Isotropic Mixing Research Articles

Full Correlations across Broad NMR Spectra by Two-Field Total Correlation Spectroscopy.

Total correlation spectroscopy (TOCSY) is a key experiment to assign nuclear magnetic resonance (NMR) spectra of complex molecules. Carbon-13 TOCSY experiments are essential to assign signals of protein side chains. However, the performance of carbon-13 TOCSY deteriorates at high magnetic fields since the necessarily limited radiofrequency irradiation fails to cover the broad range of carbon-13 frequencies. Here, we introduce a new concept to overcome the limitations of TOCSY by using two-field NMR spectroscopy. In two-field TOCSY experiments, chemical shifts are labelled at high field but isotropic mixing is performed at a much lower magnetic field, where the frequency range of the spectrum is drastically reduced. We obtain complete correlations between all carbon-13 nuclei belonging to amino acids across the entire spectrum: aromatic, aliphatic and carboxylic. Two-field TOCSY should be a robust and general approach for the assignment of uniformly carbon-13 labelled molecules in high-field and ultra-high field NMR spectrometers beyond 1000 MHz.

Anisotropy Enhanced Phase Separation in Polymer Dispersed Liquid Crystals

Phase separated blends of polymers and low molecular weight liquid crystals, commonly known as polymer dispersed liquid crystals in short PDLCs, are investigated. These materials offer a realm of applications in modern technologies, including sensors, commutable windows, display devices and telecommunication systems. A particular attention is given to the effects of anisotropy of the liquid crystal on the phase behavior under equilibrium and non equilibrium conditions. The theoretical formalism used is based on the lattice model of isotropic mixing, combined with standards theories of nematic and smectic-A orders. Considering the equilibrium phase behavior, we find that the nematic order enhances the polymer / solvent phase separation, and that the osmotic pressure shows substantial changes for relatively small polymer volume fractions. We find that the anisotropy enhanced phase separation is more pronounced for a smectic-A liquid crystal, and the miscibility gap is widened. The kinetics of swelling by nematic LCs is examined using a linear solvent diffusion process, with a rate of swelling directly related to the derivative of the osmotic pressure. An abrupt swelling / de-swelling transition is found, due to overwhelming effects of the anisotropic interaction beyond the threshold LC concentration. Anisotropy enhanced phase separation is also investigated in the method of synthesis based on the polymerization induced phase separation mechanism. We find that the kinetics of separation during early stages of polymerization is faster, due to the anisotropic interaction of the low molecular weight solvent. The kinetics speed up is favored by the long range viscous flow effects due to hydrodynamic interactions. A limited selection of experimental data in the literature is chosen to validate some theoretical predictions obtained from the present formalisms.

Effect of nanoparticles on the [formula omitted] rotator phase transitions of alkanes

Experimental studies have shown that nanoparticles play an important role on the rotator phase transitions of n-alkanes. A phenomenological model for predicting the RII-RI-RV phase transitions in mixtures of alkanes and nanoparticles has been proposed by combining Flory-Huggins free energy of isotropic mixing and Landau free energy. The impact of nanoparticles on the RII-RI-RV phase transitions and their transition temperatures is discussed by means of phenomenological theory. The possibility of the tricritical behavior of the RI-RV phase transition in the mixtures of alkanes and nanoparticles is discussed. The theoretical predictions are in good qualitative agreement with available experimental results.

Novel selective TOCSY method enables NMR spectral elucidation of metabolomic mixtures

Complex mixture analysis is routinely encountered in NMR-based investigations. With the aim of component identification, spectral complexity may be addressed chromatographically or spectroscopically, the latter being favored to reduce sample handling requirements. An attractive experiment is selective total correlation spectroscopy (sel-TOCSY), which is capable of providing tremendous spectral simplification and thereby enhancing assignment capability. Unfortunately, isolating a well resolved resonance is increasingly difficult as the complexity of the mixture increases and the assumption of single spin system excitation is no longer robust. We present TOCSY optimized mixture elucidation (TOOMIXED), a technique capable of performing spectral assignment particularly in the case where the assumption of single spin system excitation is relaxed. Key to the technique is the collection of a series of 1D sel-TOCSY experiments as a function of the isotropic mixing time (τm), resulting in a series of resonance intensities indicative of the underlying molecular structure. By comparing these τm-dependent intensity patterns with a library of pre-determined component spectra, one is able to regain assignment capability. After consideration of the technique’s robustness, we tested TOOMIXED firstly on a model mixture. As a benchmark we were able to assign a molecule with high confidence in the case of selectively exciting an isolated resonance. Assignment confidence was not compromised when performing TOOMIXED on a resonance known to contain multiple overlapping signals, and in the worst case the method suggested a follow-up sel-TOCSY experiment to confirm an ambiguous assignment. TOOMIXED was then demonstrated on two realistic samples (whisky and urine), where under our conditions an approximate limit of detection of 0.6mM was determined. Taking into account literature reports for the sel-TOCSY limit of detection, the technique should reach on the order of 10μM sensitivity. We anticipate this technique will be highly attractive to various analytical fields facing mixture analysis, including metabolomics, foodstuff analysis, pharmaceutical analysis, and forensics.

Triple quantum filtered spectroscopy of homonuclear three spin-1/2 systems employing isotropic mixing

We report the design and performance evaluation of novel pulse sequences for triple quantum filtered spectroscopy in homonuclear three spin-1/2 systems, employing isotropic mixing (IM) to excite triple quantum coherence (TQC). Our approach involves the generation of combination single quantum coherences (cSQC) from antisymmetric longitudinal or transverse magnetization components employing isotropic mixing (IM). cSQC’s are then converted to TQC by a selective 180° pulse on one of the spins. As IM ideally causes magnetization to evolve under the influence of the spin coupling Hamiltonian alone, TQC is generated at a faster rate compared to sequences involving free precession. This is expected to be significant when the spins have large relaxation rates. Our approach is demonstrated experimentally by TQC filtered 1D spectroscopy on a 1H AX2 system (propargyl bromide in the presence of a paramagnetic additive), as well as a 31P linear AMX system (ATP in agar gel). The performance of the IM-based sequences for TQC excitation are compared against the standard three pulse sequence (Ernst et al., 1987) and an AX2 spin pattern recognition sequence (Levitt and Ernst, 1983). The latter reaches the unitary bound on TQC preparation efficiency starting from thermal equilibrium in AX2 systems, not considering relaxation. It is shown that in systems where spins relax rapidly, the new IM-based sequences indeed perform significantly better than the above two known TQC excitation sequences, the sensitivity enhancement being especially pronounced in the case of the proton system investigated. An overview of the differences in relaxation behavior is presented for the different approaches. Applications are envisaged to Overhauser DNP experiments and to in vivo NMR.

Polymer phase transition in n‐lauryl methacrylate monoliths

AbstractMonolithic materials prepared from a mixture of n‐lauryl methacrylate (LMA) and ethylene glycol dimethacrylate (EGDMA) dedicated to nano‐liquid chromatography separation were synthesized using in situ UV polymerization in 75 µm inner diameter capillary tubing. A mixture of cyclohexanol and ethylene glycol was used as a porogen to control porosity. While the preparation conditions yielded satisfactory analytical results, values of pertinent parameters turned out to be critical for obtaining columns with efficient separation. In particular, the impact of two key parameters was studied here in an attempt to identify optimal preparation conditions: (a) different concentrations of the crosslinker EGDMA and (b) different porogen compositions while the monomer to porogen ratio was kept constant. Resulting monolithic phases were characterized in terms of permeability, mean pore diameter and swelling using three different eluents (water, acetonitrile and a mixture at maximum viscosity). First, the LMA/EGDMA monolithic phases present peculiar morphology and hydrodynamic properties for 37% by weight of EGDMA, as reflected by the peak observed for their permeability and mean pore diameter. Swelling experiments revealed the coexistence of two phases in the monolithic structure: a highly crosslinked rigid phase which was insensitive to swelling in the presence of solvent and a weakly crosslinked flexible phase exhibiting significant swelling, with a transition to such a biphasic behavior taking place at 37% by weight of EGDMA. The effects of porogen composition and network swelling properties are presented based on a combination of the Flory − Huggins theory of isotropic mixing in polymer solutions and the Flory − Rehner theory of rubber elasticity in the affine network approximation. © 2016 Society of Chemical Industry

Time-optimal excitation of maximum quantum coherence: Physical limits and pulse sequences.

Here we study the optimum efficiency of the excitation of maximum quantum (MaxQ) coherence using analytical and numerical methods based on optimal control theory. The theoretical limit of the achievable MaxQ amplitude and the minimum time to achieve this limit are explored for a set of model systems consisting of up to five coupled spins. In addition to arbitrary pulse shapes, two simple pulse sequence families of practical interest are considered in the optimizations. Compared to conventional approaches, substantial gains were found both in terms of the achieved MaxQ amplitude and in pulse sequence durations. For a model system, theoretically predicted gains of a factor of three compared to the conventional pulse sequence were experimentally demonstrated. Motivated by the numerical results, also two novel analytical transfer schemes were found: Compared to conventional approaches based on non-selective pulses and delays, double-quantum coherence in two-spin systems can be created twice as fast using isotropic mixing and hard spin-selective pulses. Also it is proved that in a chain of three weakly coupled spins with the same coupling constants, triple-quantum coherence can be created in a time-optimal fashion using so-called geodesic pulses.

The multicomponent doping of surface layers of materials under the influence of ion beams with a broad energy spectrum

The paper discusses the various factors that influence the efficiency of ion mixing. It was found that in the base of penetration of atoms multilayer films in polycrystalline substrate is the process of energy transfer from ions and primary knocked-on atom (PKA) of films to subsequent displacement cascade. At the same time the penetration of implanted atoms to great depths determined by the density of defects, radiation-stimulated migration of interstitial atoms and their physico-chemical interaction with the atoms of the matrix, which can be described by the model of an isotropic mixing. It is shown that doping atoms of the multilayer films, possibly the formation of gradient layers, which are determined by radiation traces in the substrate implanted atoms and their migration under irradiation by the ion beam with a broad energy spectrum.

Asynchronous through-bond homonuclear isotropic mixing: application to carbon-carbon transfer in perdeuterated proteins under MAS.

Multiple-bond carbon-carbon homonuclear mixing is a hurdle in extensively deuterated proteins and under fast MAS due to the absence of an effective proton dipolar-coupling network. Such conditions are now commonly employed in solid-state NMR spectroscopy. Here, we introduce an isotropic homonuclear (13)C-(13)C through-bond mixing sequence, MOCCA, for the solid state. Even though applied under MAS, this scheme performs without rotor synchronization and thus does not pose the usual hurdles in terms of power dissipation for fast spinning. We compare its performance with existing homonuclear (13)C-(13)C mixing schemes using a perdeuterated and partially proton-backexchanged protein. Based on the analysis of side chain carbon-carbon correlations, we show that particularly MOCCA with standard 180-degree pulses and delays leading to non-rotor-synchronized spacing performs exceptionally well. This method provides high magnetization transfer efficiency for multiple-bond transfer in the aliphatic region compared with other tested mixing sequences. In addition, we show that this sequence can also be tailor-made for recoupling within a selected spectral region using band-selective pulses.

Theoretical Studies of the Effects of Magnetic Field on the Phase Transition of Swollen Liquid Crystal Elastomers

The effect of magnetic fields on the swelling of liquid crystal elastomers (LCE) dissolved in liquid crystal (LC) solvent have been studied. The Flory-Huggins model used to calculate the free energy of an isotropic mixing and the Maier-Saupe model used to calculate the free energy of a nematic mixing. Numerical integration method used to calculate the orientational order parameter and the total free energy of system (consists of : nematic free energy, elastic free energy, isotropic mixing free energy and magnetic free energy) and the calculation results graphed as a function of temperatures for various magnetic fields and as function of magnetic fields for various of temperatures. We find that the magnetic field shifts the transition points towards higher temperatures, increases the energy transition, and induces an isotropic phase to paranematic phase.

Using MUSIC and CC(CO)NH for backbone assignment of two medium-sized proteins not fully accessible to standard 3D NMR.

The backbone assignment of medium-sized proteins is rarely as straightforward as that of small proteins, and thus often requires creative solutions. Here, we describe the application of a combination of standard 3D heteronuclear methods with CC(CO)NH and a variety of MUltiplicity Selective In-phase Coherence transfer (MUSIC) experiments. Both CC(CO)NH and MUSIC are, in theory, very powerful methods for the backbone assignment of proteins. Due to low sensitivity, their use has usually been linked to small proteins only. However, we found that combining CC(CO)NH and MUSIC experiments simplified the assignment of two challenging medium-sized proteins of 13 and 19.5 kDa, respectively. These methods are to some extent complementary to each other: CC(CO)NH acquired with a long isotropic mixing time can identify amino acids with large aliphatic side chains. Whereas the most sensitive MUSIC experiments identify amino acid types that cannot be detected by CC(CO)NH, comprising the residues with acid and amide groups, and aromatic rings in their side chains. Together these methods provide a means of identifying the majority of peaks in the 2D 15N HSQC spectrum which simplifies the backbone assignment work even for proteins, e.g., small kinases, whose standard spectra resulted in little spectral resolution and low signal intensities.

Conditionally Statistical Description of Turbulent Scalar Mixing at Subgrid-Scales

Conditionally filtered passive scalar density function (FDF), dissipation (CFD), diffusion (CFDIF) and conditionally filtered velocity (CFV) are studied by using direct numerical simulation (DNS) data of homogeneous isotropic turbulent mixing with an imposed mean scalar gradient. The velocity and scalar fields are statistically stationary and have a Taylor micro-scale Reynolds number of 209. A three-dimensional box filter with a filter width of 21 scalar dissipation scales is employed to obtain filtered variables. The PDFs of conditioning variables, filtered scalar ( $\left \langle \phi \right \rangle _L $ ), and subgrid-scale (SGS) scalar variance ( $\left \langle \phi ^{\prime \prime 2} \right \rangle _L$ ), are shown and are in reasonably good agreement with the experimental results of a one-dimensional filter. Since the three-dimensional filter has been used, the PDF of the filtered scalar exhibits perfect symmetry, and this significantly affects the symmetry of succeeding conditionally filtered statistics on $\left \langle \phi \right \rangle _L$ such as FDF, CFD, and CFDIF. The FDF, CFD, and CFDIF also show visible dependence on $\left \langle \phi ^{\prime \prime 2} \right \rangle _L$ . When $\left \langle \phi ^{\prime \prime 2} \right \rangle _L$ varies from large to small, CFD varies from bell-shaped to U-shaped curves, and CFDIF varies from an inverse-S curve to a linear line; these are in accordance with the evolution of the FDF varying from bimodal to Gaussian. It suggests that the SGS mixing could be compared with global binary mixing in the view of the conditional description and this also suggests that it would reveal the non-equilibrium characteristics of the SGS scalar mixing; this SGS scalar mixing is supposed to be caused by diffusion-layer-like structures. In addition, the CFV remains linear regardless of the value of $\left \langle \phi ^{\prime \prime 2} \right \rangle _L$ . These results numerically confirm two distinct regimes of SGS mixing observed in previous experimental studies and extend the experimental results into isotropic turbulence, which suggests that the characteristics of the SGS scalar are intrinsic behavior of SGS mixing, independent of flow types.

Aliphatic chain length by isotropic mixing (ALCHIM): determining composition of complex lipid samples by ¹H NMR spectroscopy.

Quantifying the amounts and types of lipids present in mixtures is important in fields as diverse as medicine, food science, and biochemistry. Nuclear magnetic resonance (NMR) spectroscopy can quantify the total amounts of saturated and unsaturated fatty acids in mixtures, but identifying the length of saturated fatty acid or the position of unsaturation by NMR is a daunting challenge. We have developed an NMR technique, aliphatic chain length by isotropic mixing, to address this problem. Using a selective total correlation spectroscopy technique to excite and transfer magnetization from a resolved resonance, we demonstrate that the time dependence of this transfer to another resolved site depends linearly on the number of aliphatic carbons separating the two sites. This technique is applied to complex natural mixtures allowing the identification and quantification of the constituent fatty acids. The method has been applied to whole adipocytes demonstrating that it will be of great use in studies of whole tissues.

Pulse design for broadband correlation NMR spectroscopy by multi-rotating frames

We present a method for designing radio-frequency (RF) pulses for broadband or multi-band isotropic mixing at low power, suitable for protein NMR spectroscopy. These mixing pulses are designed analytically, rather than by numerical optimization, by repeatedly constructing new rotating frames of reference. We show how pulse parameters can be chosen frame-by-frame to systematically reduce the effective chemical shift bandwidth, but maintain most of the effective J-coupling strength. The effective Hartmann-Hahn mixing condition is then satisfied in a multi-rotating frame of reference. This design method yields multi-band and broadband mixing pulses at low RF power. In particular, the ratio of RF power to mixing bandwidth for these pulses is lower than for existing mixing pulses, such as DIPSI and FLOPSY. Carbon-carbon TOCSY experiments at low RF power support our theoretical analysis.

Accelerated spin dynamics in NMR driven by homonuclear scalar couplings

The influence of homonuclear scalar couplings on multiple quantum coherences evolving under isotropic mixing is investigated. In suitable clusters of four or more spins-1/2 (or two or more spins >½), zero and multiple quantum coherences are shown to evolve at high frequencies that involve multiples of the coupling constant. This behavior under spin lock is consequent on the cluster being in a superposition of spin coupling eigenstates that differ in composite spin by more than one unit. The resulting accelerated spin dynamics also offers opportunity to measure unresolved couplings in MRS and in high resolution NMR.

Phase diagrams and morphology of polymer-dispersed liquid crystals: An analysis

Thermal and morphological studies of polymer-dispersed liquid crystals (PDLCs) for various compositions of liquid crystalline material 4-undecyloxybenzoic acid (UDBA) in two polymer matrices, polyvinylidenefluoride-co-hexafluoropropylene P(VdF–HFP) or polyethylene oxide (PEO), have been carried out using differential scanning calorimetry (DSC) and polarising optical microscopy (POM). Phase diagrams for different series of PDLCs have been analysed using the Flory–Huggins theory of isotropic mixing and the Maier–Saupe–McMillan theory, to include the anisotropic contributions. Mesogenic transitions of UDBA are observed to be greatly influenced when dispersed in these polymers. The morphologies and miscibility studies of these PDLCs suggest that UDBA is highly miscible in PEO, only partially miscible in poly(methyl methacrylate) (PMMA), but almost immiscible in P(VdF–HFP).

Elaboration of Side-Chain Liquid-Crystalline Elastomers and Study of Their Swelling Behavior in Anisotropic Solvents

A poly [6-(4′-cyanophenyl-4″-phenoxy) alkyl acrylate] side chain liquid crystalline elastomer and a poly [n-butylacrylate] network were synthesized via photopolymerization using 1,6-hexanediol diacrylate as a crosslinking agent. The materials obtained were characterized by differential scanning calorimetry and polarized optical microscopy. In particular, the latter method allowed to follow the swelling behavior of the polymer networks in a low molar weight liquid crystal (LC) and to observe the formation of LC gels. In the case of the anisotropic network, a stable nematic gel phase and a miscibility gap were observed, in relationship with the nematic-isotropic transition temperatures of both LC-solvent and LC-gel. The experimental phase diagrams were well reproduced by a mean field model combining the Flory-Rehner model for isotropic mixing and the Maier Saupe theory for nematic ordering.

Ion mixing in multilayer films and the doping of the near-surface layers of polycrystalline substrates under irradiation by ion beams with a wide energy spectrum

Various factors influencing ion mixing efficiency are considered. It is established that the energy transfer from ions and primary knocked-on atom films to subsequent displacement cascades underlies the penetration of atoms from multilayer films into a polycrystalline substrate. The penetration of implanted atoms to great depths is in this case determined by the defect density, the radiation induced migration of the implanted atoms, and their interaction with the atoms of the matrix. All of these factors can be described in terms of the isotropic mixing model. It is shown that when doped by atoms from multilayer films, gradient layers form that are determined by the range of radiation in the substrate of the atoms to be implanted and their migration under irradiation by an ion beam with a wide range of energies.

Phase Properties of Interpenetrating Polymer Networks and Liquid Crystals

The phase behavior of interpenetrating acrylic polymer network/liquid crystal systems was investigated. The crosslinked polymers were obtained by UV-curing of initial solutions containing a reactive monomer, a crosslinker and a photoinitiator. Experimental phase diagrams were established using polarizing optical microscopy observations of the variation of equilibrium swelling of polymer/liquid crystal systems with temperature. The experimental data were rationalized within a combination of the Flory-Rehner theory of isotropic mixing and the Maier-Saupe theory of nematic ordering.

Thermophysical Analysis of Smectic A Domains Confined into a Thermoplastic Polymer Matrix

Cloud point measurements and calorimetric properties were considered for binary systems of polystyrene/4cyano-4′-n-octylbiphenyl (8CB), and deuterated polystyrene/8CB. The experimental phase diagrams were established using polarizing optical microscopy and differential scanning calorimetry. The experiments were realized in the range of temperatures from 25 to 75°C, in which the 8CB presents a smectic A structure, a nematic and an isotropic phase. The phase behavior of both systems were analyzed using a combination of the Flory-Huggins lattice model for isotropic mixing and the Maier-Saupe-McMillan theories for anisotropic ordering. A good agreement was obtained between theoretical considerations and experimental data.