Chain Conformational Disorder Research Articles

Free volumes in mixed-tacticity poly(3-hydroxybutyrate) determined by viscosimetry and their correlations with structural features and mechanical properties

Abstract The viscosities of poly(3-hydroxybutyrate) (PHB) with mixed tacticities were determined as functions of temperature and shearing rate via plate–plate viscosimetry, and fitted by a combined Herschel–Bulkley–Cross–Doolittle model while applying Carvalho et al.’s single-point correction. They were compared to values of slit viscosimetry, obtained during processing at a temperature of 428 K. In both cases, measured values and fitted model parameters as a function of tacticity exhibited a discontinuous behaviour at a fraction of meso diads of 0.7. To further investigate, we calculated values of vibrational and excess free volume. We found that these correlate with structural and external properties, some of which were reported in earlier works, namely the paracrystalline contents and specific volumes of the materials at room temperature, as well as their energies of fracture during tensile testing. These, in turn, correlate with the statistical averages of relative lengths of chain segments of similar a- or iso-tacticity. Hence, we found that expressing changing tacticities simply by a continuous progression of the fraction of meso diads is insufficient to trace associated discontinuities of the flow behaviour in mixed-tacticity PHB. We conclude that free volume is generated by polymer chain conformational disorder, which should be treated on the oligomeric chain segment level.

Confinement and nucleation effects in poly(ethylene oxide) melt-compounded with neat and coated sepiolite nanofibers: Modulation of the structure and semicrystalline morphology

Composites have been prepared by melt compounding poly(ethylene oxide) with neat sepiolite, sepiolite coated with polyethylene glycol and with d-α-tocopherol polyethylene glycol 1000 succinate. It has been found that the nature of the sepiolite surface and its ratio in the composite enable the modulation of interfacial (nucleation) and confinement phenomena, both of which have distinct and opposed effects on the polymer’s crystalline phase. The modulation of both phenomena allows to tune the final morphology and structure of the polymer, this tuning including spherulite size, orientation of the crystalline lamellae, tortuosity of the crystalline phase, chain conformational disorder, glass transition temperature and pre-melting α′relaxation. Some of these modifications are thickness dependent, what is frequently found when the semicrystalline morphology of polymers is altered by its confinement into micrometric domains. Basic properties of the polymer such as elastic modulus and gas permeability are seen to depend very strongly on the morphology and structure modulation.

Crystal structure and microstructure of cholesteryl oleyl carbonate

The crystal structure as well as the microstructure, i.e., size and strain, of crystallites of cholesteryl oleyl carbonate was determined from X-ray powder diffraction data. The X-ray line broadening was analyzed through the refinement of TCH-pseudo-Voigt function parameters (isotropic effects) and the refinement of multipolar functions, i.e., symmetrized cubic harmonics (anisotropic effects). The crystal structure turns out to be primitive monoclinic, space group Pc, type I monolayer having two molecules per unit cell with parameters: a=18.921±0.006Å, b=12.952±0.003Å, c=9.276±0.002Å and β=91.32±0.03°. The average size of a well ground specimen of crystallites was 60nm. The average micro-strain, e.g., 45×10−4 has been tentatively attributed to fatty chain conformational disorder. The unit cell parameters, including the lamellar thickness, of COC crystal is very closely similar to those of another, structurally similar cholesterol ester, e.g., cholesteryl oleate (CO) crystal, space group P21, type II monolayer. Type I monolayer structure has been established for COC on the basis of the intensity calculations of the XRD profiles of both CO and COC. The dipolar and structural disorder in a 4:1 molar, binary mixture of CO and COC can be accommodated in an induced smectic phase with a lamellar thickness, which is nearly equal to that of pure CO or pure COC.

Wetting properties of dioleoyl-phosphatidyl-choline bilayers in the presence of trehalose: an X-ray diffraction study

The combined effect of trehalose and temperature on the wetting properties of L- α -dioleoyl-phosphatidyl-choline (DOPC) model membranes in excess aqueous solutions has been analyzed by X-ray diffraction and extended electron density map reconstruction. At room temperature, DOPC in excess water forms a fluid lamellar L α phase. In the presence of trehalose, no phase transitions occur, but repeat and intermembrane distances increase considerably. Electron density maps show that trehalose in solution promotes a straightening of the hydrocarbon chain packing and a reduction of the molecular average area at the polar–apolar interface. Accordingly, the increased intermembrane distance is interpreted as a clear indication of a sugar screening effect of the van der Waals attractive contribution in the lamellar stacks, which overcomes the eventual decreasing of the repulsive fluctuations due to the hardening of the bilayer. By contrast to the thermally induced membrane swelling observed in excess water, in the presence of trehalose the DOPC bilayer thickness and repeat and intermembrane distances decrease continuously with temperature. While the thermal dependence of bilayer thickness is a consequence of the chain conformational disorder promoted by temperature, the changes in the intermembrane distance can be explained only assuming a trehalose-induced re-setting of the long-range force balance. The final results confirm the complex mechanism by which trehalose stabilize lipid bilayers.

Nonisothermal order–disorder phase transition of alkylammonium ions in nanoconfined space

To understand the effect of confined space (interlayer space in montmorillonite (MMT)) on the nonisothermal ordering transition (chain packing) kinetics and disorder transition (chain melting) behavior including the conformational changes of the chain segment of the cationic surfactants, we have characterized MMT modified with dioctadecyl dimethylammonium (DC 18DM) ions (MMT-DC 18DM) using temperature-modulated differential scanning calorimeter (TMDSC), wide-angle X-ray diffraction (WAXD) and Fourier transform infrared spectroscopy (FTIR) techniques. For MMT-DC 18DM, the chain conformational disorder–order phase transition took place during the cooling process. The transition peak was much broader and it appeared at lower temperature ( T c) when compared to the crystallized dioctadecyl dimethylammonium bromide (DC 18DM-Br), as a reference. In MMT-DC 18DM, the formation of gauche conformers was enhanced and the chains were not as densely packed as in crystalline DC 18DM-Br. The normal crystallization took place in the bulk during the nonisothermal crystallization of DC 18DM-Br. The confined ions (DC 18DMs) in one or two dimensional order contributed to the nonisothermal chain packing for a higher cooling rate of 5.0–20.0 °C/min. The observed chain packing in confined space at different T c ranges (cooling rate > 5.0 °C/min) could be explained by much lower energy barrier.

Imaging Hindered Rotations of Alkoxy Species on TiO2(110)

We present the first scanning tunneling microscopy (STM) study of the rotational dynamics of organic species on any oxide surface. Specifically, variable-temperature STM and dispersion-corrected density functional theory (DFT-D) are used to study the alkyl chain conformational disorder and dynamics of 1-, 2-, 3- and 4-octoxy on rutile TiO(2)(110). Initially, the geminate pairs of the octoxy and bridging hydroxyl species are created via octanol dissociation on bridging-oxygen (O(b)) vacancy defects. The STM images provide time-averaged snapshots of octoxy species rotating among multiple energetically nearly degenerate configurations accessible at a given temperature. In the calculations we find that the underlying corrugated potential energy surface is a result of the interplay between attractive van der Waals dispersion forces, leading to weak attractive C...Ti and repulsive C...O(b) interactions which lead to large barriers of 50-70 kJ mol(-1) for the rotation of the octoxy alkyl chains across the O(b) rows. In the presence of the geminate hydroxyl groups we find that the relative populations of the various conformations as well as the rotational barriers are perturbed by the presence of geminate hydroxyl due to additional C...hydroxyl repulsions.

Interactions of a Rhodococcus sp. biosurfactant trehalose lipid with phosphatidylethanolamine membranes

Trehalose lipids are an important group of glycolipid biosurfasctants mainly produced by rhodococci. Beside their known industrial applications, there is an increasing interest in the use of these biosurfactants as therapeutic agents. We have purified a trehalose lipid from Rhodococcus sp. and made a detailed study of the effect of the glycolipid on the thermotropic and structural properties of phosphatidylethanolamine membranes of different chain length and saturation, using differential scanning calorimetry, small and wide angle X-ray diffraction and infrared spectroscopy. It has been found that trehalose lipid affects the gel to liquid crystalline phase transition of phosphatidylethanolamines, broadening and shifting the transition to lower temperatures. Trehalose lipid does not modify the macroscopic bilayer organization of saturated phosphatidylethanolamines and presents good miscibility both in the gel and the liquid crystalline phases. Infrared experiments evidenced an increase of the hydrocarbon chain conformational disorder and an important dehydrating effect of the interfacial region of the saturated phosphatidylethanolamines. Trehalose lipid, when incorporated into dielaidoylphosphatidylethanolamine, greatly promotes the formation of the inverted hexagonal H II phase. These results support the idea that trehalose lipid incorporates into the phosphatidylethanolamine bilayers and produces structural perturbations which might affect the function of the membrane.

Effects of dirhamnolipid on the structural properties of phosphatidylcholine membranes

Rhamnolipids are biosurfactants produced by Pseudomonas aeruginosa which are well known for their potential industrial and environmental uses. Rhamnolipids have gained considerable interest in recent years due to their potential use in cosmetics and pharmaceutics. They also show broad biological activities and have potential applications as therapeutic agents. The amphiphilic nature of rhamnolipids points to the membrane as their hypothetical site of action. We have purified dirhamnolipid and studied its interaction with phosphatidylcholine membranes, using differential scanning calorimetry, X-ray diffraction and infrared spectroscopy. It has been found that dirhamnolipid greatly affects the gel to liquid crystalline phase transition of phosphatidylcholines, broadening and shifting the transition to lower temperatures. Dirhamnolipid increases the interlamellar repeat distance of phosphatidylcholines and reduces the long-range order of the multilamellar systems. The phospholipid hydrocarbon chain conformational disorder is increased and the packing of the phospholipid molecules is perturbed in the presence of dirhamnolipid. The above evidence supports the idea that dirhamnolipid intercalates into the phosphatidylcholine bilayers and produces structural perturbations which might affect the function of the membrane.

Raman spectroscopic study of polycrystalline mono- and polyunsaturated 1-eicosanoyl-d(39)-2-eicosenoyl-sn-glycero-3-phosphocholines: bilayer lipid clustering and acyl chain order and disorder characteristics.

Polycrystalline lipid samples of a series of mono- and polyunsaturated, double bond positional isomers of 1-eicosanoyl-d(39)-2-eicosenoyl-sn-glycero-3-phosphocholines [C(20-d(39)):C(20:1 Delta(j))PC, with j = 5, 8, 11, or 13; C(20-d(39)):C(20:2 Delta(11,14))PC; and C(20-d(39)):C(20:3 Delta(11, 14,17))PC] were investigated using vibrational Raman spectroscopy to assess the acyl chain packing order-disorder characteristics and putative bilayer cluster formation of the isotopically differentiated acyl chains. Perdeuteration of specifically the saturated sn-1 acyl chains for these bilayer systems enables each chain's intra- and intermolecular conformational and organizational properties to be evaluated separately. Various saturated chain methylene CD(2) and carbon-carbon (C&bond;C) stretching mode peak height intensity ratios and line width parameters for the polycrystalline samples demonstrate a high degree of sn-1 chain order that is unaffected by either the double bond placement or number of unsaturated bonds within the sn-2 chain. In contrast, the unsaturated sn-2 chain spectral signatures reflect increasing acyl chain conformational disorder as either the cis double bond is generally repositioned toward the chain terminus or the number of double bonds increases from one to three. The lipid bilayer chain packing differences observed between the sn-1 and sn-2 chains of this series of monounsaturated and polyunsaturated 20 carbon chain lipids suggest the existence of laterally distributed microdomains predicated on the formation of highly ordered, saturated sn-1 chain clusters.

Structural and interaction properties of β‐Lactoglobulin as studied by FTIR spectroscopy

Summary New aspects of β‐Lactoglobulin (BLG) structure which have not previously been found by FT–IR spectroscopy are presented. The conformational changes of BLG and its heat‐induced denaturation have been studied at low concentration and different pHs. First, it was found that the spectra of BLG in solution are concentration‐dependent. Below 1%, they reveal one component in the 1620–1635 cm−1 region while above this concentration, two components are present. These changes are related to the modifications of the quaternary structure of BLG, i.e. to the monomeric or dimeric forms. As shown at high temperature (85 °C), this concentration (1%) represents the threshold of protein aggregation. Second, the thermal behaviour of BLG at pH 7.4 and 4.4 is compared. The results suggest that the denaturation process and the intermolecular interactions in aggregates are different and lead to two types of aggregate. These observations are in agreement with the formation of two gel microstructures near neutral pH and near the pI of BLG as observed by Stading & Hermasson (1990). Finally, interactions between BLG and two zwitterionic phospholipids have been investigated. Dipalmitoylphosphatidylcholine (DPPC) is unaffected in the presence of BLG, suggesting that no interaction occurs. In contrast, BLG increases the lipid chain conformational disorder of milk sphinglomyelin (SM) as a consequence of hydrophobic interactions of BLG with SM. Since this effect occurs at and above the gel‐to‐liquid‐crystalline phase transition, it is suggested that membrane fluidity plays an important role in these interactions.

H NMR Relaxation Study of Chain Motions and Solid-State Ordering in Stiff, Random Copolymers

Fully-aromatic, thermotropic, liquid crystalline random copolyesters of 4-hydroxybenzoic acid (HBA) and 6-hydroxy-2-naphthoic acid (HNA) were studied at elevated temperatures with 1H NMR. Proton T1 measurements provide high-frequency characterization of the glass transition. Proton rotating frame relaxation time (T1ρ) measurements near room temperature are sensitive to HBA rotational motions; these motions are expected when there is monomer length-scale chain conformational disorder. In melt-quenched samples, this disorder is spatially homogeneous (i.e., the material solidifies without crystallization). Annealing below the nominal solidification temperature (Ts) results only in a slight, uniform lessening of this disorder. For samples annealed above Ts, proton T1ρ measurements show a spatially distinct component in which monomeric rotational motions are suppressed. Spin−diffusion experiments indicate this component has a mean thickness of 10 ± 2 nm. 1H-13C (cross polarization-magic-angle spinning) CP-MAS measurements indicate that these ordered regions are enriched in HBA. The fraction and HNA content of the “cocrystallites” decrease at higher annealing temperatures. We discuss a new conceptual model of solid ordering in highly sequence-frustrated, stiff HBA/HNA copolyesters that is consistent with these and previously reported data.

Study by infrared spectroscopy of the interaction of bovine myelin basic protein with phosphatidic acid.

The effect of bovine myelin basic protein (MBP) on dimyristoylphosphatidic acid (DMPA) and phosphatidic acid prepared from egg yolk phosphatidylcholine (EPA) has been investigated by transmission and attenuated total reflectance (ATR) Fourier transform infrared spectroscopy. Interaction of MBP with DMPA and EPA dispersions decreases the lipid acyl chain conformational disorder as a consequence of hydrophobic interactions of the protein with the lipids. Since these effects are more important for EPA dispersions than for DMPA, MBP is believed to penetrate more into EPA bilayers. This could be due to the fact that the hydrogen bond network formed by the charged polar headgroups of EPA is weaker than that of DMPA. This is supported by the spectra of the phosphate region showing that the phosphate groups of EPA are less hydrogen bonded than DMPA. In the presence of MBP, the hydrogen bond network is replaced by electrostatic interactions of the protein with the polar headgroups of the lipid. Infrared spectra of the polar headgroup region also show evidence that MBP enhanced the second ionization state of the phosphate group at neutral pH, this effect being more important for EPA than for DMPA bilayers. Also, infrared spectra of the lipid carbonyl stretching region show evidence that MBP limits the accessibility of water molecules to the interfacial part of the lipid bilayer. Finally, ATR measurements on oriented films of lipid/protein complexes indicate that the penetration of the protein into the lipid bilayer is followed by a reorientation of the lipid acyl chains toward the normal to the bilayer in the case of EPA.(ABSTRACT TRUNCATED AT 250 WORDS)