Head Groups Of Amphiphiles Research Articles

The Molecular Recognition of Dipeptide by Oligoglycyl Head Group of Amphiphile: A Quantum Chemical Study

In the present work, we presented an analysis of the unusual recognition specificity exhibited by marked difference in the binding behavior of dipeptide with amphiphilic head group when subtle relative change of N-terminal and C-terminal of the dipeptide are made. Recently, in a series of detailed experiments, binding of aqueous dipeptides, GlyX and X/Gly (X = Leu, Phe, Pro, Ala; X/ = Leu, Phe) with dialkyl oligoglycyl amphiphiles is studied [X. Cha, K. Ariga, M. Onda, and T. Kunitake, J. Am. Chem. Soc. 117, 11833 (1995)]. It is observed that GlyX are specifically bound to 2C18BGly2NH2 while X/Gly are insignificantly bound. We first studied the conformational energy variation of GlyPhe, PheGly and model of 2C18BGly2NH2 amphiphile using semi-empirical and ab-initio methods in vacuum. Using the individual energy optimized monomer structure of amphiphile and peptide, we studied the binding energy of optimized GlyPhe: amphiphile pair and PheGly: amphiphile pair structures at 1:1 and 1:2 ratio at the same level of theory using a population of structures. Binding of GlyPhe is favorable over the binding of PheGly at various levels of theory (semi-empirical and ab-initio). It is noted that the hydrogen bonding pattern in the GlyPhe binding is more effective than that in the PheGly binding. In the population of low energy structures, PheGly: amphiphile structures have more exposed area around the hydrophobic Phe group than the GlyPhe: amphiphile structures. Relatively more PheGly: amphiphile structures have intermolecular orientation unsuitable to contribute to the population of head group structures relevant in aqueous interface. Summarizing, significantly better binding capacity of GlyPhe over the PheGly with amphiphile, is due to the difference in hydrogen bonding interaction pattern, hydrophobic effect and possible orientations of the amphiphile and peptide at interface, relevant to the condensed phase monolayer structure. All the three factors cooperatively lead to favorable recognition of GlyPhe over PheGly as observed in experiment. A calculation of the ratio of the amphiphile-peptide bound complex to the initial concentration of the amphiphile indicates that the diffusional process at the peptide interfaces could be significantly influenced by hydrogen bonding.

A Low Temperature Phase Transition in Langmuir-Blodgett Films

The influences of temperature on the SFG spectra of Langmuir-Blodgett films of cadmium stearate, ferric stearate, stearic acid and octadecanamide are reported. Upon cooling, all films display reversible discontinuous shifts of approximately 8 cm (-1) in the r+, r- and rfermi modes of the terminal methyl groups at approximately 150 K. Reversible changes in the relative intensities of these methyl group peaks, most pronounced in the PPP spectra, are also observed and attributed to a change in the environment of the methyl group that accompanies a discontinuous transition in the ordering of their alkyl chains. The onset of new spectral features at higher frequency is attributed to the observation of ordered water molecules contained within the films. The correlation between the onset of the water features and the onset of the reversible, discontinuous, spectroscopic changes of the amphiphiles argues for a causal connection between the two. In addition to the discontinuous behavior upon cooling, monolayer films of stearic acid and octadecanamide display activity of methylene modes upon exposure to vacuum. Films displaying SFG-active methylene groups at room temperature had them gradually become completely SFG-inactive by 100 K. Heating the films to room temperature revealed that the methylene group activity was reversible. Monolayer films of cadmium stearate and ferric stearate do not display this methylene activity upon exposure to vacuum, suggesting that this behavior may be linked to solvation of the amphiphile's headgroup. These observations suggest that water plays a key role in the stability and structure of LB supported monolayers, and have important implications to those interested in low temperature (cryogenic) effects of biological systems.

Polymeric Amphiphile Branching Leads to Rare Nanodisc Shaped Planar Self-Assemblies

Self-assembly is fundamental to the biological function of cells and the fabrication of nanomaterials. However, the origin of the shape of various self-assemblies, such as the shape of cells, is not altogether clear. Polymeric, oligomeric, or low molecular weight amphiphiles are a rich source of nanomaterials, and controlling their self-assembly is the route to tailored nanosystems with specific functionalities. Here, we provide direct evidence that a particular molecular architecture, polymeric branching, leads to a rare form of self-assembly, the planar nanodisc. Cholesterol containing self-assemblies formed from amphiphilic linear or branched cetyl poly(ethylenimine) (Mn approximately 1000 Da) or amphiphilic cetyl poly(propylenimine) dendrimer derivatives (Mn approximately 2000 Da) show that branching, by reducing the hydrophilic headgroup area, alters the shape of the self-assemblies transforming closed 60 nm spherical bilayer vesicles to rare 50 nm x 10 nm planar bilayer discs. Increasing the hydrophilic headgroup area, by the inclusion of methoxy poly(ethylene glycol) moieties into the amphiphilic headgroup, transforms the planar discs to 100 nm spherical bilayer vesicles. This study provides insight into the key role played by molecular shape on molecular self-organization into rare nanodiscs.

Indole, the aromatic element of tryptophan, as a pi-donor and amphiphilic headgroup

Abstract Indole, the aromatic sidechain element of tryptophan, may play many biological roles. It may serve as an anchor or organizing element, a hydrogen bond donor, or a Lewis basic donor in the cation–pi sense to alkali metal or ammonium cations. Organic chemistry has been used to develop model systems that probe these interactions and give experimental data concerning their importance.

Ion induced lamellar-lamellar phase transition in charged surfactant systems

We propose a model for the liquid-liquid (L(alpha)-->L(alpha(') )) phase transition observed in osmotic pressure measurements of certain charged lamellae-forming amphiphiles. The model free energy combines mean-field electrostatic and phenomenological nonelectrostatic interactions, while the number of dissociated counterions is treated as a variable degree of freedom that is determined self-consistently. The model, therefore, joins two well-known theories: the Poisson-Boltzmann theory for ionic solutions between charged lamellae and the Langmuir-Frumkin-Davies adsorption isotherm modified to account for charged adsorbing species. Minimizing the appropriate free energy for each interlamellar spacing, we find the ionic density profiles and the resulting osmotic pressure. While in the simple Poisson-Boltzmann theory the osmotic pressure isotherms are always smooth, we observe a discontinuous liquid-liquid phase transition when the Poisson-Boltzmann theory is self-consistently augmented by the Langmuir-Frumkin-Davies adsorption. This phase transition depends on the area per amphiphilic head group, as well as on nonelectrostatic interactions of the counterions with the lamellae and interactions between counterion-bound and counterion-dissociated surfactants. Coupling the lateral phase transition in the bilayer plane with electrostatic interactions in the bulk, our results offer a qualitative explanation for the existence of the L(alpha)-->L(alpha(') ) phase transition of didodecyldimethylammonium bromide (DDABr), but the transition's apparent absence for the chloride and the iodide homologs. More quantitative comparisons with experiment require better understanding of the microscopic basis of the phenomenological model parameters.

Surfactant tail length-dependent lipase activity profile in cationic water-in-oil microemulsions

The catalytic activity of Chromobacterium viscosum lipase (CV-lipase) was estimated across varying surfactant tail lengths (C-10–C-18) in water-in-oil (w/o) microemulsions of cationic surfactants containing four different hydroxyethyl-substituted head groups. An attempt to find a correlation, if any, between the activity of interfacially solubilized lipase and the varying surfactant tails was made for the first time in micellar enzymology. The second-order rate constant, k 2 , in lipase-catalyzed hydrolysis of p-nitrophenyl- n-hexanoate at pH 6.0 and 25 °C shows an improvement in enzyme activity (∼30–140%) across different head groups of amphiphiles with increasing tail lengths in varying solution compositions. Improvement of enzyme activity is prominent in ascending from C-10 to C-14/C-16, depending on the nature of polar head group. The hydrolytic activity of lipase in different surfactant (50 mM)/water/isooctane/ n-hexanol with varying z= [alcohol]/[surfactant] (6.4 or 4.8) was amplified by 25–250% with increment in surfactant tail length in comparison with widely used cationic w/o microemulsions having solution compositions ( z = 16 ) . As a notable outcome of this research, we found w/o microemulsions of 25 mM tetradecyltrimethylammonium bromide/water/isooctane/ n-hexanol ( z = 8 ) producing the highest ever activity of lipase in any w/o microemulsions.

Inhibition Effect of Hydroxamic and Phosphonic Acids Langmuir-Blodgett Films on Iron Corrosion in Sodium Perchlorate Solution

Nanolayers, prepared by Langmuir-Blodgett techniques, are potential replacements of currently applied surface inhibitors. Protective effects of octadecanoyl hydroxamic- and octadecyl phosphonic acid Langmuir-Blodgett films were studied in neutral sodium perchlorate solution. The protection efficiency of these monolayers on an Armco iron surface was characterized by electrochemical methods. Both types of nanolayers effectively decreased the corrosion processes at the metal surface. The difference between the efficiencies of the applied layers could be explained on the basis of interactions between their amphiphile head groups with the Armco iron surface. The size of the head group of octadecyl phosphonic acid is larger than that of octadecanoyl hydroxamic acid, which can be attributed to the better protective efficiency of octadecanoyl hydroxamic acids.

Solubility of amphiphiles in membranes: influence of phase properties and amphiphile head group

The solubilities of two fluorescent lipid amphiphiles with comparable apolar structures and different polar head groups, NBD-hexadecylamine and RG-tetradecylamine (or -octadecylamine), were compared in lipid bilayers at a molar ratio of 1/50 at 23 °C. Bilayers examined were in the solid, liquid-disordered, or liquid-ordered phases. While NBD-hexadecylamine was soluble in all the examined bilayer membrane phases, RG-tetradecylamine was stably soluble only in the liquid-disordered phase. RG-tetradecylamine insolubility in solid and liquid-ordered phases manifests itself as an aggregation of the amphiphile over a period of several days and the kinetics of aggregation were studied. Solubility of these amphiphiles in the different phases examined seems to be related to the dipole moment of the amphiphile (in particular, of the polar fluorophore) and its orientation relative to the dipolar potential of the membrane. We propose that amphiphilic molecules inserted into membranes (including lipid-attached proteins) partition into different coexisting membrane phases based upon: (1) nature of the apolar structure (chain length, degree of saturation, and chain branching as has been proposed in the literature); (2) magnitude and orientation of the dipole moment of the polar portion of the molecules relative to the membrane dipolar potential; and (3) hydration forces that are a consequence of ordering of water dipoles at the membrane surface.

Ligand accessibility as means to control cell response to bioactive bilayer membranes.

We report a new method to create a biofunctional surface in which the accessibility of a ligand is used as a means to influence the cell behavior. Supported bioactive bilayer membranes were created by Langmuir-Blodgett (LB) deposition of either a pure poly(ethylene glycol) (PEG) lipid, having PEG head groups of various lengths, or 50 mol % binary mixtures of a PEG lipid and a novel collagen-like peptide amphiphile on a hydrophobic surface. The peptide amphiphile contains a peptide synthetically lipidated by covalent linkage to hydrophobic dialkyl tails. The amphiphile head group lengths were determined using neutron reflectivity. Cell adhesion and spreading assays showed that the cell response to the membranes depends on the length difference between head groups of the membrane components. Cells adhere and spread on mixtures of the peptide amphiphile with the PEG lipids having PEG chains of 120 and 750 molecular weight (MW). In contrast, cells adhered but did not spread on the mixture containing the 2000 MW PEG. Cells did not adhere to any of the pure PEG lipid membranes or to the mixture containing the 5000 MW PEG. Selective masking of a ligand on a surface is one method of controlling the surface bioactivity.

Aggregate formation from 3-alkylindoles: amphiphilic models for interfacial helix anchoring groups

Indole can function as an amphiphile headgroup, but the counter-intuitive observation that 3-substituted indoles form less stable aggregates than the N-substituted isomers has been addressed by use of a Langmuir–Blodgett trough.

Azacrown Ethers as Amphiphile Headgroups: Formation of Stable Aggregates from Two- and Three-Armed Lariat Ethers

Ten 18-membered ring lariat ether compounds have been prepared as one-, two-, and three-armed derivatives of aza-, diaza-, and triaza-18-crown-6. They include N-[[(3-cholestanyloxy)carbonyl]methyl]aza-18-crown-6, 1, N-tetradecylaza-18-crown-6, 2, N,N‘-dibutyl-4,13-diaza-18-crown-6, 3, N,N‘-dinonyl-4,13-diaza-18-crown-6, 4, N,N‘-didodecyl-4,13-diaza-18-crown-6, 5, N,N‘-ditetradecyl-4,13-diaza-18-crown-6, 6, N,N‘-dioctadecyl-4,13-diaza-18-crown-6, 7, N,N‘-bis[[(3-cholestanyloxy)carbonyl]methyl]-4,13-diaza-18-crown-6, 8, N,N‘-bis[[(3-cholestanyloxy)carbonyl]decyl]-4,13-diaza-18-crown-6, 9, and N,N‘,N‘‘-tri-n-hexyl-4,10,16-triaza-18-crown-6, 10. Compounds 2, 8, and 9 are previously unreported. Aqueous suspensions of these monomers were sonicated, and the first evidence for stable aggregates formed from diaza and triaza lariat ethers (3−10) was obtained. The formation of aggregates from 3 or 10 is especially notable since the side chains are butyl or hexyl, respectively. The aggregates were studied by a combin...

Competition between Monovalent and Divalent Counterions in Surfactant Systems

Competition between mono- and divalent ions in the association of counterions to the headgroups of amphiphiles was studied in one surfactant system with organic counterions (piperidine+/piperazine2+octanesulfonate) and one with inorganic counterions (Na+/Ca2+octyl sulfate). By conductivity and13C NMR chemical shift measurements the critical micelle concentration (CMC) was found to decrease drastically when small amounts of divalent counterions were present in the system. Self-diffusion coefficients of surfactant ions and organic counterions were measured in the micellar phase by the Fourier transform pulsed-gradient spin-echo (FT-PGSE) NMR method. The degree of counterion binding in the micellar system with piperidine+/piperazine2+counterions was obtained from FT-PGSE NMR measurements. It was observed that the divalent counterions were more strongly bound than the monovalent counterions. The experimental results were compared with theoretical Poisson–Boltzmann calculations. The cell model was used to study the electrostatic effects. Good agreement between electrostatic theory and experiment was observed; however, an attractive force exists between the monovalent piperidine counterions and the micelle, probably because of hydrophobic interactions.

Light-scattering study of phase transitions in aqueous solutions of nonionic amphiphiles.

The dynamical behavior of water in aqueous solutions of the nonionic polyoxyethylene surfactant ${\mathrm{C}}_{10}$${\mathrm{H}}_{21}$(${\mathrm{OCH}}_{2}$${\mathrm{CH}}_{2}$${)}_{5}$OH has been studied using Raman and depolarized Rayleigh-wing scattering, with particular attention to changes due to the structural phase transitions apparent at relatively large amphiphile concentrations. The frequencies and intensities of the two principal components evident in Raman spectra of the water O-H stretching band change at the hexagonal to isotropic phase transition. The width and intensity of the single resolvable line in the depolarized Rayleigh spectra similarly change at this transition: the relaxation time of the rotational water mode increases in the hexagonal phase. The changes are all consistent with an increase in the hexagonal phase of the proportion of water bound to the oxyethylene head groups: the environment of water in the system differs significantly from that in bulk water. At the lamellar to isotropic phase transition very much smaller changes were apparent, but only in the Raman scattering data. This lesser sensitivity suggests that in the lamellar phase almost all of the water is bound to the amphiphile head groups, as in the isotropic phase at such volume fractions. The changes in water dynamics reflect the structural changes in the solutions at the phase transitions, in particular the differing packing constraints in the various phases.

Interaction and orientation of a chiral solute in cholesteric lyotropic mesophases

Abstract Cholesteric lyotropic mesophases are prepared by addition of 2,3:4,6-di-O-isopropylidene-α-L-sorbofuranose or diacetone-sorbose, DAS, to nematic matrices based on several amphiphiles. The orientation of the DAS solute is investigated by deuterium NMR of the labelled DAS-d 6 derivative. This compound was obtained by exchanging one acetone ligand for perdeuteriated acetone. The resulting deuteriated methyl groups are observed as coincident or distinguishable doublets. Quadrupolar splitting values are used for the calculation of parameters of the solute's Saupe ordering matrix, defined in a molecule fixed coordinate system. Decylsulphate lyomesophases containing both caesium and sodium counter-ions are also investigated. For these mesophases, caesium-133 NMR spectra of the counter-ion and deuterium NMR of the residual HDO are also obtained. The present work points out that the DAS ordering in lyomesophases is dependent on the charge and nature of the amphiphile headgroup. The exchange of Na+ for Cs+ counter ions in decylsulphate mesophases affects the anisotropic properties of the liquid crystalline medium, changing the ordering of the DAS solute.

Stabilization of Asphaltenes in Aliphatic Solvents Using Alkylbenzene-Derived Amphiphiles. 1. Effect of the Chemical Structure of Amphiphiles on Asphaltene Stabilization

Stabilization of crude oil asphaltenes in apolar alkane solvents was investigated using a series of alkylbenzene-derived amphiphiles as the asphaltene stabilizers. In this paper (i.e., part I), we present the study on the influences of the chemical structure of these amphiphiles on the effectiveness of asphaltene solubilization and on the strength of asphaltene-amphiphile interaction using both UV/vis and FTIR spectroscopies. The results showed that the amphiphile's effectiveness of asphaltene stabilization was primarily controlled by the polarity of the amphiphile's head group and the length of the amphiphile's alkyl tail. Increasing the acidity of the amphiphile's head group could promote the amphiphile's ability to stabilize asphaltenes by increasing the acid-base attraction between asphaltenes and amphiphiles. On the other hand, although decreasing the amphiphile's tail length increased the asphaltene-amphiphile attraction slightly, it still required a minimum tail length (six carbons for p-alkylphenol amphiphiles) for amphiphiles to form stable steric layers around asphaltenes. We also found additional acidic side groups of amphiphiles could further improve the amphiphile's ability to stabilize asphaltenes. The effect of the molecular weight of alkane solvents on the amphiphile's ability to stabilize asphaltenes was also studied. 18 refs., 12 figs., 3 tabs.

Combination of polycation/polyanion self-assembly and Langmuir-Blodgett transfer for the construction of superlattice films

The Langmuir-Blodgett transfer (LB) of amphiphile monolayers from the air /water interface onto solid supports is a powerful tool to create periodic lattices in many varieties, usually composed of bilayers. By self-assembly adsorption (SA) of polyelectrolytes, uniform molecular films with very exact thickness adjustment are obtaincd. The combination of Langmuir-Blodgett transfer and polycation/polyanion self-assembly allows a precise extension of the LB lattice, and a superlattice is formed by combination of LB and SA units. The superlatticc was realized in a multicomponent film, in which additional polyelectrolyte sheets are inserted between the amphiphile headgroups

Lamellar liquid crystal and other phases in the system sodium dodecyl sulfate, hexylamine, water, and heptane

Phase equilibria have been studied for the system composed of sodium dodecyl sulfate, hexylamine, heptane, and water. Greatest emphasis has been placed on the lamellar liquid crystal region and X-ray diffraction measurements. Equations are developed or modified to allow determination of the degree of penetration of water into the amphiphilic double layer as a function of water or amphiphile concentration. It is found that the interfacial area occupied by the polar or ionic head groups of amphiphiles increases with increasing water content.