Measurement Of Translational Diffusion Coefficient Research Articles

Theoretical analysis of flow effects in spatially encoded diffusion NMR.

The measurement of translational diffusion coefficients by nuclear magnetic resonance (NMR) spectroscopy is essential in a broad range of fields, including organic, inorganic, polymer, and supramolecular chemistry. It is also a powerful method for mixture analysis. Spatially encoded diffusion NMR (SPEN DNMR)" is a time efficient technique to collect diffusion NMR data, which is particularly relevant for the analysis of samples that evolve in time. In many cases, motion other than diffusion is present in NMR samples. This is, for example, the case of flow NMR experiments, such as in online reaction monitoring and in the presence of sample convection. Such motion is deleterious for the accuracy of DNMR experiments in general and for SPEN DNMR in particular. Limited theoretical understanding of flow effects in SPEN DNMR experiments is an obstacle for their broader experimental implementation. Here, we present a detailed theoretical analysis of flow effects in SPEN DNMR and of their compensation, throughout the relevant pulse sequences. This analysis is validated by comparison with numerical simulation performed with the Fokker-Planck formalism. We then consider, through numerical simulation, the specific cases of constant, laminar, and convection flow and the accuracy of SPEN DNMR experiments in these contexts. This analysis will be useful for the design and implementation of fast diffusion NMR experiments and for their applications.

Translational diffusion of unfolded and intrinsically disordered proteins.

Translational (or self-diffusion) coefficient in dilute solution is inversely proportional to the size of a diffusing molecule, and hence self-diffusion coefficient measurements have been applied to determine the effective hydrodynamic radii for a range of native and nonnative protein conformations. In particular, translational diffusion coefficient measurements are useful to estimate the hydrodynamic radius of natively (or intrinsically) disordered proteins in solution, and, thereby, probe the compactness of a protein as well as its change when environmental parameters such as temperature, solution pH, or protein concentration are varied. The situation becomes more complicated in concentrated solutions. In this review, we discuss the translational diffusion of disordered proteins in dilute and crowded solutions, focusing primarily on the information provided by pulsed-field gradient NMR technique, and draw analogies to well-structured globular proteins and synthetic polymers.

Understanding the Microscopic Behavior of Binary Mixtures of Ionic Liquids through Various Spectroscopic Techniques.

In recent times, it has been shown that certain binary mixtures of pure ionic liquids having appropriate chemical composition can behave like a new chemical entity. However, current knowledge about the microscopic behavior of these interesting systems is rather limited. The present study is undertaken with an objective to understand the microscopic behavior in terms of intermolecular interaction, structure, and dynamics of these systems. In the present study, few (IL + IL) mixtures are chosen with a common cation and a variation of anion. The investigations are also carried out by taking individual pure ILs so that the difference in the behavior of pure IL and (IL + IL) mixtures is understood. Initially, the systems have been investigated by studying the thermophysical properties of the concerned mixtures. The synergistic effect between combining pure ILs through photochromism has also been studied. These mixtures have been investigated further through steady-state and time-resolved fluorescence spectroscopy, electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), and fluorescence correlation spectroscopy (FCS). Interestingly, time-resolved fluorescence data also pointed out that (IL + IL) mixtures are not only spatially heterogeneous but also dynamically heterogeneous. EPR measurements have suggested that the micropolarity ( ET(30)) of the (IL + IL) mixture is close to that of aliphatic polyalcohols. Measurements of translational diffusion coefficients of the diffusing species through NMR and FCS studies have provided an idea about the nanostructural organization within (IL + IL) binary mixtures. The analysis of data essentially reveals that the mixtures of ILs that are used in the current study do not behave like a nonideal solution. The behavior of the IL mixtures is observed to be more like quasi-ideal type.

Measuring translational diffusion of 15N-enriched biomolecules in complex solutions with a simplified 1H-15N HMQC-filtered BEST sequence.

Pulsed-field gradient nuclear magnetic resonance has seen an increase in applications spanning a broad range of disciplines where molecular translational diffusion properties are of interest. The current study introduces and experimentally evaluates the measurement of translational diffusion coefficients of 15N-enriched biomolecules using a 1H-15N HMQC-filtered band-selective excitation short transient (BEST) sequence as an alternative to the previously described SOFAST-XSTE sequence. The results demonstrate that accurate translational diffusion coefficients of 15N-labelled peptides and proteins can be obtained using this alternative 1H-15N HMQC-filtered BEST sequence which is implementable on NMR spectrometers equipped with probes fitted with a single-axis field gradient, including most cryoprobes dedicated to bio-NMR. The sequence is of potential use for direct quantification of protein or peptide translational diffusion within complex systems, such as in mixtures of macromolecules, crowded solutions, membrane-mimicking media and in bicontinuous cubic phases, where conventional sequences may not be readily applicable due to the presence of intense signals arising from sources other than the protein or peptide under investigation.

Understanding the Microscopic Behavior of the Mixture of Ionic Liquid/Ethylene Glycol/Lithium Salt through Time-Resolved Fluorescence, Nuclear Magnetic Resonance (NMR), and Electron Paramagnetic Resonance (EPR) Studies.

The present study has been undertaken with an aim to find out the suitability of a binary solvent system, comprising an anionic liquid, 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([OHEMIM][NTf2]) and ethylene glycol (EG), toward lithium ion battery applications. For this purpose, the behavior in terms of structure, intermolecular interaction, and dynamics of several solvent systems, [OHEMIM][NTf2], [OHEMIM][NTf2]-LiNTf2(lithium bis(trifluorimethylsulfonyl)imide), [OHEMIM][NTf2]-EG, and [OHEMIM][NTf2]-EG-LiNTf2, is investigated by carrying out steady state and time-resolved fluorescence, nuclear magnetic resonance (NMR), and electron paramagnetic resonance (EPR) measurements. Both steady-state fluorescence and EPR studies have pointed out that the micropolarities of [OHEMIM][NTf2]-EG-LiNTf2 are close to those of neat RTIL. However, studies on rotational dynamics have revealed that the structural organization of [OHEMIM][NTf2]-LiNTf2 is significantly influenced upon addition of EG. Interestingly, the average solvation time is observed to be relatively faster in [OHEMIM][NTf2]-EG-LiNTf2 than those in other solvent systems. Since average solvation time and conductivity are inversely related to each other, the present observation indicates that the introduction of EG is helpful in increasing the electrical conductivity of [OHEMIM][NTf2]-EG-LiNTf2. Translational diffusion coefficient measurements in [OHEMIM][NTf2]-EG-LiNTf2 and [OHEMIM][NTf2]-LiNTf2 through NMR spectroscopy have also indicated the suitability of [OHEMIM][NTf2]-EG-LiNTf2 as a potential electrolytic medium for battery applications.

Analysis of cellulose nanocrystal rod lengths by dynamic light scattering and electron microscopy

A method to characterize the length (L) of rod-like cellulose nanocrystal (CNC) particles is described. It is based on dynamic light scattering (DLS) and electron microscopy. Measurement of translational diffusion coefficient from DLS along with diameter measurements from electron microscopy is used in Broersma translational diffusion coefficient equation to calculate the CNC particle L. For wood based CNC rods, diameter of 15 nm measured by scanning electron microscopy (SEM) and translational diffusion coefficient of 5.21 × 10−12 m2/s measured by DLS give particle L of 271 nm from Broersma’s relation. This one and other calculated L values for various rod-like particles are in good agreement with the L of the particles measured either by transmission or SEM.

Translational Diffusion Measurements by Microcoil NMR in Aqueous Solutions of the Fos-10 Detergent-Solubilized Membrane Protein OmpX

Aqueous solutions of the detergent Fos-10 (n-decylphosphocholine) without and with addition of the integral membrane protein (IMP) OmpX (outer membrane protein X) have been characterized using pulsed field gradient-stimulated echo (PFG-STE) NMR experiments for measurements of translational diffusion coefficients. Effective diffusion coefficients for Fos-10 micelles in the absence of OmpX were obtained by observation of NMR signals from 10-bromodecan-1-ol that had been inserted into the micelles, and in the presence of OmpX by NMR observation of the protein. It is thus shown that solutions of Fos-10-reconstituted OmpX can be quantitatively described as a mixture of Fos-10 monomers, uniform Fos-10 micelles, and uniform OmpX-containing Fos-10 micelles, with Fos-10 monomers in fast exchange between the pools of these three species. This result establishes an avenue for efficient determination of the effective translational diffusion coefficients of IMP-containing detergent micelles based on observation of the intense detergent NMR signals, which is also applicable with unlabeled IMPs. This monitoring of the species present in a given IMP solution contributes to improved guidelines for rational selection of detergent and buffer conditions in structural studies of integral membrane proteins.

Osteogenesis Imperfecta Model Peptides: Incorporation of Residues Replacing Gly within a Triple Helix Achieved by Renucleation and Local Flexibility

Missense mutations, which replace one Gly with a larger residue in the repeating sequence of the type I collagen triple helix, lead to the hereditary bone disorder osteogenesis imperfecta (OI). Previous studies suggest that these mutations may interfere with triple-helix folding. NMR was used to investigate triple-helix formation in a series of model peptides where the residue replacing Gly, as well as the local sequence environment, was varied. NMR measurement of translational diffusion coefficients allowed the identification of partially folded species. When Gly was replaced by Ala, the Ala residue was incorporated into a fully folded triple helix, whereas replacement of Gly by Ser or Arg resulted in the presence of some partially folded species, suggesting a folding barrier. Increasing the triple-helix stability of the sequence N-terminal to a Gly-to-Ser replacement allowed complete triple-helix folding, whereas with the substitution of Arg, with its large side chain, the peptide achieved full folding only after flexible residues were introduced N-terminal to the mutation site. These studies shed light on the factors important for accommodation of Gly mutations within the triple helix and may relate to the varying severity of OI.

NMR Study of the Reversible Trapping of SF6 by Cucurbit[6]uril in Aqueous Solution

The complexation of sulfur hexafluoride (SF(6)), a highly potent greenhouse gas, by cucurbit[6]uril (CB) was studied at various temperatures in Na(2)SO(4) aqueous solutions by (19)F and (1)H NMR. CB shows a remarkable affinity for SF(6), suggesting that it is a suitable molecular container for the design of materials tailored for SF(6) trapping. At 298 K, the equilibrium constant characterizing the inclusion of SF(6) by CB is 3.1 x 10(4) M(-1) and the residence time of SF(6) within the CB cavity is estimated to be of the order of a few seconds. The enthalpic and entropic contributions to the free energy of encapsulation were determined and are discussed. This work also reports on the interest of SF(6) in the framework of the spin-spy methodology. The advantages and drawbacks of solution-state (19)F NMR of SF(6) with respect to (129)Xe NMR are discussed. SF(6) comes forward as a versatile and informative spin-spy molecule for probing systems in solution because its detection limit by (19)F NMR reaches the micromolar range with standard equipment and because quantitative integral measurements, relaxation time measurements, and demanding experiments, such as translational diffusion coefficient measurements, are easily carried out in addition to chemical shift measurements. Solution-state (19)F NMR of SF(6) emerges as a promising alternative to (129)Xe NMR for probing cavities and for other applications relying on the encapsulation of an NMR active gaseous probe.

Measurement of Translational Diffusivity by Microchannel Multiphase Flow

A method is proposed for the rapid measurement of translational diffusion coefficient by employing the unique multiphase flow of perfectly miscible incompressible fluids in microchannels. The molecule of interest is injected, as a bolus or continuously, into the middle phase of a three phase flow system. Subsequently, the phase or stream is depleted of molecule by diffusion into the adjacent streams through the common interface. By monitoring the elution profile of the molecule, one may obtain estimates of the diffusion coefficient under fixed experimental conditions. Transport of molecule in the microchannel is described by a transient convection-diffusion-dispersion model. Analytical and numerical solutions are provided to illustrate the change of elution profile with stream velocity and other design parameters. The diffusion coefficient is estimated from a graphical plot. It is shown that diffusion coefficient may be measured without prior knowledge of dispersion. Experimental data is presented for the proteins, fibrinogen and insulin. A number of advantages over the T-sensor and Taylor-Aris methods are highlighted.

Scanning Microphotolysis: Three-Dimensional Diffusion Measurement and Optical Single-Transporter Recording

Scanning microphotolysis (SCAMP) is a combination of fluorescence microphotolysis and confocal laser scanning microscopy. A laser scanning microscope is equipped with an optical switch able to modulate the power or/and wavelength of the laser beam in less than a microsecond while a dedicated computer program is employed to precisely coordinate scanning process and laser beam modulation. By these means it becomes possible to vary the power or/and wavelength of the laser beam during scanning at a precision of one resolution element. Patterns of almost arbitrary design can be written into the object by photolysis, e.g., photobleaching or photoactivation. The dissipation of the photolysis pattern by diffusion or other types of molecular transport can be followed at confocal resolution and used to characterize the transport process. SCAMP can be employed in conjunction with single-photon or multiphoton excitation. Furthermore, it can be easily installed on virtually any confocal laser scanning microscope. We summarize at first the conceptual and practical basis of SCAMP. Then, two novel applications are discussed: (i) measurements of translational diffusion coefficients in truly three-dimensional systems at diffraction-limited resolution, and (ii) optical recording of single transporters in membrane patches.

DNA conformational dynamics in the presence of catanionic mixtures

DNA conformational behavior in the presence of non-stoichiometric mixtures of two oppositely charged surfactants, cetyltrimethylammonium bromide and sodium octyl sulfate, was directly visualized in an aqueous solution with the use of a fluorescence microscopy technique. It was found that in the presence of cationic-rich catanionic mixtures, DNA molecules exhibit a conformational transition from elongated coil to compact globule states. Moreover, if the catanionic mixtures form positively charged vesicles, DNA is adsorbed onto the surface of the vesicles in a collapsed globular form. When anionic-rich catanionic mixtures are present in the solution, no change in the DNA conformational behavior was detected. Cryogenic transmission electron microscopy, as well as measurements of translational diffusion coefficients of individual DNA chains, supported our optical microscopy observations.

Unconstrained optimization method for interpreting the concentration and temperature dependence of the linewidths of interacting nitroxide spin labels. Application to the measurement of translational diffusion coefficients of spin-labeled phospholipids in membranes

A modified Gauss-Newton optimization procedure has been used to fit an analytical expression for the total peak-to-peak, first-derivative linewidth to the concentration and temperature dependence of the ESR spectra of interacting nitroxide spin labels. The measurements were restricted to the low-concentration regime in which the contributions to the Lorentzian linewidth, ΔH pp L from both exchange and magnetic dipole-dipole interactions are linearly dependent on the spin-label concentration. The temperature dependences of these two contributions are inversely related via the translational diffusion coefficient, the temperature dependence of which was characterized by a single effective activation energy. The intrinsic Lorentzian linewidths were determined by linear regression on the concentration dependence, within the optimization procedure. The concentration dependence of the Gaussian linewidth, δH pp G, which arises from unresolved proton hyperfine structure, was characterized by an empirical switching function proposed by B. L. Bales and D. Willett ( J. Chem. Phys., 80, 2997 (1984)). This simulates the effects of exchange narrowing by turning off the Gaussian broadening and turning on an additional Lorentzian broadening in a manner which is determined by the relative sizes of the exchange frequency and the effective proton hyperfine splitting. The total peak-to-peak, δH pp tot, linewidth is then related to the Lorentzian and Gaussian contributions via the relation ( δH pp G/ δH pp tot) 2 + δH pp L/ δH pp tot = 1, (S. N. Dobryakov and Ga. S. Lebedev, Sov. Phys. Dokl., 13, 873 (1969)). The method has been applied to the determination of the translational diffusion coefficients of spin-labeled phosphatidylcholine in dioleoyl phosphatidylcholine bilayers over the concentration range 0.5–2.5 mol% spin label, and the temperature range 5–80°C, and also to study of the diffusion of a small spin-labeled fatty acid in water.

Influence of the regulatory light chain of fast skeletal muscle myosin on its interaction with actin in the presence and absence of ATP

The effect of myosin LC2 modifications (phosphorylation or selective proteolytic removal of a seven-residue N-terminal peptide) and partial or complete removal of the whole LC2 was studied under various conditions. (1) Actin binding in the absence of ATP is not influenced by the nature of the myosin species (phosphorylated, dephosphorylated or devoid of LC2). (2) A 50% inhibition of K +/EDTA-ATPase was obtained with actin concentrations hardly different when phosphorylated and dephosphorylated myosins were compared (of the order of 5 μM), whereas both myosin devoid of LC2 and myosin in which the LC2 N-terminal peptide has been removed required significantly higher concentrations of actin (13.0 ± 2 and 12.0 ± 2.0 μM, respectively). (3) Dissociation of the actomyosin complex at high ionic strength with nucleotides is not infuenced by phosphorylation. (4) Actin activation of Mg 2+-ATPase is enhanced when LC2 is phosphorylated; no activation enhancement is observed with myosin devoid of LC2. (5) Translational diffusion coefficient measurements of myosin in high-ionic-strength solutions indicate a tendency for LC2-deprived myosin to form autoassociation oligomers. It thus appears that a structural modification (partial cleavage or removal of LC2) induces important structural changes in myosin, pointing to a role for LC2 in the intrinsic conformation of the molecule and its interaction potentialities. Effects of LC2 removal at high ionic strength are best explained by interactions bearing no relationship to physiological functions. A physiologically significant effect of LC2 phosphorylation requires a minimum degree of organization (actomyosin complex) to be expressed in which LC2 could play the role of a return-spring in the cross-bridge mechanism.

Hydrodynamics, size, and shape of bacteriophage T4D tails and baseplates.

AbstractWe have used translational diffusion coefficient measurements and subunit hydrodynamic theory to determine the dimensions and shape of bacterioophage T4D baseplates and tails. The diffusion coefficient of the baseplate, measured by quasielastic laser light scattering (QLS), was determined previously by Wagenknecht and Bloomfield to be D = 8.56 × 10−8 cm2/s. For the tail, we found D = 5.88 × 10−8 cm2/s by QLS, and D = 6.02 × 10−8 cm2/s by combining sedimentation coefficient and molecular weight in the Svedberg equation. These values, which have an uncertainty of ±2.7%, when combined with subunit hydrodynamic theory, enabled us to refine estimates of dimensions obtained by electron microscopy. For the hexagonal baseplate, the vertex‐to‐vertex distance is about 480 Å, the thickness is 160 Å, and there are six extended short fibers 320‐Å long and 40 Å in diameter. When a baseplate of these dimensions is attached to a tail tube‐sheath‐connector complex 1050‐Å long and 240 Å in diameter, the calculated D is 5.93 × 10−8 cm2/s, within 1% of experiment. This combined use of electron microscopy and hydrodynamics, using the former to ascertain shape, and the latter to obtain solution dimensions, is a powerful approach to the structure of biomolecular complexes.

Quasielastic light-scattering measurements of diffusion coefficients in polystyrene solutions

Abstract Measurement of translational diffusion coefficients by laser light scattering has been proposed as a convenient, rapid, and accurate method for characterization of macromolecular species in solution. We have determined diffusion coefficients at infinite dilution in various solutions of a series of narrow molecular weight polystyrene samples and found that the data are consistent with only one of two studies reported in the literature. The concentration dependence of the diffusion coefficient for molecules of different particle weight is investigated, and the infinite dilution values are used to access the unperturbed hydrodynamic dimensions and the excluded volume. The diffusion data are compared with the corresponding intrinsic viscosity values and related to the particle weight. Finally a simple method for evaluating macromolecular polydispersity is described and applied to two polystyrene samples with broad molecular weight distribution.

Solute diffusion in polymer networks. Part 3.—Hydroxyethylcellulose gels; solvent effects and fluorescence depolarisation measurements

Measurements of translational diffusion coefficients of oligosaccharides have been made as a function of temperature in a dilute hydroxyethylcellulose gel swollen in water, dimethyl formamide (DMF) and dimethyl sulphoxide (DMSO). The diffusion coefficients in the gel phase are substantially lower than the values for free diffusion in the bulk solvents in all cases. The influences of solvent character, temperature and polymer concentration on this reduction are discussed.Fluorescence depolarisation measurements were made over a corresponding temperature interval in the same gel system using fluorescein freely dispersed in the gel phase and also conjugated to the matrix polymer. The results are discussed in terms of the microscopic viscosities of the systems.

The Use of Stable Free Radicals to Determine Motion in Polymeric Systems

Abstract The application of thermally and chemically stable nitroxide free radicals to a variety of motional problems in polymeric systems has been investigated. Solvents do not cause significant motion in glassy polymers until the glass transition is lowered to the temperature of measurement. With nonsolvents the results depend on the nature of the sample preparation as well as on the polymer-solvent pair. Molecular motion at the surface of a polymer immersed in a nonsolvent was investigated by means of surface-labeled latex spheres. The application of nitroxides as spin labels to the measurement of translational diffusion coefficients is explored and discussed. Finally, the application of the technique to adsorption and motion of surfactants and polymers onto surfaces is reported.