Longer Mixing Times Research Articles

The Identification and Location of Succinyl Residues and the Characterization of the Interior Arabinan Region Allow for a Model of the Complete Primary Structure of Mycobacterium tuberculosis Mycolyl Arabinogalactan

The complex cell wall of Mycobacterium tuberculosis is the hallmark of acid fast bacteria and is responsible for much of its physiological characteristics. Hence, much effort has been made to determine its primary structure. Such studies have been hampered by its extreme complexity. Also, its insolubility leads to difficulties determining the presence or absence of base labile groups. We have used an endogenous arabinase to solubilize the arabinan region of the cell wall and have shown using mass spectrometry and NMR that succinyl esters are present on O2 of the inner-branched 1,3,5-alpha-d-arabinofuranosyl residues. In addition, an inner arabinan region of 14 linear alpha-1,5 arabinofuranosyl residues has been identified. These and earlier results now allow the presentation of a model of the entire primary structure of the mycobacterial mycolyl arabinogalactan highlighted by three arabinan chains of 31 residues each.

Enhanced relative BOLD signal changes in T2‐weighted stimulated echoes

The origin of the stimulus/task-induced signal changes in spin echo (SE) functional MRI (fMRI) at high magnetic fields is dynamic averaging due to diffusion in the presence of field gradients surrounding deoxyhemoglobin-containing microvasculature. The same mechanism is expected to be operative in stimulated echoes (STE). Compared to SE-fMRI, however, STE-fMRI has the potential for larger diffusion weighting and consequently larger stimulus/task-induced signal changes as a result of an additional delay, the mixing time, T(M). In the present study, functional signal changes were quantified for both primary echo (PRE) and STE as a function of echo and mixing time. The relative blood oxygenation level dependent (BOLD) signal changes in STE were larger than in PRE at the same echo time and increased with both mixing and echo time. The contrast-to-noise ratio (CNR) of the STE, however, is close to the CNR of the PRE, indicating an increase of physiological noise with longer mixing times. In addition, the signal attenuation due to diffusion in the presence of magnetic field gradients near blood vessels was modeled using Monte Carlo simulations. They support the hypothesis that the sensitivity of the STE to fluctuations of susceptibility-induced magnetic field gradients near microvasculature is enhanced as a result of an extended diffusion time.

1H and 13C solid-state NMR of Gossypium barbadense (Pima) cotton

The interaction of water with cellulose and its influence on the nuclear spin dynamics in Gossypium barbadense (Pima) cotton were investigated by 1H and 13C solid-state NMR techniques. 1H spin diffusion results from a Goldman-Shen experiment indicate that the water is multilayered. 1H MAS experiments provide evidence of a range of correlation times for the water, indicative of molecular motion ranging from restricted to relatively mobile. The 1H spin-lattice relaxation time varies with water content and is different for static and MAS conditions. By coupling the Goldman-Shen sequence with 13C CP/MAS, cross-polarization from the molecularly mobile water protons distributes magnetization throughout the cellulose (as opposed to enhancing 13C resonances from only the crystalline or the amorphous domains or from only the surface of the cellulose). However, spatial localization of the combined Goldman-Shen- 13C CP/MAS experiment using both short mixing and contact times yields a spectrum consistent with predominantly the I β polymorph of cellulose. Longer mixing times and the same, short contact time yield a spectrum that is indicative of an increased I α polymorph content in the crystallite interiors relative to the smaller values found with short mixing times.

Four-dimensional heteronuclear correlation experiments for chemical shift assignment of solid proteins

Chemical shift assignment is the first step in all established protocols for structure determination of uniformly labeled proteins by NMR. The explosive growth in recent years of magic-angle spinning (MAS) solid-state NMR (SSNMR) applications is largely attributable to improved methods for backbone and side-chain chemical shift correlation spectroscopy. However, the techniques developed so far have been applied primarily to proteins in the size range of 5-10 kDa, despite the fact that SSNMR has no inherent molecular weight limits. Rather, the degeneracy inherent to many 2D and 3D SSNMR spectra of larger proteins has prevented complete unambiguous chemical shift assignment. Here we demonstrate the implementation of 4D backbone chemical shift correlation experiments for assignment of solid proteins. The experiments greatly reduce spectral degeneracy at a modest cost in sensitivity, which is accurately described by theory. We consider several possible implementations and investigate the CANCOCX pulse sequence in detail. This experiment involves three cross polarization steps, from H to CA[i], CA[i] to N[i], and N[i] to C'[i-1], followed by a final homonuclear mixing period. With short homonuclear mixing times (<20 ms), backbone correlations are observed with high sensitivity; with longer mixing times (>200 ms), long-range correlations are revealed. For example, a single 4D experiment with 225 ms homonuclear mixing time reveals approximately 200 uniquely resolved medium and long-range correlations in the 56-residue protein GB1. In addition to experimental demonstrations in the 56-residue protein GB1, we present a theoretical analysis of anticipated improvements in resolution for much larger proteins and compare these results in detail with the experiments, finding good agreement between experiment and theory under conditions of stable instrumental performance.

Thermo‐Mechanical Degradation of LDPE‐Based Nanocomposites

AbstractThermo‐mechanical degradation of LDPE‐based nanocomposites was studied by mainly investigating the rheological properties. For all of the investigated processing conditions, the viscosity of the nanocomposites was higher than that of the pure‐LDPE matrix, but on increasing the severity of the mixing conditions, the difference between the viscosity of the nano‐filled polymer and that of the pure LDPE decreased. The X‐ray traces of the nanocomposites suggest that intercalation has been achieved during the melt, when less‐severe processing conditions were used. At severe processing conditions (longer mixing time, high temperature and shear stress) the thermo‐mechanical degradation was accelerated, possibly due to the loss of mass from the organoclay galleries. The variations of the viscosity in the presence of two organo‐modified montmorillonite (MMt) clays were compared to the ones observed with a MMt clay at different processing conditions.magnified image

Friction of fullerene-like WS2 nanoparticles: effect of agglomeration

Inorganic fullerene-like (IF) solid lubricant nanoparticles with extremely useful tribological characteristics have been realized, offering a plethora of new applications for these nanomaterials. The IF nanoparticles were found to be in the aggregated state. The main goal of this work is to elucidate the effect of the mixing time of IF-WS2 nanomaterial in the oil on the size of the IF aggregates and their influence on the friction and wear. The fraction of small aggregate increases and that of the large aggregates decreases with longer mixing time. Consequently, the spread of the tribological results diminishes with the lengthening of the mixing time. The reproducibility of the friction results for the pairs lubricated with oil +IF nanoparticles is determined by distribution of the IF aggregates in the lubricant and the size of the solid lubricant aggregates.

Nonadiabatic Effects in the Two-Dimensional Infrared Spectra of Peptides: Application to Alanine Dipeptide

A method of simulating two-dimensional infrared spectra accounting for nonadiabatic effects is presented. The method is applied to the amide I modes of a dipeptide. The information necessary to construct the time-dependent Hamiltonian for the system is extracted from molecular dynamics simulations using a recently published ab initio-based model. It is shown that the linear absorption spectrum agrees with experiment only if the nonadiabatic effects are accounted for. The two-dimensional infrared spectrum is predicted for a range of mixing times. It is shown that population transfer between the amide I site vibrations affects the anisotropy at longer mixing times. It is also demonstrated that the population transfer can, to a good approximation, be extracted from the simulated spectra using a procedure that should also be applicable to experimental spectra.

Revisiting the initial rate approximation in kinetic NOE measurements

The nuclear Overhauser effect (NOE) is undoubtedly one of the most useful tools in NMR spectroscopy and is widely used in solving structural and conformational problems of small organic molecules and macromolecular systems alike. In particular, measurement of the kinetics of the NOE, often facilitated by selective 1D NOE buildup experiments, can generate invaluable quantitative distance information for the molecule being investigated. In practice, analysis of such kinetic NOE data routinely assumes a first-order approximation of the initial buildup rate. However, often times such an approximation holds true only for the shortest mixing times. As shown by Macura and others, the linear range of the NOE buildup obtained from 2D NOESY and exchange experiments can be substantially extended by simply scaling the NOE cross-peaks against the corresponding diagonal peaks. In this note, we demonstrate through a detailed analysis that the same approach can be applied to the analysis of 1D NOE data obtained with the DPFGSE NOE pulse sequence, one of the most widely used selective 1D NOE experiments today. We show that this approach allows the inclusion of data points acquired with much longer mixing times in the analysis and thus considerably improves the accuracy of the measured cross-relaxation rates and internuclear distances, while considerably simplifying the data analysis. Similar results can be obtained for the rotating frame DPFGSE ROE experiment.

Sedimentation of IF-WS 2 aggregates and a reproducibility of the tribological data

Inorganic fullerene-like (IF) solid lubricant nanoparticles and nanotubes with extremely useful mechanical and tribological characteristics have been realized, offering a plethora of new applications for these nanomaterials. The IF nanoparticles were found to be in the aggregated state. It is expected that the size of the aggregates and their distribution determine the penetration and entrapping of IF nanopowder into the interface. The main goal of the present work is to elucidate the effect of the mixing time of IF-WS 2 nanomaterial in the oil on the size of the IF aggregates and their influence on the friction and wear. The fraction of small aggregates increases and that of the large aggregates decreases with longer mixing time. Consequently, the spread of the tribological results diminishes with the lengthening of the mixing time. The reproducibility of the friction results for the pairs lubricated with oil +IF nanoparticles is determined by distribution of the IF aggregates in the lubricant and the size of the solid lubricant aggregates.

Investigation of Portland Cement Concrete Mix Consistency and Concrete Performance Using a Two-Stage Mixing Process

The objective of this study was to investigate the effects of different mixing processes, particularly a two-stage mixing process, on fresh and hardened characteristics of pastes and concrete. Characteristics studied for pastes included rheological effects (i.e., yield stress, thixotropy, viscosity, and peak stress) and compressive strength for mixes prepared in a Hobart mixer and a high-shear mixer. Parameters measured for the concrete study included fresh concrete air and slump, compressive strength, and hardened concrete air void characteristics. The paste study results show that for increased mixing time, the compressive strength is not significantly influenced. The rheology results show that longer mixing times with a high-shear mixer lead to lower viscosity, thixotropy, and peak stress; this indicates better-mixed slurry when compared with a shorter mixing time and the Hobart mixer. The fresh concrete made with the two-stage mixing process shows reduction in air content, and the hardened concrete sh...

Investigation of Portland Cement Concrete Mix Consistency and Concrete Performance Using a Two-Stage Mixing Process

The objective of this study was to investigate the effects of different mixing processes, particularly a two-stage mixing process, on fresh and hardened characteristics of pastes and concrete. Characteristics studied for pastes included rheological effects (i.e., yield stress, thixotropy, viscosity, and peak stress) and compressive strength for mixes prepared in a Hobart mixer and a high-shear mixer. Parameters measured for the concrete study included fresh concrete air and slump, compressive strength, and hardened concrete air void characteristics. The paste study results show that for increased mixing time, the compressive strength is not significantly influenced. The rheology results show that longer mixing times with a high-shear mixer lead to lower viscosity, thixotropy, and peak stress; this indicates better-mixed slurry when compared with a shorter mixing time and the Hobart mixer. The fresh concrete made with the two-stage mixing process shows reduction in air content, and the hardened concrete shows little change in compressive strength when compared with the concrete produced with the one-step mixing process. Rapid air void analysis results indicate that the two-stage mixing method may be beneficial for mixes containing portland cement as a sole binder in freeze–thaw durability.

Morphology and dynamics of the poly(ϵ‐caprolactone)‐b‐poly(L‐lactide) diblock copolymer and its inclusion compound with α‐cyclodextrin: A solid‐state 13C NMR study

AbstractA biodegradable diblock copolymer of poly(ϵ‐caprolactone) (PCL) and poly(L‐lactide) (PLLA) was synthesized and characterized. The inclusion compound (IC) of this copolymer with α‐cyclodextrin (α‐CD) was formed and characterized. Wide‐angle X‐ray diffraction showed that in the IC crystals α‐CDs were packed in the channel mode, which isolated and restricted the individual guest copolymer chains to highly extended conformation. Solid‐state 13C NMR techniques were used to investigate the morphology and dynamics of both the bulk and α‐CD‐IC isolated PCL‐b‐PLLA chains. The conformation of the PCL blocks isolated within the α‐CD cavities was similar to the crystalline conformation of PCL blocks in the bulk copolymer. Spin–lattice relaxation time (T1C) measurements revealed a dramatic difference in the mobilities of the semicrystalline bulk copolymer chains and those isolated in the α‐CD‐IC channels. Carbon‐observed proton spin–lattice relaxation in the rotating frame measurements (T1ρH) showed that the bulk copolymer was phase‐separated, while, in the IC, exchange of proton magnetization through spin‐diffusion between the isolated guest polymer chains and the host α‐CD was not complete. The two‐dimensional solid‐state heteronuclear correlation (HetCor) method was also employed to monitor proton communication in these samples. Intrablock exchange of proton magnetization was observed in both the bulk semicrystalline and IC copolymer samples at short mixing times; however, even at the longest mixing time, interblock proton communication was not observed in either sample. In spite of the physical closeness between the isolated included guest chains and the host α‐CD molecules, efficient proton spin diffusion was not observed between them in the IC. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2086–2096, 2005

The effect of varying the mixing formula on the quality of a yeast sweet bread and also on the process conditions, as studied by surface response methodology

SummaryThe objective of this work was to evaluate the effect of different ingredients on the mixing and fermentation times required to attain good bread quality. Results showed that variation in almost all ingredients affected mixing time; shortening being the exception. Longer mixing times were required as the concentration of ingredients was increased. Water‐soluble ingredients had a synergistic effect. The same behaviour was observed with total fermentation time, sugar, salt and milk powder having major effects. The only change in crumb colour was caused by egg. Crust colour was only affected by the presence of milk and sugar. It was concluded that it was possible to get good quality bread by varying the process conditions according to the formulation used. Non‐hygroscopic ingredients had a lesser effect on mixing and fermentation times. Predictive equations were obtained.

Two-dimensional spin-exchange solid-state NMR study of the crystal structure of cellulose II

A two-dimensional (2D) 13C– 13C spin-exchange NMR experiment was applied to the uniformly 13C-enriched cellulose II in order to obtain interatomic distance information in the cellulose II crystal. A radio frequency-driven recoupling (RFDR) rotor-synchronized π pulse sequence was incorporated during the mixing time of the 2D experiment. The 2D spin-exchange NMR recorded with a short mixing time (0.80–2.58 ms) provided the correlations between a pair of strongly coupled 13C spins such as neighbor carbon nuclei. This spectrum enabled us to assign all 13C resonance lines of two kinds of anhydroglucose residues A and B in the structure of cellulose II. On the basis of the 13C resonance assignment of residues A and B, the interatomic distances from each C1 to the other carbon nuclei were compared by measuring the 2D spectra recorded with longer mixing times (5.12–20.48 ms). As a result, it was revealed that the respective residues A and B are composed of independent chains (– A– A– and – B– B– repeating units) and that there are no – A– B– repeating units in the chain. This experimental technique is expected to be applicable to conformation analysis of polysaccharides as well as the other cellulose polymorphs.

Precision 1H-1H distance measurement via 13C NMR signals: utilization of 1H-1H double-quantum dipolar interactions recoupled under magic angle spinning conditions.

We applied the POST-C7 DQ-dipolar recoupling pulse sequence to the measurement of (1)H-(1)H distances with high precision. The spectral resolution is enhanced by detecting the (1)H magnetization via (13)C signals. A least-squares fitting of the build-up curve of the transferred magnetization to the exact numerical simulations yielded a (1)H(alpha)-(1)H(beta) distance of 248 +/- 4 pm for fully (13)C-labeled L-valine. This distance agrees with the neutron diffraction study. The negative transferred magnetization clearly indicates that the direct DQ (1)H-(1)H dipolar couplings have the largest effect. The signal for the magnetization transfer builds up rapidly by the direct (1)H-(1)H dipolar coupling, and decreases to zero at longer mixing time when the relayed magnetization transfer becomes significant. This large intensity change of the signal leads to the high precision in the distance measurement. We inspected factors that limit the effective bandwidth of the POST-C7 recoupling for the (1)H and (13)C homonuclear spin systems. The spin interactions at times shorter than the cycle time of the C7 sequence were also evaluated to measure the distances. The carbon-detected 2D (1)H DQ mixing experiment was demonstrated for the measurement of multiple (1)H-(1)H distances.

Effect of a Change in Crystal Polymorph on the Degree of Adhesion Between Micronized Drug Particles and Large Homogenous Carrier Particles During an Interactive Mixing Process

This study reports the effect of a change in crystal form on the quality of interactive mixtures prepared with homogenous sugar beads and the three polymorphs of chloramphenicol palmitate (CAP). Six mixtures containing micronized CAP polymorph powder (0.5%) and sugar beads (99.5%) were mixed in a Turbula® mixer at 27 rpm or 54 rpm for 2.5, 5, 10, or 20 min. Three of the six mixtures was screened to remove the unmixed drug powder. The content uniformity (CV %) of the three screened and three unscreened mixtures was determined by determining the variation in drug content of 10 randomly taken samples from each mixture. Comparison of the amount of drug screened and the content uniformity of the mixtures showed that at short mixing times and 27 rpm and longer mixing times of 10 and 20 min at 54 rpm, the three crystal forms formed interactive mixtures with statistically the same content uniformity. In contrast at 54 rpm and mixing for 5 min and shorter, form A formed less homogeneous interactive mixtures compared with forms B and C. Consistently at both mixing speeds a larger amount of form A, compared with forms B and C, was screened from the mixtures. The results showed that the affinity between forms B and C and the carrier sugar beads is higher than that between form A and the sugar beads. Therefore, changing the crystal form of a drug influences the affinity between the drug and a carrier when preparing interactive mixtures.

Complex processing–morphology interrelationships during the reactive compatibilization of blends of poly(butylene terephthalate) with epoxide‐ containing rubber

AbstractReactive processing of blends of poly(butylene terephthalate) (PBT) with the ethene–(methyl acrylate)–(glycidyl methacrylate) terpolymer (E–MA–GMA) is known to present a very complex reactivity since two competitive reactions take place spontaneously during melt blending, that is, blend compatibilization and rubber‐phase crosslinking. In this article, the effects of several processing parameters, such as the shear rate, the processing temperature, and the matrix viscosity, on the reactive processing of those blends were investigated in terms of the blend morphology and of the amount of copolymer formed at the blend interface. It was shown that the morphology development could be divided in two successive regimes: In the early stages of the mixing process, the particle size is essentially determined by the physical dispersion process, that is, breakup and coalescence, while, at longer mixing times, a further decrease in particle size is obtained as a result of the compatibilization reactions. The shift between the two regimes is progressive and intimately related to the processing conditions. Despite such a complexity, not only the blend morphology but also the elastic properties of the rubber particles can be controlled in a broad range by an adequate adjustment of the relative kinetics between both physical and chemical processes. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 703–718, 2004

Preparation of clay-dispersed poly(styrene- co-acrylonitrile) nanocomposites using poly(ϵ-caprolactone) as a compatibilizer

Poly(styrene- co-acrylonitrile) (SAN)/organoclay nanocomposites were prepared using poly(ϵ-caprolactone) (PCL) as a compatibilizer. Two-step mixing sequence was used to prepare the SAN nanocomposites: PCL/organoclay master batches with different degree of intercalation were prepared by melt mixing first, and then they were melt-mixed with SAN, where PCL is miscible with SAN. The intercalation behavior of PCL in the master batches was investigated in terms of the type of organic modifier and mixing conditions such as shear rate, mixing temperature and mixing time. Longer mixing time and lower mixing temperature were required to prepare exfoliated PCL master batches. As the degree of exfoliation of clays becomes better, the stiffness reinforcement effect of the organoclays increases in both PCL/organoclay master batches and their blends with SAN.

RF excitation profiles with FAIR: impact of truncation of the arterial input function on quantitative perfusion.

This study investigates the impact of imaging coil length and consequent truncation of the arterial input function on the perfusion signal contrast obtained in the flow-sensitive alternating inversion recovery (FAIR) perfusion imaging measurement. We examined the difference in perfusion contrast achieved with head, head and neck, and body imaging coils based on the hypothesis that the standard head coil provides a truncated input function compared with that provided by the body coil and that this effect will be accentuated at long inversion times. The TI-dependent cerebral response of the FAIR sequence was examined at 1.5 T by varying the TI from 200 to 3500 msec with both the head and whole body coils (n = 5) as well as using a head and neck coil (n = 3). Difference signal intensity DeltaM and quantitative cerebral blood flow (CBF) were plotted against TI for each coil configuration. Despite a lower signal-to-noise ratio, relative CBF was significantly greater when measured with the body or head and neck coil compared with the standard head coil for longer inversion times (two-way ANOVA, P < or = 0.002). This effect is attributed to truncation of the arterial input function of labeled water by the standard head coil and the resultant inflow of unlabeled spins to the image slice during control image acquisition, resulting in overestimation of CBF. The results support the conclusion that the arterial input function depends on the anatomic extent of the inversion pulse in FAIR, particularly at longer mixing times (TI > 1200 msec at 1.5 T). Use of a head and neck coil ensures adequate inversion while preserving SNR that is lost in the body coil.

Phenoxy/Hytrel blends. I. Miscibility and melt-state reactions

Polyhydroxy ether of bisphenol A (phenoxy)/Hytrel (Hy) blends were obtained by melt mixing throughout the composition range and at several processing times. Interchange reactions took place and were followed by the torque behavior, solubility tests, and FTIR spectra. After a mixing time of 15 min, they gave rise to basically unreacted products, but at longer mixing times, they produced both branched and crosslinked products. The blends were miscible at all compositions due to the presence of specific interactions, as was seen because of the presence of a single Tg by DSC that was confirmed by DMTA. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 185–193, 1998