摘要:

AbstractThe self‐assembly of lipid molecules into lamellar liquid‐crystalline bilayer membranes was studied using a statistical thermodynamic method elaborating aScheutjens‐Fleer‐like approach. In the modelling much attention was paid to the two‐tail character and the complex head‐group structure of the lipids that comprise the bilayer.Zwitterionic head groups like phosphatidylcholine (PC) are, on average, oriented nearly flat onto the interface. At not too low ionic strengths, we find that the PC head group has two preferred conformations. Ionic head groups like phosphatidylserine (PS) are tilted towards the solution. These two predictions are consistent with experimental data.Both co‐ and counter‐ions penetrate the head‐group region, compensating for part of the head‐group charges. Usually the counter‐ions adsorb preferentially. However, at high salt concentrations, no positive adsorption of ions is found in dimyristoylphosphatidylserine (DMPS) bilayers. Due to the lateral pressure in the head‐group area, all ions are depleted. Since negative ions are more depleted than positive ones, charge compensation is still achieved.In order to investigate how the membrane structure can be modulated, we modified the lipids by adding a hydrophilic chain onto the end of the PC head group. There are two effects. First, the increase of the bulkiness of the head group reduces the number of lipids per unit area in the membrane so that the size of the hydrophobic core diminishes. Second, we predict that the orientation of the PC part of the head group is altered non‐monotonically. Increasing the electrostatic screening promotes the out‐of‐plane tilting of the head groups toward entropica

ISSN:0165-0513    

DOI:10.1002/recl.19941130402

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractA microemulsion system has been developed that simultaneously destroys three environmental contaminants by combined oxidative/hydrolytic pathways. The method is rapid, cheap, and mild. No special equipment (stirrer, heater, photolyzer, etc.) is required. Self‐organization converts a seemingly intractable 8‐component mixture into a useful system, a tactic discovered by living cells long

ISSN:0165-0513    

DOI:10.1002/recl.19941130403

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractWe have tested the predictions of four thermodynamic models of micelle formation against experimental enthalpies of micelle formation of model surfactants across a range of temperatures. The mass action (MA) approach agrees best with experimental observations.The change in thermodynamic parameters of micelle formation with temperature has been interpreted according to a two‐step model of hydrogen bonds in water. This model explains the trends correctly but is open to criticism regarding the choice of parameters.We give a brief review of our own computer simulations of self‐assembly which suggest that non‐hydrogen‐bonded liquids are solvents in which self‐assembly can take place too. Other experimental evidence lends support to the conclusion that a sufficiently large difference between the solvent and solute cohesiveness parameters is enough for self‐assembly to occur i

ISSN:0165-0513    

DOI:10.1002/recl.19941130404

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractThe synthesis and aggregation behaviour of simple dialkylamine‐imidazole and dialkylamine‐pyrazole surfactants is described. In the presence of one equivalent of HCl the imidazole surfactants form stable vesicular dispersions. Addition of copper(II) dichloride to these dispersions leads to formation of metallo‐vesicles in which four imidazole groups are coordinated to one copper(II) ion. These metallo‐vesicles are unstable and are slowly converted into stacked bilayers, which precipitate. The pyrazole surfactants on the other hand do not form vesicles but give, with one molar equivalent of HCl, water‐insoluble stacked bilayers. The pyrazole moieties in these stacked bilayers do not coordinate copper(II) ions. This behaviour is most probably due to the formation of an intramolecular hydrogen bond in the mono‐protonated pyrazole surfactant molecule. The imidazole surfactants cannot form such a hy

ISSN:0165-0513    

DOI:10.1002/recl.19941130405

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractThe aggregation behaviour of a synthetic C4phospholipid, containing two phosphate groups and two stearoyl groups, has been studied. The surfactant formed different types of aggregates in water depending on the concentration, the temperature and the treatment of the samples. Tilted bilayers were found at high surfactant concentrations, whereas at low surfactant concentrations intercalated bilayers could be identified. The intercalated structures could be converted into very small vesicles upon sonication. Tube‐like aggregates were formed after storage at low temperature, which coagulated into thread‐like superstructu

ISSN:0165-0513    

DOI:10.1002/recl.19941130406

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractThe assembling properties of single‐chain ammonium amphiphiles containing benzylideneaniline and salicylideneaniline units as rigid segments in the formation of aqueous bilayers, cast multi‐bilayers, and surface monolayers were studied. Electron microscopy indicated that aggregate morphologies observed in water, such as vesicles and fibers, were dependent on the nature of the aromatic unit and the alkyl‐chain length of the component amphiphiles. The patterns of molecular assembly in bilayers were examined by absorption spectroscopy of aqueous bilayers and X‐ray diffraction of cast multi‐bilayer films. The patterns were virtually identical to those of the corresponding azobenzene bilayers, and commonly determined by combinations of alkyl‐chain lengths. This alkyl‐chain combination was crucial for formation of stable surfa

ISSN:0165-0513    

DOI:10.1002/recl.19941130407

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractHybrid molecular assemblies were prepared as combinations of a synthetic lipid, involving an L‐alanine residue interposed between an anionic head group and a hydrophobic double‐chain segment, with macrocyclic hosts, cage‐type cyclophanes having a rigid molecular framework and peptide cyclophanes bearing four flexible branches. Each host was completely incorporated into a relatively polar domain close to the vesicular surface, as confirmed by ultrasensitive differential‐scanning calorimetry and dynamic‐light‐scattering measurements as well as by gel‐filtration chromatography and electronic‐absorption spectroscopy. Perturbation effects of the hosts on the membrane structure were characterized on the basis of steady‐state fluorescence polarization, evaluated using a fluorescent probe, 1‐[4‐(trimethylammonio)phenyl]‐6‐phenyl‐1,3,5‐hexatriene iodide, and thermodynamic parameters associated with the phase transition between gel and liquid‐crystalline states. The cage‐type cyclophanes caused a disturbance of the orderly bilayer lipid membrane in the liquid‐crystalline phase due to their rigid conformational framework, while the peptide cyclophanes generated little effect on the lipid fluidity. The hybrid molecular assembly formed with the lipid and the cage‐type host exercised effective molecular recognition toward α‐amino acids with an aromatic moiety through hydrophobic, π‐π and chirality‐based interactions; L‐ and D‐tryptophan were strongly incorporated into the host cavities as confirmed by1H‐NMR spectroscopy. On the other hand, the hybrid assembly showed no capacity to bind al

ISSN:0165-0513    

DOI:10.1002/recl.19941130408

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractTwo steroid cyclophanes, having individually L‐lysine and L‐aspartate residues as connector units interposed between a 1,6,20,25‐tetraaza[6.1.6.1]paracylophane skeleton and four cholate moieties, were designed and synthesized. The cationic steroid cyclophane, having L‐lysine residues, binds anionic and non‐ionic guests very efficiently, while it has no capacity to bind a guest with a positive charge in aqueous solution. On the other hand, the anionic steroid cyclophane, bearing L‐aspartate residues, shows good binding affinity toward hydrophobic guests in aqueous solution regardless of their charged states. Aggregate morphology of the cationic and anionic peptide lipids, involving an L‐alanine residue interposed between a charged head moiety and a hydrophobic double‐chain segment, in the sonicated vesicular state was not perturbed significantly upon formation of hybrid assemblies with the steroid cyclophanes in 2.5 mol%. Even though the anionic bilayer vesicle interacts only weakly with anionic guests, the corresponding hybrid assembly formed with the cationic steroid cyclophane is capable of marked molecular recognition of anionic guests, along with shape‐sensitive discrimination, through electrostatic and hydrophobic interactions in aqueous solution. In a similar manner, the cationic bilayer membrane alone is incapable of binding a cationic guest. However, the guest‐binding ability is not much enhanced in the presence of the anionic steroid cyclophane. Consequently, the cationic steroid cyclophane can act as an efficient cell‐surface receptor model for anionic guests while the anionic steroid cyclophane is not a good receptor model when both are embedded

ISSN:0165-0513    

DOI:10.1002/recl.19941130409

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractThe micellar and lyotropic phase behavior of three isomeric bola‐amphiphiles1, 2and3based onpara‐, meta‐andortho‐phenylene derivatives with hydrophilic pyridinium headgroups was investigated. The micellar behavior was characterized by surface‐tension and conductivity measurements: while thepara‐substituted amphiphile1and theorthoisomer3aggregated to micelles, themeta‐substituted amphiphile2exhibited stepwise, but discontinous self‐association. This is explained by the mass‐action and phase‐separation model. The existence of “premicellar” aggregates and the discontinous aggregation of2is evidenced in addition by a difference‐spectro‐photometric measurement. The low surface activity and the calculated areas per molecule of 100–200 Å2at the air‐water interface agree with a loop‐like conformation of all three bola‐amphiphiles1–3.Lyotropic mesophases were found in the high‐concentration regime only for theparaisomer1, while2formed crystalline‐isotropic phases and3isotropic phases only. The mesophase of1was characterized by X‐ray measurements as lamellar.The addition of 1‐hexanol as a co‐surfactant led to the induction of a lyotropic mesophase for2. The role of the cosurfactant consists of filling the wedge between the two hyd

ISSN:0165-0513    

DOI:10.1002/recl.19941130410

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY

摘要:

AbstractThe effects of the salting‐out effectiveness of electrolytes and of the salting‐out resistance of surfactants on the physical stability of lamellar phase systems, consisting of sodium dodecylbenzenesulphonate, a co‐surfactant, an electrolyte and water, have been investigated. A correlation was found between (i) the electrolyte concentration necessary to induce salting‐out/phase separation of a micellar solution of the co‐surfactant and (ii) the maximum electrolyte concentration which can be incorporated into a lamellar phase system formed from the co‐surfactant before physical instability is observed. High concentrations of salting‐out electrolyte can be added to lamellar phase systems before instability is observed, when the co‐surfactant has a high resistance against salt

ISSN:0165-0513    

DOI:10.1002/recl.19941130411

出版商:WILEY‐VCH Verlag    

年代:1994

数据来源: WILEY