anionic surfactant, as in the case using n-alkylamines. A wormhole-like mesostructured calcium phosphate can be obtained using dicarboxyl N-lauroyl- l-glutamic acid, whose headgroup is larger than that of palmitic acid. Similar mesostructured product was obtained using 4-dodecyldiethylenetriamine with a large headgroup containing two primary amine groups. Interactions of carboxyl and primary amino groups in the surfactant molecules with inorganic species are quite important for the formation of mesostructured calcium phosphates. The Ca/P molar ratio of mesostructured calcium phosphates was strongly affected by the molecular structure of surfactants containing carboxyl and primary amino groups. Ca-rich materials can be obtained using carboxylic acid-type surfactants (Ca/P approximately 1.7) rather than amine-type surfactants (Ca/P approximately 1.0).

Strong Deaggregating Effect of a Novel Polyamino Resorcinarene Surfactant on Gold Nanoaggregates under Microwave Irradiation

Gold nanoparticles were prepared by ethylene glycol (EG) reducing gold chloride under microwave irradiation. The EG-stabilized gold colloids varied from red to blue with increasing amounts of EG, due to particle aggregation. Addition of the macrocyclic polyamine 2,8,14,20-tetranonyl-4,6,10,12,16,18,22,24-octa(1-aminoethylcarbamoyl)methoxyresorcinarene (TNMR) reversed nanoparticle aggregation under microwave irradiation and greatly improved their dispersion stability in aqueous solutions. These effects are likely due to the amphiphilic nature of TNMR, which has a large hydrophilic headgroup with eight amino groups and four hydrophobic chains. Moreover, the large and flexible hydrophilic groups containing more N and O atoms in the TNMR molecule has a strong stretching and penetrating ability in the aqueous solution, and TNMR molecules can easily form a bilayer protecting structure on the surface of gold nanoparticles, which plays a critical role in the color-change process of the EG-stabilized gold colloid.

Effects of the phenolic lipid 3-pentadecylphenol on phospholipid bilayer organization

3-Pentadecylphenol (PDP) belongs to a natural group of amphiphilic phenols and plays a lot of different interesting functions. Effects on the phase behavior of bilayer mixtures of DPPC (dipalmitoyl phosphatidylcholine) or MPPC (l-myristoyl-2-palmitoyl-phosphatidylcholine) and PDP has been investigated using electron microscopy, differential scanning calorimetry (DSC) and fluorescence spectroscopy. Addition of 3-pentadecylphenol increases the phase transition temperature T m. Above 20 mole % PDP in DPPC the phase behavior becomes increasingly complex. Coexistence of various phases within the bilayer with different composition and structural properties is suggested and is responsible for the changes of the physico-chemical properties of the lipid bilayer. Excellent analogy between fluorescence and calorimetric data demonstrate that phase effects are primarily caused by the abundance of the large phenolic headgroup. DPPC and MPPC bilayers are similarly affected but MPPC is more sensitive to added PDP.

Debye-Hückel theory of mixed charged-zwitterionic lipid layers

Modeling electrostatic properties of macroions such as charged lipid membranes is especially simple within linear Debye-Hückel theory where analytical solutions are often available. Charged lipid layers typically also contain zwitterionic lipids that possess a large headgroup dipole. We incorporate the presence of zwitterionic lipids into the Debye-Hückel description and derive analytical expressions for the free energies of isolated and interacting lipid layers. Our approach accounts for two major characteristic features of zwitterionic lipids, the firm linkage of the dipole's negatively charged phosphate end to the lipid chain and the comparatively large conformational freedom of the opposite, positively charged, dipole end. This leads to differences in structural properties of mixed anionic and cationic lipid layers that are known from experiment and that the Debye-Hückel description qualitatively recovers. Most notably, this includes the different dependencies of the average cross-sectional area per lipid on the composition of mixed anionic and cationic membranes. In addition, we study the predictions of Debye-Hückel theory regarding the electrostatic influence of zwitterionic lipids on the stability of non-ideally mixed membranes as well as on the adsorption energy of oppositely charged macroions.

Structure and Thermotropic Behavior of the Staphylococcus aureus Lipid Lysyl-Dipalmitoylphosphatidylglycerol

We have characterized the structural and thermotropic properties of one of the most important lipids in the cell membrane of Staphylococcus aureus, lysyl-dipalmitoylphosphatidylglycerol (lysyl-DPPG). applying differential scanning calorimetry and small- and wide-angle x-ray scattering. Microcalorimetry revealed that under physiological conditions (phosphate buffer, 20 mM NaPi, 130 mM NaCl, pH 7.4), the synthetic lysyl-DPPG resembles the features of the parent dipalmitoylphosphatidylglycerol (DPPG) with respect to its melting behavior. However, in contrast to DPPG, lowering the pH did not significantly affect the main transition temperature (∼40°C) of lysyl-DPPG, which can be explained by its difference in protonization because of the lysine group. X-ray experiments yielded the first information on chain packing and morphology of lysyl-DPPG. We found that lysyl-DPPG forms an interdigitated lamellar phase below the chain-melting transition. This can be explained by the large headgroup area of lysyl-DPPG as a result of its charged lysine group, especially if the headgroup is arranged parallel to the bilayer plane. Additionally, lysyl-DPPG degradation products, such as lysine and free fatty acids, had significant influences on the melting behavior and led to a multicomponent melting transition. Our results indicate that the degradation of lysyl-DPPG takes place mainly during the hydration process but also depends on lipid storage time, pH, and thermal treatment. Detailed temperature-resolved experiments at pH 5.0 demonstrated the formation of a lamellar gel phase with tilted hydrocarbon chains and a ripple phase, coexisting with the interdigitated lysyl-DPPG bilayers.

Effect of Headgroup on DNA−Cationic Surfactant Interactions

The interaction behavior of DNA with different types of hydroxylated cationic surfactants has been studied. Attention was directed to how the introduction of hydroxyl substituents at the headgroup of the cationic surfactants affects the compaction of DNA. The DNA-cationic surfactant interaction was investigated at different charge ratios by several methods like UV melting, ethidium bromide exclusion, and gel electrophoresis. Studies show that there is a discrete transition in the DNA chain from extended coils (free chain) to a compact form and that this transition does not depend substantially on the architecture of the headgroup. However, the accessibility of DNA to ethidium bromide is preserved to a significantly larger extent for the more hydrophilic surfactants. This was discussed in terms of surfactant packing. Observations are interpreted to reflect that the surfactants with more substituents have a larger headgroup and therefore form smaller micellar aggregates; these higher curvature aggregates lead to a less efficient, "patch-like" coverage of DNA. The more hydrophilic surfactants also presented a significantly lower cytotoxicity, which is important for biotechnological applications.

Thermodynamics of organic cation exchange selectivity in smectites

AbstractThe selectivities of clay minerals for larger organic cations over smaller ones have been attributed to favorable clay-organic interactions in clay interlayers and to hydrophobic effects resulting from (partial) dehydration of organic cations in the clay interlayers, but the magnitudes of these energy components have not been estimated. The objective of this study was to differentiate and quantify the contributions of clay-phase and aqueous-phase energy changes to the overall thermodynamics of cation exchange, and thereby to determine which forces control the general selectivity of smectites for organic cations. We compiled literature measurements and estimates for the free energies of overall cation exchange reactions and also for the free energies of organic cation hydration. Our study suggests that organic cation-exchange thermodynamics can be broken into three classes: (1) For two organic cations with identical head-groups, the difference in their cation exchange selectivities is driven almost quantitatively by the difference in their free energies of hydration. Here, the mechanism for organic cation selectivity is almost pure hydrophobic expulsion of the larger cation from water. The clay interlayer simply behaves like a subaqueous phase into which the least hydrophilic organic cations partition and the essentials of such cation exchange selectivity can be explained without any favorable clay-organic interactions. (2) For two organic cations with rather different head-groups, the difference in their cation exchange selectivities is just a small percentage of the difference in their free energies of hydration. This indicates that the clay phase interacts much more strongly with the cation having the smaller head-group, as might be expected on the basis of simple electrostatics. Here, the clay has an intrinsic strong preference for the cation with smaller head-group yet ‘selects’ for the cation with larger head-group because the aqueous-phase preference for the cation with smaller head-group is even stronger than the clay preference. (3) When the clay is already substantially loaded with organic cations, then van der Waals forces apparently can play a significant role in determining organic cation exchange selectivity differences.