Chain Asymmetry Research Articles

Dialkyl Sulfides: Novel Passivating Agents for Gold Nanoparticles

Gold nanoparticles passivated with symmetrical and unsymmetrical dialkyl sulfides (H21C10SC10H21 and H37C18SC10H21) have been synthesized via the borohydride reduction of HAuCl4 and characterized by 1H NMR, FTIR, UV-vis, Auger Electron, XPS spectroscopies, and TEM. Under equivalent conditions of formation, the size and polydispersity of the gold cores obtained was greater for dialkyl sulfide ligands (d(C10SC10) = 5.3 ± 0.8 nm; d(C18SC10) = 6.3 ± 1.1 nm) than alkanethiol ligands (d(C10H22SH) = 2.2 ± 0.1 nm). Edge-edge interparticle spacing of 2-D arrays of the nanoparticles is found to be dependent on the length of the longest alkyl chain passivating the nanoparticles and is independent of the asymmetry of the alkyl chains in the dialkyl sulfide.

Developing relational exchange

Argues that relational exchange is dependent upon both effectiveness and power considerations. The primary objective is to study the effect of power and effectiveness on the relational exchange process between voluntary retail chains and their vendors. In particular, proposes that relational exchange between the chain and the vendor is dependent on (1) the chain’s ability to obtain necessary coordination and limiting free‐riding within the chain system, as well as (2) creating a symmetrical dependence structure that fosters trust and long‐term commitment. The first issue is related to effectiveness, while the second issue concerns aspects of power. The hypotheses were tested on a sample of relations between voluntary retail chains and their suppliers in Norway. Results suggest that free‐riding within the chain, number of chain members and asymmetry of market position are negatively related to relational exchange.

The effect of ethanol on the phase transition temperature and the phase structure of monounsaturated phosphatidylcholines

Previous studies from our laboratories have delineated the relationship between the acyl chain asymmetry of mixed-chain phosphatidylcholines, C(X):C(Y)PC, and the effect of ethanol concentration, [EtOH], on the main phase transition temperature, Tm, and the phase structure of the lipid bilayer composed of C(X):C(Y)PC using differential scanning calorimetry and X-ray diffraction techniques [Huang and McIntosh, Biophys. J. 72 (1997) 2702–2709]. In the present work, we have extended these studies to characterize the effect of [EtOH] on the Tm and the phase structure of the lipid bilayer composed of sn-1 saturated/sn-2 monounsaturated phosphatidylcholines with various positions of the cis double bond. Specifically, five positional isomers of 1-eicosanoyl-2-eicosenoyl-sn-glycero-3-phosphocholines, C(20):C(20:1Δn)PC with n=5, 8, 11, 13 and 17, were synthesized and studied. For C(20):C(20:1Δn)PC with n=5 and 8, results from the calorimetric experiments showed that in response to various concentrations of ethanol, the change in Tm of the lipid bilayer composed of monounsaturated lipids was characterized by a sigmoidal or biphasic profile in the plot of Tm versus [EtOH]. In contrast, a continuous depression of the Tm by ethanol was observed calorimetrically for C(20):C(20:1Δn)PC with n≥11. The X-ray diffraction experiments further demonstrated that C(20):C(20:1Δ5)PC and C(20):C(20:1Δ8)PC can undergo the ethanol-induced gel-to-fully interdigitated phase transition at T<Tm. Such a transition, however, was not observed for C(20):C(20:1Δ13)PC even at a very high ethanol concentration of 100 mg/ml. These distinct different effects of [EtOH] on the phase transition temperature and the phase structure can be attributed to various positions of the cis double bond in these monounsaturated phosphatidylcholines. And the different effects of ethanol can, in fact, be explained based on the molecular structures of these monounsaturated lipids packed in the gel-state bilayer as generated by molecular mechanics simulations. To the best of our knowledge, this is the first time that the ethanol-induced fully interdigitated bilayers are observed at T<Tm for unsaturated phospholipids with well defined double bond positions in their sn-2 acyl chains.

Molecular Mechanics and Calorimetric Studies of Phosphatidylethanols

Phosphatidylethanols (PEths) are negatively charged diacyl phospholipids that are ubiquitously present in humans under the condition of alcohol intoxication. These lipids, derived in vivo from other naturally occurring phospholipids such as phosphatidylcholines (PC) via transphosphatidylation reaction as catalyzed by phospholipase D in the presence of ethanol, are well known to affect many biochemical properties of the cell membranes in humans. In this communication, we applied the combined approach of molecular mechanics (MM) simulations and high-sensitivity differential scanning calorimetry (DSC) to investigate the structure and phase transition behavior of PEth. We first determined the energy-minimized structures of tetrameric C(15):C(15)PEth arranged in two types of packing motif by the MM approach. An inwardly bent orientation of the lipid headgroup was observed; specifically, the methyl terminus of PEth's headgroup was juxtaposed intramolecularly to the C(2) atom of the sn-2 acyl chain. Clearly, this headgroup conformation was rather unique among all naturally occurring phospholipids. Subsequently, the phase transition behavior of the fully hydrated lipid bilayers prepared individually from 11 species of saturated C(X):C(Y)PEth with the same MW was studied by DSC, and the resulting Tm values were codified in terms of the normalized acyl chain asymmetry (ΔC/CL). A V-shaped Tm profile was observed in the plot of Tm versus ΔC/CL for each subclass of these lipids, suggesting two types of packing motif for C(X):C(Y)PEth at T <Tm. Moreover, it was observed that within each packing motif these Tm values were, on average, 2.0 ± 0.9°C smaller than the Tm values of the corresponding saturated PC. However, based on the unique headgroup conformation of PEth, we were able to predict that monounsaturated PEth with a cis double bond near the H2O/hydrocarbon interface would exhibit a higher Tm than the corresponding PC. Most interestingly, this prediction was indeed borne out by DSC results obtained with C(18):C(20:1Δ5)PC and C(18):C(20:1Δ5)PEth.

Single chain conformations in the system of symmetric and asymmetric diblock copolymers

A theory which describes a local structure and global properties of a diblock copolymer melt has been developed in the framework of the one-loop self-consistent approximation. We have derived expressions for the sizes of a single diblock macromolecule and its parts. Two different behaviors of single macromolecule conformations in the disordered melt have been obtained depending on the asymmetry of chains and morphologies occurring in ordered states after the order-disorder transition (ODT). In the nearly symmetric melt, 0.35 < f ≤ 0.5 (f is a composition), the blocks of both types shrink a little initially as the temperature decreases and then, at some temperature, they begin to swell. In strongly asymmetric melts, f < 0.35, the block of a macromolecule which consists of the monomers of minority type shrinks monotonically, while the other block monotonically swells. We have found nearly Gaussian behavior of the individual blocks and stretching near the chemical bond joining the blocks. Near the ODT the chains are stretched with a magnitude which is of the order of a few percent of their Gaussian sizes. We have calculated the peak position in the scattering curve as a function of the Flory-Huggins interaction parameter, composition and degree of polymerization. Less then 5% change in the size of copolymer molecules lead to a 25% shift of the scattering peak in comparison to the Gaussian limit. We have found a good quantitative agreement of our theoretical results with the experimental neutron scattering data.

Solid State Transitions of Asymmetric Catanionic Surfactants

A series of asymmetric surfactants were prepared from cationic surfactant, hexadecyltrimethylammonium bromide, and anionic surfactant, sodium alkyl sulfate (the number of carbon atoms per chain being 10, 12, or 14). The influence of the alkyl chain asymmetry on the thermal properties of formed hexadecyltrimethylammonium alkyl sulfates was investigated by means of polarizing microscopy, differential scanning calorimetry, and X-ray diffraction. Asymmetric catanionic surfactants exhibited a complex polymorphism and thermotropic mesomorphism from the stable crystalline form to the isotropic phase. On heating, successive phase transitions (several solid crystalline–solid crystalline, solid crystalline–liquid crystalline, and liquid crystalline–isotropic liquid) were observed. The number of polymorphs, in all of the bilayered structure, depended on the asymmetry between the lengths of surfactant tails. The basic lamellar thickness varied linearly with the increase of alkyl sulfate chain length. An increase of the basic lamellar thickness with temperature was determined. Polarizing microscopy revealed a characteristic texture of the smectic phase. On cooling, all compounds underwent reversibly the isotropic liquid–liquid crystalline transitions, while crystallization from melted samples was kinetically controlled.

Thermotropic phase behavior of mixed-chain phosphatidylglycerols: implications for acyl chain packing in fully hydrated bilayers

In this communication we report the first systematic investigation of the thermodynamic properties of fully hydrated mixed-chain phosphatidylglycerols (PG) using high-resolution differential scanning calorimetry (DSC). The crystal structure of dimyristoylphosphatidylglycerol shows an acyl chain conformation that is nearly opposite to that of phosphatidylcholine (PC). In PC, the sn-1 chain is straight while the sn-2 chain contains a bend; for PG, the sn-1 contains a bend while the sn-2 chain is in the all- trans conformation (R.H. Pearson, I. Pascher, The molecular structure of lecithin dihydrate, Nature, 281 (1978) 499–501; I. Pascher, S. Sundell, K. Harlos, H. Eibl, Conformational and packing properties of membrane lipids: the crystal structure of sodium dimyristoylphosphatidylglycerol, Biochim. Biophys. Acta, 896 (1987) 77–88). If the structure of PG found in the single crystal can be extrapolated to that in the fully hydrated gel-state bilayer, the observed difference in acyl chain conformations implies that modulation of the acyl chain asymmetry will have an opposite effect on the thermotropic phase behavior of PG and PC. For example, it is expected, based on the crystal structures, that C(15):C(13)PG should have a higher main phase transition temperature ( T m) than C(14):C(14)PG, and C(13):C(15)PG should have a lower T m than C(14):C(14)PG. However, our DSC studies show clearly that the expectation is not borne out by experimental data. Rather, the T m values of C(15):C(13)PG, C(14):C(14)PG, and C(13):C(15)PG are 18.2°C, 23.1°C, and 24.4°C, respectively. Several other PGs, each with a unique acyl chain composition, have also been studied in this laboratory using high-resolution DSC. It is shown that the acyl chain conformation of fully hydrated PG in general is nearly opposite to that seen in the PG crystal structure.

Hydrocarbon chain packing and the effect of ethanol on the thermotropic phase behavior of mixed-chain phosphatidylglycerols

Previous studies in this laboratory have delineated the relationship between the acyl chain asymmetry of mixed-chain phosphatidylcholines and the effect of ethanol concentration ([EtOH]) on their melting behavior (Li et al., Biophys J., 70 (1996) 2784–2794). This present investigation extends these findings to another phospholipid family by using high-resolution differential scanning calorimetry (DSC) to characterize the effect of ethanol concentration on the main phase transition temperature ( T m) of five molecular species of mixed-chain phosphatidylglycerol (PG). For C(14):C(18)PG, C(15):C(17)PG, C(16):C(16)PG, and C(17):C(15)PG, a biphasic profile in the T m versus [EtOH] plot was observed, and the minimum in the plot for each PG occurred at 33, 15, 19, and 36 mg/ml, respectively. This biphasic behavior is typical of phospholipids whose acyl chain asymmetry is fairly small. For C(18):C(14)PG, only a linear decrease in the T m was observed as a function of ethanol concentration; this effect is characteristic of highly asymmetric phospholipids. Our DSC results obtained with mixed-chain PG in the presence of ethanol demonstrate that the acyl chain asymmetry of the five lipids studied can be ranked as follows: C(15):C(17)PG<C(16):C(16)PG<C(14):C(18)PG<C(17):C(15)PG<C(18):C(14)PG, confirming independent results from this laboratory that the overall acyl chain structure of PG in the hydrated gel phase is opposite to that detected in the single crystals. In fully hydrated bilayers, the acyl chain conformation of PG is most likely to be similar to that of phosphatidylcholine.

Structural changes in human tear lipocalins associated with lipid binding

Structural and conformational changes in tear lipocalins were detected in association with ligand binding and release. Circular dichroism measurements demonstrated that ligand binding induces β structure formation, aromatic side chain asymmetry, and a more rigid state in tear lipocalins (TL). The exposure of the tyrosyl component is less in apo-TL than in holo-TL. The sole tryptophan residue, Trp 17, is buried in both holo- and apo-TL. The steady state exposure of Trp 17 is the same in holo- and apo-TL, but the dynamic exposure is two-fold greater in apo-TL. Maneuvers to unfold the protein with urea or incubation in an acidic environment resulted in increased exposure of aromatic amino acids. Electron paramagnetic resonance studies verified that lipids are liberated from TL in an acidic environment. Acidic pH promotes conformational changes in TL involving aromatic residues, particularly the conserved residue Trp 17. These changes are associated with lipid release. The liberation of lipid from the cavity of TL under acidic conditions involves a molten globule state of the protein. We postulate that TL, exposed to the steep surface pH gradient that exists at lipid–aqueous interfaces, would release lipid in association with a molten globule transition. The data suggest a plausible regulatory mechanism for lipid delivery from lipocalins at the tear film surface.

Selectively Swollen Phases of Diblock Copolymers

We study the aggregation of diblock copolymers in a selective solvent as a function of the asymmetry of the copolymer chains. When the shorter block is in a poor solvent, our mean-field approach predicts copolymer micellization in spherical or cylindrical geometries. For iarge collapsed blocks a dense lamellar phase is formed in an excess of solvent. A further increase of the size of the molten blocks leads to the formation of reverse micellar structures, where small spherical swollen cores are surrounded by a majority matrix of molten chains.

Protein stability: still an unsolved problem.

This brief review suggests that molecular packing, the efficient filling of space, may be the most generally applicable factor that leads to the unique structures of most globular proteins. While simple in concept, the details of packing can lead to very subtle effects. The mechanical properties of a protein, dynamics and deformations under stress, tend to be asymmetric. In terms of structural alterations and thermostability, responses to genetic mutations are context dependent and remain difficult to predict with any confidence. Through small shifts proteins can frequently accommodate major changes in composition of the core region without substantial alteration in the basic chain conformation. Extending a jigsaw puzzle analogy, all of the pieces (side chains) are convex, varying flexible, and cannot be packed together without leaving cavities. Although large cavities do occasionally occur, a relatively even distribution of empty space is more common, and the overall packing does seem to specify the unique native structure. While it might appear that the translation machinery of the cell could have been designed with any set of alpha amino acids, the packing requirements, while strong, must be flexible enough to permit nondestructive single site mutations. This flexibility, combined with the need to produce a unique structure, may limit the average number of allowed side chain rotamers per residue. This in turn will reduce the allowable asymmetry of the side chains in order to maintain the largest number of structural motifs. It may be hard to improve on current set of amino acids.

Probing the ethanol-induced chain interdigitations in gel-state bilayers of mixed-chain phosphatidylcholines

Using high-resolution differential scanning calorimetry (DSC), we have studied the effects of ethanol concentrations, [EtOH], on the main phase transition temperatures (T[m]) of the following mixed-chain phosphatidylcholines (PCs): C(15):C(17)PC, C(17):C(15)PC, and C(12):C(20)PC. These lipids have a common molecular weight; however, their apparent acyl chain-length differences between the sn-1 and sn-2 acyl chains, delta C, are distinctively different. The delta C values for these three mixed-chain PCs are, respectively, 0.5, 3.5, and 6.5 C-C bond lengths. DSC results show that the T(m) profiles for C(15):C(17)PC and C(17):C(15)PC bilayers in the plot of T(m) versus [EtOH] are V-shaped biphasic curves, with the minimum T(m) occurring at 50 and 73 mg/ml of ethanol, respectively. In contrast, the C(12):C(20)PC bilayer exhibits a nearly linear decrease in T(m) with increasing [EtOH]. In addition, x-ray diffraction experiments were also performed to assess the structural changes of these three mixed-chain PCs in the gel-state bilayers, at 20 degrees C, in response to high concentrations of ethanol. X-ray diffraction data indicate that, in the absence of ethanol, these three lamellar lipids are all packed in the normal (L beta') gel phase in aqueous media. In the presence of 120 mg/ml of ethanol, however, the C(15):C(17)PC and C(17):C(15)PC lamellae are packed in the fully interdigitated (L beta[I]) gel phase. The V-shaped T(m) curves detected calorimetrically for these two lipids in response to [EtOH] can thus be explained by the ethanol-induced L beta' --> L beta[I] isothermal phase transition. Interestingly, the results of x-ray diffraction study reveal, for the first time, that an ethanol-induced L beta' --> L(MI) (mixed interdigitated phase) isothermal phase transition occurs in the gel-state bilayer of highly asymmetrical C(12):C(20)PC. Therefore, the chain asymmetry is recognized to play an important role in the ethanol-induced chain interdigitation at T < T(m).

Computer Simulation of Asymmetric Polymer Mixtures

In the framework of the 3D bond fluctuation model the phase transition of asymmetric polymer mixtures is studied in the semi-grand-canonical ensemble by a Monte Carlo simulation. The asymmetry is realized by different chain lengths (N B /N A =2 or 3) of the two types of polymers. Monomers of the same kind interact via an attractive square well potential extended over the first peak of the pair correlation function, whereas monomers of different types repel each other. We determine the equation of state and discuss the dependence of the critical temperature and density on chain length and asymmetry. The results are related to predictions of the Flory-Huggins theory via the intermolecular pair correlation function of the macromolecular fluid

Chain Architecture and Asymmetry in Copolymer Microphases

Chain Architecture and Asymmetry in Copolymer Microphases

Effect of alkyl chain asymmetry on the fusion and crystallization behavior of vesicles formed from di-n-alkyl phosphates

Fusion of vesicles formed from synthetic, asymmetric (i.e mixed-chain) sodium di-n-alkyl phosphates (1-6) has been studied with a resonance energy transfer assay for lipid mixing and with transmission electron microscopy. Fusion was induced by Ca2+ ions above the Lalpha --> Lbeta phase transition temperatures of the different bilayers We ha e found that two competing events take place simultaneously but independently from each other These are fusion and formation of tubules, which have been characterized as anhydrous crystals. Both processes are fast. The asymmetric di-n-alkyl phosphates with alkyl chains that differ by only two methylene groups (1 and 2)showed initial fusion rates comparable to that of vesicles formed from (symmetric) di-n-dodecyl phosphate. With an increase in the difference in chain length (3, 4, and 5), the fusion rates increase, indicating that chain asymmetry decreases bilayer stability by reducing alkyl chain packing in the bilayer. However, when the difference in chain length becomes eight methylene groups (6), fusion is effectively suppressed. The sizes of the crystals formed from 1 to 6 turned out to be rather different, but no simple relationship could be found with the extent of asymmetry of the alkyl chains.

Analysis of the bilayer phase transition temperatures of phosphatidylcholines with mixed chains

The analysis of the chain-length dependence of the chain-melting transition temperatures of bilayers composed of lipids with identical chains (Marsh, D. 1991. Biochim. Biophys. Acta. 1062: 1-6) is extended to include lipids with chains of unequal length. The bilayer transition temperatures of saturated asymmetrical phosphatidylcholines are interpreted by assuming that the transition enthalpy and transition entropy are linearly related to the absolute value of the difference in chain length between the sn-1 and sn-2 chains, with constant end contributions. Such an assumption is supported by calorimetric data on phosphatidylcholines of constant mean chainlength and varying chain asymmetry. In particular, a symmetrical linear dependence is observed on the chain asymmetry, Deltan, which is centered around a value Deltan degrees that corresponds to the conformational inequivalence of the sn-1 and sn-2 chains. The transition temperature then takes the form: T(t) = T(t) (infinity)(n - n(H) - h' Deltan + Deltan degrees )/(n - n(s) - s' Deltan + Deltan degrees ) where n(H), n(s) are the end contributions, and h', s' are fractional deficits in the incremental transition enthalpy and entropy, respectively, arising from the overlapping regions of the longer chains. Optimization on the transition temperature data for the dependence on chain asymmetry of three series of phosphatidylcholines with constant mean chainlength, n, yields parameters that are capable of predicting the dependence of the transition temperatures on chain asymmetry for other mean chainlengths. The dependence of the transition temperature on mean chainlength for phosphatidylcholines in which the chain asymmetry is maintained constant, as well as the dependence on both mean chain length and chain asymmetry for phosphatidylcholines in which one of the two chains is maintained of constant length, are also described with high accuracy by using the same parameters.

How membrane chain-melting phase-transition temperature is affected by the lipid chain asymmetry and degree of unsaturation: an effective chain-length model

Hydrocarbon effects on the lipid chain-melting phase-transition temperature are analyzed. The membrane fluidization temperature is shown to increase with the effective chain length, which is proportional to the thickness of the well-packed hydrocarbon region. The latter, as a rule, increases with the length of the longest ordered and aligned segment on each chain. This conclusion is independent of the cause for the reduced chain packing in membrane interior: chain unsaturation (which effectively decouples the two hydrocarbon segments disjoined by a double bond) or chain asymmetry (which causes the terminal hydrocarbon segments to lose close contact) both affect the bilayer chain-melting phase-transition temperature comparably on the effective chain-length scale. Thermodynamic consequences of the trans unsaturation are approximately 50% smaller than the effects of the double bonds in the cis conformation, owing to the smaller membrane perturbation by the former double bonds. A simple quantitative model is introduced for the analysis of the phospholipid chain-melting phase behavior. This new model permits quantitative predictions of the chain-melting transition temperature solely on the basis of the known lipid chemical composition. It also explains lipid sensitivity to the hydrocarbon type and attachment. The model agreement with the experimental data is usually better than to within 99% and thus comparable to experimental scatter, even when only a few or no adjustable parameters are used. The membrane fluidization temperature is calculated for a number of potentially interesting, also as yet unexplored, phospholipids, and the biological significance of the effective chain-length concept is discussed.

Small-angle neutron scattering studies on chain asymmetry of coextruded poly(vinyl alcohol) film

La mesure du rayon de giration du PVA avant et apres coextrusion montre que la deformation du PVA est affine jusqu'a un taux d'etirage de 5, en accord avec les mesures de birefringence

Polymer shapes in two, four, and five dimensions

The average asymmetry of two-, four-, and five-dimensional linear and ring polymers, with and without excluded volume, is investigated numerically via Brownian dynamics. It is found that the average asymmetry of nonexcluded volume chains is in good agreement with theoretical predictions in all dimensions studied. The distribution of the average asymmetry is also determined. This distribution indicates that two-dimensional ring chains are likely to be nearly symmetrical. Moreover, the distribution of the maximum eigenvalues of the tensor of the moment of inertia is also in agreement with the theoretical predictions.

Phospholipid fatty acyl chain asymmetry in the membrane bilayer of isolated skeletal muscle sarcoplasmic reticulum.

We previously showed [Herbette, L. G., Blasie, J. K., DeFoor, P., Fleischer, S., Bick, R. J., Van Winkle, W. B., Tate, C. A., & Entman, M. L. (1984) Arch. Biochem. Biophys. 234, 235-242; Herbette, L. G., DeFoor, P., Fleischer, S., Pascolini, D., Scarpa, A., & Blasie, J. K. (1985) Biochim. Biophys. Acta 817, 103-122] that the phospholipid head-group distribution in the membrane bilayer of isolated sarcoplasmic reticulum is asymmetric. From these studies, both the total number of phospholipid head groups and the total lipid, as well as the head-group species for these lipids, were found to be different for each monolayer of the membrane bilayer. In this paper, we demonstrate for the first time that there is significant asymmetry in the distribution of unsaturated fatty acids between the two monolayers; i.e., the outer monolayer of the sarcoplasmic reticulum contained more unsaturated and polyunsaturated chains when compared to the inner monolayer. X-ray diffraction measurements demonstrated that the time-averaged fatty acyl chain extension for the outer monolayer was approximately 20% less than for the inner monolayer. This is consistent with the concept that the greater degree of unsaturation in the outer monolayer may provide for a decreased average fatty acyl chain extension for that layer. This architecture for the bilayer may be related to both the "resting" state mass distribution of the calcium pump protein within the membrane bilayer and possible "conformational" states of the calcium pump protein during calcium transport by the sarcoplasmic reticulum.