Mobility Of Alkyl Chains Research Articles

IR spectroscopy analysis of pancreatic lipase-related protein 2 interaction with phospholipids: 1. Discriminative recognition of mixed micelles versus liposomes

Guinea pig pancreatic lipase-related protein 2 (GPLRP2) is an interesting model enzyme that can hydrolyze a large set of acylglycerols in vitro but displays however some selectivity depending on the supramolecular structure of substrate and the presence of surfactants like bile salts. We showed that GPLRP2 hydrolyzes 1,2-dipalmitoyl phosphatidylcholine (DPPC) present in mixed micelles with sodium taurodeoxycholate (NaTDC) but not in multilamellar (MLV) and large unilamellar (LUV) vesicles of DPPC. After characterization of these lipid aggregates by dynamic light scattering (DLS), the discriminative recognition of DPPC in DPPC/NaTDC micelles versus MLV and LUV by an inactive variant (S152G) of GPLRP2 to avoid the effect of substrate hydrolysis was investigated using Fourier transform infrared spectroscopy (FTIR). IR spectra were recorded after hydrogen/deuterium exchange, at pD 6 and various temperatures to study phase transitions. We analyzed the methylene asymmetric stretching (ν(CH2)as), the carbonyl stretching (ν(CO)) and the composite polar head-group vibration bands, first to characterized differences in DPPC micelles and vesicles, and second to estimate the degree of interaction of GPLRP2 S152G with phospholipid. Our results indicate that a significant interaction between GPLRP2 S152G and DPPC is only observed when NaTDC is added to the system to form micelles and this can be explained by the different organization of DPPC in mixed micelles compared to lamellar vesicles (higher hydration of polar head, higher mobility of alkyl chains) that favors GPLRP2 penetration into the phospholipid layer.

Interfacial and mechanical properties of epoxy composites containing carbon nanotubes grafted with alkyl chains of different length

Dispersion of the nano-fillers is important in the manufacture of high performance nanocomposites. Dispersion of silane treated CNT/epoxy composites was analyzed for various alkyl chain lengths. Difference in mobility of alkyl chains of different length affects the excluded free volume and repulsive forces. An optimal chain length was observed to impart maximum mechanical and impact properties. Differences in the dispersion of CNT due to alterations in alkyl chain lengths were determined using electrical resistance measurement. Spreading of CNT/epoxy rearranged the CF tow resulting in a change in electrical resistance ratio (CERR) and the differences in alkyl chain length effected CNT spreading. Dispersion and wetting conditions with different alkyl chain lengths resulted in changes in CERR, ILSS and other proprieties. The butyl chain length was optimal of the alkyl chain lengths studied. This is attributed to the butyl chains known good mobility and steric repulsion effects compared to longer alkyl chain lengths.

Structure and dynamic properties of water saturated CTMA-montmorillonite: molecular dynamics simulations

The interlayer structure of wet organoclays has attracted great interest due to its close relation with adsorption process but still remains ambiguous at the atomic level. In this study, the structure and dynamics of cetyltrimethylammonium (CTMA+) intercalated montmorillonite (Mt) in the water saturated condition were investigated by molecular dynamics (MD) simulations. Two Mt models with different cation exchange capacity (CEC) were selected and various amounts of CTMA+ were added to achieve different loading levels based on experiments. The simulation results show that as the surfactant loading level increases, the arrangement of CTMA+ transforms from bilayer to inclined paraffin-type with a large amount of water in the interlayer space, indicating a different configuration from dry systems. The conformations of CTMA+ are influenced by water molecules and surfactant loading level. Compared to the dry models, the percentage of gauche conformations of CTMA+ decreases in the water saturated condition. In the cases of incomplete cation exchange, the confinement from silicate surface may lead to more ordered structure of alkyl chains. In the cases of excessive surfactant loading, the percentage of gauche conformation decreases due to the steric hindrance. Moreover, the ammonium head groups of CTMA+ are found to locate close to the center of six-member ring of silicate surface and coordinated with 4–6 water molecules. Their mobility is low due to the electrostatic interactions while the alkyl chains show higher mobility. Mt with a higher CEC has a stronger confining effect on both alkylammonium and water, which then reduces the mobility of alkyl chains and water molecules within the interlayer space.

First detection of lamella-gyroid-cylinder phase transition of neat polyethylene-poly(ethylene oxide) diblock copolymers on the basis of synchrotron WAXD/SAXS and infrared/Raman spectral measurements

The phase transition behaviour of polyethylene-b-poly(ethylene oxide) (PE-b-PEO) diblock copolymer with relatively short chain lengths has been studied on the basis of temperature dependent infrared and Raman spectral measurements and synchrotron WAXD/SAXS simultaneous measurements, from which the concrete structural changes were deduced successfully from the various levels of molecular chain conformation, chain packing mode and higher-order structure. The higher-order structure has been found to transform between lamella, perforated lamella, gyroid, cylinder and sphere structures. The inner structural changes occurring in the polyethylene and poly(ethylene oxide) parts have been related with these morphological changes. The morphological transition from lamella to gyroid occurs with keeping the crystalline state of polyethylene parts. This apparently curious transition can be interpreted reasonably by assuming the thermally-activated chain motion in the crystal lattice, which may play an important role as a trigger to induce the morphological change from lamella to gyroid. This idea was supported by the measurement of half-width of Raman anti-symmetric CH2 stretching band sensitive to the thermal mobility of alkyl chains.

Effect of the Phase States of Self-Assembled Monolayers on Pentacene Growth and Thin-Film Transistor Characteristics

To investigate the effects of the phase state (ordered or disordered) of self-assembled monolayers (SAMs) on the growth mode of pentacene films and the performance of organic thin-film transistors (OTFTs), we deposited pentacene molecules on SAMs of octadecyltrichlorosilane (ODTS) with different alkyl-chain orientations at various substrate temperatures (30, 60, and 90 degrees C). We found that the SAM phase state played an important role in both cases. Pentacene films grown on relatively highly ordered SAMs were found to have a higher crystallinity and a better interconnectivity between the pentacene domains, which directly serves to enhance the field-effect mobility, than those grown on disordered SAMs. Furthermore, the differences in crystallinity and field-effect mobility between pentacene films grown on ordered and disordered substrates increased with increasing substrate temperature. These results can be possibly explained by (1) a quasi-epitaxy growth of the pentacene film on the ordered ODTS monolayer and (2) the temperature-dependent alkyl chain mobility of the ODTS monolayers.

Relationship between lipid peroxidation and rigidity in L- α-phosphatidylcholine-DPPC vesicles

DPPC incorporation into egg-PC unilamellar vesicles reduces their oxidation rate beyond that expected from the unsaturated lipid dilution. Addition of the unsaturated lipids produces changes in the physical properties of the inner parts of the lipid bilayer, as sensed by fluorescence anisotropy of DPH, and in the hydrophilic/hydrophobic region, as sensed by the generalized polarization of laurdan. DPPC (30 mol%) incorporation into egg-PC vesicles produces a decrease in alkyl chain mobility in the inner part of the bilayer, evaluated by the increase of DPH fluorescence anisotropy, and a rise of the generalized polarization value of laurdan in the bilayer interface. It also leads to a decrease in the rate of water efflux promoted by a hypertonic shock. Oxidation of PC LUVs, promoted by AAPH, as sensed by oxygen uptake and MDA formation, leads to qualitatively similar results than DPPC addition: rigidification at the inner part and the surface of the liposomes, and a lower rate of water permeation. It is suggested that these changes could contribute to the observed decrease in oxidation rate with conversion.

Thermally Induced Reorganization of Redshifted Band in Merocyanine–Cd Arachidate Mixed Langmuir–Blodgett Films

The effect of heat treatment at 60–70°C has been studied for merocyanine (MS)–Cd arachidate mixed Langmuir–Blodgett (LB) films. MS chromophores are found to be reorganized by the heat treatments to form a novel phase of a redshifted band with spectra sharper than those of the J-band in the as-deposited films. The reorganization of MS seems to be closely related with the mobility of alkyl chains in the LB system, suggesting that the mild heat treatments will introduce another possible method of modifying the properties of films.

A new method for evaluation of the mobility of silica-grafted alkyl chains by suspension-state 1H NMR

Although molecular mobility is usually probed by measurement of the T1 relaxation time, it is not the case for methylene groups of grafted acyl chains because each methylene group has a different mobility and hence, a different T1 relaxation time depending on the distance from the junction between organic molecule and nonorganic particle. Therefore, we devised a very simple method that can semiquantitatively determine the extent of mobility in the grafted acyl chains. Silica-supported poly(octadecylacrylate) (Sil-ODAn), polymeric octadecylsilyl silica (polymeric ODS), and monomeric octadecylsilyl silica (monomeric ODS) were studied by comparing the intensity of NMR peaks from grafted molecules with the same amount of nongrafted molecules. In the case of Sil-ODAn, with a gradual increase in temperature, the intensity of the proton signals (1H NMR) of the octadecyl moieties (mainly methylene groups) rose dramatically. This dramatic rise was at the same temperature as that of an endothermic peak detectable in its DSC thermogram, indicating a relatively complete solid to liquid phase transition. Sil-ODAn, as the stationary phase in RP-HPLC, showed analogous temperature dependencies of the separation factor between naphthacene and triphenylene (as a simple indicator of shape selectivity). Using this method we determined the liquid phase percentage (LPP) in Sil-ODAn, polymeric ODS, and monomeric ODS at various temperatures. We suggest LPP as a semiquantitative index of mobility for grafted organic layers.Key words: comb-shaped polymer, nanocomposite, alkyl chain mobility, NMR, liquid chromatography.

Molecular Simulation of the Interlayer Structure and the Mobility of Alkyl Chains in HDTMA+/Montmorillonite Hybrids

Molecular simulation techniques are used to find the basal spacing of organoclay on the basis of the energy minimum, using the canonical NVT ensemble. Then, the interlayer structure and mobility of alkyl chains are explored based on the interlayer atomic density profiles. Besides the basic lateral-monolayer arrangement, lateral-bilayer accompanied by partial a pseudo-trilayer and a transition structure between the two basic lateral models are observed. The later provides an excellent explanation about the reflection at 16 angstroms on XRD patterns in the literature. The atomic density profiles reveal that nitrogen atoms show stronger layering behavior than carbon atoms do. Our simulation demonstrates that the molecular mobility of the confined alkyl chains decreases from lateral-monolayer to lateral-bilayer with the increase of the intercalated surfactant. This is in accordance with the suggestion deduced from experiments. Furthermore, our simulation indicates that the mobility of the alkyl chains strongly depends on the surfactant arrangement rather than the surfactant packing density.

Investigation on conformational order and mobility of DiamondBond-C18 and C18-alkyl modified silica gels by Fourier transform infrared and solid-state NMR spectroscopy

The effect of surface coverage and solid supports on the conformational order of alkyl chains of commercially available carbon clad zirconia based supports and synthesised C18-alkyl modified silica based supports are probed in the dry state for the first time using variable temperature Fourier transform infrared (FT-IR) and solid-state 13C NMR spectroscopy. From FT-IR spectroscopy, the conformational order of alkyl chains tethered to the substrates is examined by the analysis of CH 2 symmetric and anti-symmetric stretching bands. Through solid-state 13C NMR spectroscopy, the order is inferred from the relative intensity of the main methylene carbon resonance assigned to trans and trans-gauche conformations. It is found that molecules tethered to the graphite layer experience a strongly diamagnetic component of the highly anisotropic magnetic susceptibility of the graphite lattice, which reflects upfield shift in the 13C NMR spectra of commercially available octadecyl-modified carbon clad zirconia based column materials. The present results prove that temperature, surface coverage and solid supports have an influence on the conformational order and mobility of alkyl chains tethered to the carbon clad inorganic metal oxides.

Characterization of organic phases in the interlayer of montmorillonite using FTIR and 13C NMR

The molecular conformation and mobility of the intercalated surfactant molecules cetyltrimethylammonium bromide (CTMAB) have been studied using Fourier transform infrared spectroscopy (FTIR) and high-resolution single-pulse 13C magic angle spinning nuclear magnetic resonance ( 13C SP MAS NMR) spectroscopy. The conformation and mobility of alkyl chains were found to be a function of the surfactant concentration. The splitting of the methylene scissoring mode at 1473–1463 cm −1 and the rocking mode at 730–720 cm −1 in FTIR are considered to be diagnostic of the packing density increase of the intercalated surfactants within the clay gallery. Compared with the 13C SP MAS NMR spectrum of CTMAB in the bulk state, 1–3 ppm upfield chemical shifts for end-methyl ( δC 16) and methylene ( δC 15, δC 2–14) of the intercalated surfactant molecules in the hybrids indicate a freer conformational situation. For these hybrids, the conformational freedom decreases with increased of surfactant concentration. In addition, on ∼2 ppm downfield shift for the C 1 carbon atom in the hybrids with higher surfactant content suggests a special local environment. This study demonstrates the different mobility of carbon atoms in the intercalated alkyl chain.

Elasticity of vesicles assessed by electron spin resonance, electron microscopy and extrusion measurements

The composition of vesicles determines the physical state and elasticity of their bilayers. Fatty acid spin labels were incorporated into vesicles, composed of the single chain non-ionic surfactant octaoxyethylenelaurate-ester (PEG-8-L), the sucrose laurate-ester l-595 and cholesterol sulfate (CS) to monitor local dynamic properties of lipid molecules in vesicle bilayers and to study the elasticity of vesicle bilayers. Studies with the spin label probes 5-, 12- and 16-doxyl stearic acid (DSA) indicated that both the order parameter and the rotational correlation times increased when the doxyl group was positioned closer to the headgroup region. These findings indicate that the fluidity of membranes decreased near the headgroup region. Comparing 16-DSA incorporated in vesicle formulations with either 30 or 70 mol% showed no difference in alkyl chain mobility as was reflected by the order parameter. The rotational correlation times, however, showed a slowdown from 0.38 to 0.71 and 1.13 ns when the PEG-8-L molar content was decreased from 100 to 70 and 30 mol% for PEG-8-L:L-595:CS vesicles, respectively. Extrusion measurements indicated an increase in elasticity of vesicle bilayers as the molar content of PEG-8-L was increased from 10 to 90 mol%. Incorporation of cholesterol sulfate stabilizes vesicles and thereby, decreases the elasticity. The increased elasticity correlated excellent with a reduction in the rotational correlation times observed. In conclusion, these results demonstrate that when the molar content of the single chain non-ionic surfactant PEG-8-L in vesicles is increased the elasticity is enhanced and the rotational correlation time is reduced. The enhanced elasticity might contribute to an optimal design of vesicles as drug carriers for transdermal application.

An Infrared Study on Order-Disorder Transitions in Evaporated Films of 2-Dodecyl-, 2-Pentadecyl-, and 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane. A Comparison of the Transitions with Those in the Corresponding Langmuir-Blodgett Films

Order-disorder transitions of evaporated films of 2-dodecyl-, 2-pentadecyl-, and 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (dodecyl-TCNQ, pentadecyl-TCNQ, and octadecyl-TCNQ) have been investigated by using infrared spectroscopy. Four kinds of evaporated films have been prepared for octadecyl-TCNQ ; two of them have been deposited on CaF 2 plates at 5 and 15 °C and the other two on Au-evaporated glass slides at the same temperatures. For all the evaporated films of octadecyl-TCNQ, a CH 2 symmetric stretching band of the hydrocarbon chain shows an abrupt upward shift near 125 °C, revealing that the order-disorder transition occurs near 125 °C irrespective of the substrates used and temperatures at which the sample has been deposited. However, the molecular orientation of the hydrocarbon chain in the evaporated films depends upon the substrates and temperatures. The molecular axis of the hydrocarbon chain is nearly parallel with the substrate surface with its molecular plane nearly perpendicular to the surface in the evaporated films of octadecyl-TCNQ deposited on the CaF 2 plate and Au-evaporated glass slide at 5 °C. On the other hand, it is tilted with respect to the normal in the films prepared on the Au-evaporated glass slide and CaF 2 plate at 15°C, respectively. There is no or little pretransitional change in the frequency of the CH 2 stretching symmetric band, but its bandwidth undergoes pretransitional alterations for all the octadecyl-TCNQ evaporated films. These observations lead us to the conclusion that the alkyl chain mobility begins to increase before their conformational disorder takes place. As in the case of corresponding Langmuir-Blodgett (LB) films, the order-disorder transition of the octadecyl-TCNQ evaporated film deposited on the CaF 2 plate at 15 °C occurs at a higher temperature (125 °C) than that (121 °C) of the dodecyl-TCNQ evaporated film prepared under the same conditions, but the evaporated film of pentadecylTCNQ with the odd-numbered hydrocarbon chain exhibits a transition temperature (121 °C) similar to that of the dodecyl-TCNQ evaporated film with the shorter even-numbered hydrocarbon chain. Cyclic temperature treatment up to 120 °C performed for the octadecyl-TCNQ evaporated film deposited on the CaF 2 plate at 5 °C changes the molecular orientation largely ; the hydrocarbon chain becomes nearly perpendicular to the substrate surface after the cycle to 120 °C.

Molecular properties of a stratum corneum model lipid system: large unilamellar vesicles

Stratum corneum lipids are relatively complex, and there is little detailed understanding of their chemical and physical properties at the molecular level. Large unilamellar vesicles (LUVs) with lipid compositions similar to those of stratum corneum were prepared at pH 9 with commercially available lipids. This system was used as a model system for molecular studies of stratum corneum lipids. LUVs were chosen as the model system as they are comparatively more stable and can be characterized more quantitatively in terms of lipid concentration, surface area, and volume than model systems such as lipid mixture suspensions, lipid films, and small unilamellar vesicles. Results from freeze-fracture and cryo electron microscopy studies of our LUVs showed spherical vesicles. Quasi-elastic light scattering measurements revealed a narrow size distribution, centering around 119 nm. At room temperature, the LUVs were stable for several weeks at pH 9 and for more than 15 h but less than 24 h at pH 6. Differential scanning calorimetry measurements indicated broad endothermic transitions centered near 60-65 degrees C, closely matching the transition temperature reported for stratum corneum lipid extracts. Spin probes, 5-doxylstearic acid and 12-doxylstearic acid, were used for electron paramagnetic resonance (EPR) studies of the molecular dynamics of the lipids. EPR results indicated more restricted motion near the polar headgroup region than near the center of the alkyl chain region. Motional profiles of the spin labels near the polar headgroup and within the alkyl chain region in the LUVs were obtained as a function of temperature, ranging from 25 to 90 degrees C. We also found that the partitioning between the lipid and aqueous phases for each spin probe was temperature dependent and was generally correlated with phase transitions observed by differential scanning calorimetry and with alkyl chain mobility observed by EPR. Thus, this LUV system is well suited for additional molecular studies under different experimental conditions.

The stratum corneum lipid thermotropic phase behavior

The stratum corneum, the outermost layer of mammalian skin, is considered the least permeable skin layer to the diffusion of water and other solutes. It is generally accepted that the intercellular lipid multilayer domain is the diffusional pathway for most lipophilic solutes. Fluidization of the lipid multilayers is believed to result in the loss of barrier properties of the stratum corneum. Current investigations address the lipid thermotropic phase behavior in terms of lipid alkyl chain packing, mobility and conformational order as measured by Fourier transform infrared (FTIR) spectroscopy. A solid-solid phase transition is observed with increased alkyl chain mobility followed by a gel to liquid-crystalline phase transition near 65°C. These results further elucidate the role of lipid fluidity that may contribute to the transport properties of the stratum corneum.

IR study of the cholesteryl n-alkanoates conformation mobility

IR-spectroscopy methods have been used to study conformational mobility of alkyl chains of cholesteryl alkanoates in different phases. It is shown that cholesteryl alkanoates may be divied into three groups with similar conformation and packing of alkyl chains in solid state and mesomorphism types within each of them.

Slow motions undergone by surfactant molecules in micelles. A 1H and 14N nuclear magnetic relaxation study

Surfactant motion in spherical micelles of the system cetyltrimethylammonium bromide/D 2O has been investigated by 1H and 14N longitudinal relaxation at different frequencies. Such measurements allow extraction of the correlation time characterizing the overall reorientation of the surfactant molecule, which includes micelle tumbling and lateral diffusion around the micelle. The proton data, which reflect the alkyl chain mobility, require the definition of a local director, distinct from the normal to the aggregate surface, thus making possible the occurrence of an additional slow motion. Conversely 14N data can be analyzed accord- ing to the classical two-step model; this yields a correlation time associated with the slow motion of as ≈5 ns leading to a value of 4× 10 −7 cm 2 s −1 for the lateral diffusion coefficient.

Mobility of alkyl chains in quaternary ammonium salts

Selective 13C relaxation times have been measured in the tetraalkylammonium ions of general formula N(Rm )4 +, with m = 1, 2, 3, 4, 6 and 8 carbon atoms. The experimental data are related to the frequencies of the conformational transitions involving specific carbon atoms, averaged over the possible conformations of the moving chain. The transition rates are calculated from a detailed model of random walk among conformational sites, requiring computation of the geometry-dependent hydrodynamic friction and the cooperative effects resulting from coupling of the rotational coordinates.

Alkyl chain flexibility in discotic columnar mesophases

A steric model with a single adjustable parameter is employed to compute configurationally averaged quadrupolar splittings of deuterium labeled alkyloxy chains in a homologous series of hexaalkyloxytriphenylenes (THEn). A comparison with new experimental data [Goldfarb et al., J. Chem. Phys. (in press)] permits inferences about the alkyl chain flexibility in discotic mesogens. The parameter, a control of the constraints on the chain mobility, is first optimized by fitting deuterium NMR data in a homolgous series of nematogens with labeled alkyloxy chains. The same optimum value of the parameter is subsequently used to model the quadrupolar splittings in the columnar phase of THEn discotigens and succeeds in qualitatively accounting for the experimental observations. This result indicates that alkyl chain mobility is quite comparable in these two classes of liquid crystals. The distinctly different features of the quadrupolar splitting patterns observed for nematogens and columnar discotigens are shown to originate from the different orientation of the molecular order tensor relative to the mean direction of chain propagation in the two types of mesogens.

A highly alpha-helical structure protein in sarcoplasmic reticulum membranes.

AMONG the various protein fractions isolated from sarco-plasmic reticulum (SR) membranes, MacLennan et al.1 demonstrated the existence of a proteolipid of molecular weight about 12,000, that is, a protein which is soluble in 2 : 1 chloroform–methanol mixture2,3. Recent spin-labelling (P. L. and M. D. Barratt, unpublished) and black-film experiments (P. L. and D. E. Graham, unpublished) have provided evidence that the proteolipid resides in the hydrocarbon interior of the phospholipid bilayer and reduces the alkyl-chain mobility. The apparent lack of any specific action in the Ca2+-transport process of SR suggests that the proteolipid merely provides structural support, possibly by stabilising the vectorial character of the transport system within the dynamic phospholipid matrix. Considering the widespread presence of proteolipids in biological membranes3, structural information about this type of intrinsic protein is of particular interest. With intrinsic membrane proteins the choice of methods for obtaining such direct information is restricted by the fact that in the native state the protein is associated with lipid molecules in aggregates of comparatively large dimensions. In this situation data on the secondary structure from measurement of the circular dichroism may be used to infer a model for the arrangement of the protein within the membrane.