Diffusion Measurements To Understand Dynamics and Structuring in Solutions Involving a Homologous Series of Ionic Liquids.

The self-diffusion coefficients of each of the components in mixtures containing pyridine and each of the homologous series 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imides in acetonitrile were determined using NMR diffusometry (i. e., Pulsed Gradient Spin Echo). The nature of solvation was found to change significantly with the proportion of salt in the mixtures. Increased diffusion coefficients (when corrected for viscosity) for the molecular components were observed with increasing proportion of ionic liquid and with increasing alkyl chain length on the cation. Comparison of the molecular solvents suggests increased interactions in solution of the pyridine with other components of the mixture, consistent with the proposed interactions shown previously to drive changes in reaction kinetics. Discontinuities were seen in the diffusion data for each species in solution across different ionic liquids between the hexyl and octyl derivatives, suggesting a change in the structuring in solution as the alkyl chain on the cation changes and demonstrating the importance of such when considering homologous series.

Evaluating the Stability of Cellulose Nanofiber Pickering Emulsions Using MRI and Relaxometry.

Magnetic resonance imaging (MRI) relaxometry and diffusion methods were used to highlight the instability mechanisms of oil-in-water Pickering emulsions stabilized by cellulose nanofibers (CNFs). Four different Pickering emulsions using different oils (n-dodecane and olive oil) and concentrations of CNFs (0.5 and 1.0 wt %) were systematically investigated over a period of one month after emulsification. The separation into a free oil, emulsion layer, and serum layer and the distribution of flocculated/coalesced oil droplets in several hundred micrometers were captured in MR images using fast low-angle shot (FLASH) and rapid acquisition with relaxation enhancement (RARE) sequences. The components of the Pickering emulsions (e.g., free oil, emulsion layer, oil droplets, and serum layer) were observable by different voxelwise relaxation times and apparent diffusion coefficients (ADCs) and reconstructing in the apparent T1, T2, and ADC maps. The mean T1, T2, and ADC of the free oil and serum layer corresponded well with MRI results for pure oils and water, respectively. Comparing the relaxation properties and translational diffusion coefficients of pure dodecane and olive oil obtained from NMR and MRI resulted in similar T1 and ADC but significantly different T2 depending on the sequence used. The diffusion coefficients of olive oil measured by NMR were much slower than dodecane. The ADC of the emulsion layer for dodecane emulsions did not correlate with the viscosity of the emulsions as the CNF concentration increased, suggesting the effects of restricted diffusion of oil/water molecules due to droplet packing.

Response to Comment on "Following Molecular Mobility during Chemical Reactions: No Evidence for Active Propulsion" and "Molecular Diffusivity of Click Reaction Components: The Diffusion Enhancement Question".

In their Comment (DOI: 10.1021/jacs.2c02965) on two related publications by our groups (J. Am. Chem. Soc. 2021, 143, 20884-20890; DOI: 10.1021/jacs.1c09455) and another (J. Am. Chem. Soc. 2022, 144, 1380-1388; DOI: 10.1021/jacs.1c11754), Huang and Granick discuss the diffusion NMR measurements of molecules during a copper-catalyzed azide-alkyne cycloaddition (CuAAC) "click" reaction. Here we respond to these comments and maintain that no diffusion enhancement was observed for any species during the reaction. We show that the relaxation agent does not interfere with the CuAAC reaction kinetics nor the diffusion of the molecules involved. Similarly, the gradient pulse length and diffusion time do not affect the diffusion coefficients. Peak overlap was completely removed in our study with the use of hydrazine as the reducing agent. The steady-state assumption does not hold for these diffusion measurements that take several minutes, which is the reason monotonic gradient orders are not suitable. Finally, we discuss the other reactions where similar changes in diffusion have been claimed. Our conclusions are fully supported by the results represented in our original JACS Article and the corresponding Supporting Information.

Preferential freezing avoidance localised in anthers and embryo sacs in wintering Daphne kamtschatica var. jezoensis flower buds visualised by magnetic resonance imaging.

To explore diversity in cold hardiness mechanisms, high resolution magnetic resonance imaging (MRI) was used to visualise freezing behaviours in wintering Daphne kamtschatica var. jezoensis flower buds, which have naked florets and no bud scales. MRI images showed that anthers remained stably supercooled to the range from -14 to -21°C or lower while most other tissues froze by -7°C. Freezing of some anthers detected in MRI images between -14 and -21°C corresponded with numerous low temperature exotherms and also with the 'all-or-nothing' type of anther injuries. In ovules/pistils, only embryo sacs remained supercooled at -7°C or lower, but slowly dehydrated during further cooling. Cryomicroscopic observation revealed ice formation in the cavities of calyx tubes and pistils but detected no ice in embryo sacs or in anthers. The distribution of ice nucleation activity in floral tissues corroborated the tissue freezing behaviours. Filaments likely work as the ice blocking barrier that prevents ice intrusion from extracellularly frozen calyx tubes to connecting unfrozen anthers. Unique freezing behaviours were demonstrated in Daphne flower buds: preferential freezing avoidance in male and female gametophytes and their surrounding tissues (by stable supercooling in anthers and by supercooling with slow dehydration in embryo sacs) while the remaining tissues tolerate extracellular freezing.

A volumetric and intra-diffusion study of solutions of AlCl3 in two ionic liquids - [C2TMEDA][Tf2N] and [C4mpyr][Tf2N].

Intra-diffusion coefficients (DSi) have been measured for the ionic liquid constituent ions and aluminium-containing species in aluminium chloride (AlCl3) solutions in the ionic liquids 1-(2-dimethyl-aminoethyl)-dimethylethylammonium bis(trifluoromethylsulfonyl)amide ([C2TMEDA][Tf2N]) and N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([C4mpyr][Tf2N]), to investigate whether spectroscopically detected interactions between the ions and AlCl3 affect these properties. Such electrolyte solutions are of interest for the electrowinning of aluminium. The temperature, composition and molar volume dependences are investigated. Apparent (Vϕ,1) and partial molar (V1) volumes for AlCl3 have been calculated from solution densities. For [C2TMEDA][Tf2N] solutions, Vϕ,1 increases with increasing solute concentration; for [C4mpyr][Tf2N] solutions, it decreases. In pure [C2TMEDA][Tf2N], the cation diffuses more quickly than the anion, but this changes as the AlCl3 concentration increases. In the [C4mpyr][Tf2N] solutions, the intra-diffusion coefficient ratio remains equal to that for the pure ionic liquid and the aluminium species diffuses at approximately the same rate as the anion at each composition. The intra-diffusion coefficients can be fitted to the Ertl-Dullien free volume power law by superposing the iso-concentration curves with concentration dependent, but temperature independent, molar volume offsets. This suggests that they are primarily dependent on the molar volume and secondarily on a colligative thermodynamic factor due to dilution by AlCl3. AlCl3 complexation by [Tf2N]- and [C2TMEDA]+, confirmed by 27Al, 15N and 19F NMR spectroscopy, seems to play a minor role. Our results indicate that the application of free volume theories might be fruitful in the study of the transport properties of ionic liquid solutions and mixtures.

Following Molecular Mobility during Chemical Reactions: No Evidence for Active Propulsion.

The reported changes in self-diffusion of small molecules during reactions have been attributed to "boosted mobility". We demonstrate the critical role of changing concentrations of paramagnetic ions on nuclear magnetic resonance (NMR) signal intensities, which led to erroneous measurements of diffusion coefficients. We present simple methods to overcome this problem. The use of shuffled gradient amplitudes allows accurate diffusion NMR measurements, even with time-dependent relaxation rates caused by changing concentrations of paramagnetic ions. The addition of a paramagnetic relaxation agent allows accurate determination of both diffusion coefficients and reaction kinetics during a single experiment. We analyze a copper-catalyzed azide-alkyne cycloaddition "click" reaction, for which boosted mobility has been claimed. With our methods, we accurately measure the diffusive behavior of the solvent, starting materials, and product and find no global increase in diffusion coefficients during the reaction. We overcome NMR signal overlap using an alternative reducing agent to improve the accuracy of the diffusion measurements. The alkyne reactant diffuses slower as the reaction proceeds due to binding to the copper catalyst during the catalytic cycle. The formation of this intermediate was confirmed by complementary NMR techniques and density functional theory calculations. Our work calls into question recent claims that molecules actively propel or swim during reactions and establishes that time-resolved diffusion NMR measurements can provide valuable insight into reaction mechanisms.

Explicit phenomenological solutions for magnetization exposed to an arbitrary NMR diffusion steady state pulse sequence.

Explicit phenomenological solutions to recurrence relations for the bulk transverse and longitudinal magnetization found using the Torrey-Bloch equations with relaxation effects are used to investigate nuclear magnetic resonance (NMR) diffusion measurements. Of particular interest are steady state NMR (self-)diffusion measurements that reduce experimental time that can extend the techniques to quickly reacting systems. The solutions for bulk transverse and longitudinal magnetization presented here are used to investigate the average behavior of the transverse and longitudinal magnetization in forming a steady state and are used to derive new expressions for the steady state longitudinal magnetization. These solutions can be applied to a noninteracting spin 1/2 ensemble undergoing free diffusion exposed to an arbitrary NMR pulse sequence containing arbitrary magnetic field gradient waveforms. The closed algebraic form method presented here has an advantage over iterative procedures for calculating transverse and longitudinal magnetization for the analysis and development of steady state pulse sequences. Previous theoretical results for steady state diffusion measurements are also reproduced. The Mathematica code for these solutions is provided in the supplementary material.

Controlled Diffusion of Photoswitchable Receptors by Binding Anti-electrostatic Hydrogen-Bonded Phosphate Oligomers.

Dihydrogen phosphate anions are found to spontaneously associate into anti-electrostatic oligomers via hydrogen bonding interactions at millimolar concentrations in DMSO. Diffusion NMR measurements supported formation of these oligomers, which can be bound by photoswitchable anion receptors to form large bridged assemblies of approximately three times the volume of the unbound receptor. Photoisomerization of the oligomer-bound receptor causes a decrease in diffusion coefficient of up to 16%, corresponding to a 70% increase in effective volume. This new approach to external control of diffusion opens prospects in controlling molecular transport using light.

Applications of WaterControl to TOCSY and COSY experiments.

WaterControl is a solvent suppression method based on WATERGATE and PGSTE and is very efficient in selectively reducing the solvent signal in 1D pulse-acquire and 2D NOESY of protein solutions. In this study, the WaterControl technique was appended to two common 2D NMR methods used in resonance assignment of proteins, namely TOCSY and CLIP-COSY. Similar to that observed in regular 1D pulse-acquire and 2D NOESY, the incorporation of WaterControl in these 2D methods led to excellent solvent suppression superior to that obtained using W3- or W5-based WATERGATE sequences. The water signal was essentially eliminated in the TOCSY and CLIP-COSY with WaterControl while useful cross peaks around the water resonance at ω2 were preserved. This is in contrast to the 2D spectra obtained from the corresponding WATERGATE containing sequences, where these cross peaks in the ω2 region are usually suppressed together with the water resonance. These new WaterControl sequences provide significantly improved water suppression thereby facilitating protein NMR studies.

Thiol-water proton exchange of glutathione, cysteine, and N-acetylcysteine: Implications for CEST MRI.

Amide-, amine-, and hydroxyl-water proton exchange can generate MRI contrast through chemical exchange saturation transfer (CEST). In this study, we show that thiol-water proton exchange can also generate quantifiable CEST effects under near-physiological conditions (pH = 7.2 and 37°C) through the characterization of the pH dependence of thiol proton exchange in phosphate-buffered solutions of glutathione, cysteine, and N-acetylcysteine. The spontaneous, base-catalyzed, and buffer-catalyzed exchange contributions to the thiol exchange were analyzed. The thiol-water proton exchange of glutathione and cysteine was found to be too fast to generate a CEST effect around neutral pH due to significant base catalysis. The thiol-water proton exchange of N-acetylcysteine was found to be much slower, yet still in the fast-exchange regime with significant base and buffer catalysis, resulting in a 9.5% attenuation of the water signal at pH 7.2 in a slice-selective CEST NMR experiment. Furthermore, the N-acetylcysteine thiol CEST was also detectable in human serum albumin and agarose phantoms.

Applications for Transition-Metal Chemistry in Contrast-Enhanced Magnetic Resonance Imaging.

Contrast-enhanced magnetic resonance imaging (MRI) is an indispensable tool for diagnostic medicine. However, safety concerns related to gadolinium in commercial MRI contrast agents have emerged in recent years. For patients suffering from severe renal impairment, there is an important unmet medical need to perform contrast-enhanced MRI without gadolinium. There are also concerns over the long-term effects of retained gadolinium within the general patient population. Demand for gadolinium-free MRI contrast agents is driving a new wave of inorganic chemistry innovation as researchers explore paramagnetic transition-metal complexes as potential alternatives. Furthermore, advances in personalized care making use of molecular-level information have motivated inorganic chemists to develop MRI contrast agents that can detect pathologic changes at the molecular level. Recent studies have highlighted how reaction-based modulation of transition-metal paramagnetism offers a highly effective mechanism to achieve MRI contrast enhancement that is specific to biochemical processes. This Viewpoint highlights how recent advances in transition-metal chemistry are leading the way for a new generation of MRI contrast agents.

Time-Resolved Diffusion NMR Measurements for Transient Processes.

A general procedure for measurement of time-resolved diffusion coefficients of molecular species by NMR is described, including the use of methanol for fast temperature-independent gradient calibration.

Hydrogen isotope replacement changes hydration and large scale structure, but not small scale structure, of agarose hydrogel networks.

Agarose samples of low (Ag1) and high (Ag2) O -methyl content on position 6 of the galactose residue were studied in H2O and D2O. Differential scanning calorimetry, turbidity and rheological measurements showed a [Formula: see text] 2 ° C shift in the coil-to-helix transition temperature, indicating higher helix stability in D2O. The differential scanning calorimetry data could be superimposed using a temperature shift factor, suggesting similar extents of helix aggregation in both solvents. Small angle X-ray scattering of H2O and D2O gels were essentially identical, indicating no change in the small scale ( [Formula: see text] 0.05-20 nm) network structure on isotopic exchange. Larger ([Formula: see text] 1 µm) scale heterogeneities were more pronounced in deuterium gels. The 1HT2 relaxation times were measured at different H/D ratios. These relaxation times were analyzed using a model assuming regular solution mixing of H2O, HDO and D2O between the solvent and gel phases. The fit results suggested that H2O has higher affinity for the agarose network than HDO and D2O. The difference, however, was much larger for the Ag2 sample. This finding implies that the higher hydrophobic effect observed in D2O affects the hydration state much more strongly for the more hydrophobic (and more polarizable) agarose sample Ag2. As a consequence, Ag2 (but not Ag1) gels retained more H2O than D2O. In contrast, the bulk rheology of either hydrogel was not affected by the isotopic exchange.

Erratum to: Hydrogen isotope replacement changes hydration and large scale structure, but not small scale structure, of agarose hydrogel networks.

With reference to eq. (5) in the original article, the authors have neglected to specify the description of some terms in the text. They correct the mistake in the present erratum.

Enhanced Diffusion of Molecular Catalysts is Due to Convection.

Intriguing reports of enhanced diffusion in enzymes and molecular catalysts have spurred significant interest in experimental and theoretical investigations, and the mechanism of this phenomenon is the topic of lively debate. Here we use time-resolved diffusion NMR methods to measure the diffusion coefficients (D) of small molecule species involved in chemical reactions with high temporal resolution. We show the enhanced diffusion of small molecules cannot be explained by reaction velocity, and that apparent measurements of enhanced diffusion by small molecules appear to be caused by bulk fluid flow processes such as convection.

A Simple and Effective Binomial Block Based Pulse Sequence Capable of Suppressing Multiple NMR Signals.

A binomial-like block based multiple suppression NMR pulse sequence, termed MULTI-GATE-FSB, that is simple to implement with outstanding suppression performance for multiple solvent signals (or multiple resonances) is investigated. The sequence was tested on two water-alcohol solvent systems, and a standard lysozyme sample, with suppression of three or four regions (though it is extendable to any number of regions). The suppression of all solvent signals was possible in the alcohol-water systems tested with both long and short recycle delays and without the requirement for lengthy presaturation pulses. Such a sequence holds promise not only for LC-NMR applications and solvent suppression but for multiple suppression applications in general (e.g., analysis of impurities/components).

Ice Nucleation Activity in Plants: The Distribution, Characterization, and Their Roles in Cold Hardiness Mechanisms.

Control of freezing in plant tissues is a key issue in cold hardiness mechanisms. Yet freeze-regulation mechanisms remain mostly unexplored. Among them, ice nucleation activity (INA) is a primary factor involved in the initiation and regulation of freezing events in plant tissues, yet the details remain poorly understood. To address this, we developed a highly reproducible assay for determining plant tissue INA and noninvasive freeze visualization tools using MRI and infrared thermography. The results of visualization studies on plant freezing behaviors and INA survey of over 600 species tissues show that (1) freezing-sensitive plants tend to have low INA in their tissues (thus tend to transiently supercool), while wintering cold-hardy species have high INA in some specialized tissues; and (2) the high INA in cold-hardy tissues likely functions as a freezing sensor to initiate freezing at warm subzero temperatures at appropriate locations and timing, resulting in the induction of tissue-/species-specific freezing behaviors (e.g., extracellular freezing, extraorgan freezing) and the freezing order among tissues: from the primary freeze to the last tissue remaining unfrozen (likely INA level dependent). The spatiotemporal distributions of tissue INA, their characterization, and functional roles are detailed. INA assay principles, anti-nucleation activity (ANA), and freeze visualization tools are also described.

Shortening NMR experimental times.

Conventionally, arrayed nuclear magnetic resonance experiments, such as diffusion and relaxation, are performed with the same number of scans (NS) at each iteration despite the signal-to-noise ratio being more than sufficient for many of the iterations. Here, we propose a simple yet effective approach that significantly shortens experimental times by varying NS through the arrayed experiments while keeping the signal-to-noise ratio essentially the same and retaining experimental accuracy.

NOESY-WaterControl: a new NOESY sequence for the observation of under-water protein resonances.

Highly selective and efficient water signal suppression is indispensable in biomolecular 2D nuclear Overhauser effect spectroscopy (NOESY) experiments. However, the application of conventional water suppression schemes can cause a significant or complete loss of the biomolecular resonances at and around the water chemical shift (ω2). In this study, a new sequence, NOESY-WaterControl, was developed to address this issue. The new sequence was tested on lysozyme and bovine pancreatic trypsin inhibitor (BPTI), demonstrating its efficiency in both water suppression and, more excitingly, preserving water-proximate biomolecular resonances in ω2. The 2D NOESY maps obtained using the new sequence thus provide more information than the maps obtained with conventional water suppression, thereby lessening the number of experiments needed to complete resonance assignments of biomolecules. The 2D NOESY-WaterControl map of BPTI showed strong bound water and exchangeable proton signals in ω1 but these signals were absent in ω2, indicating the possibility of using the new sequence to discriminate bound water and exchangeable proton resonances from non-labile proton resonances with similar chemical shifts to water.

The protective effect of apolipoprotein in models of trophoblast invasion and preeclampsia.

Preeclampsia is a hypertensive disorder of pregnancy. It is associated with abnormal placentation via poor placental invasion of the uterine vasculature by trophoblast cells, leading to poor placental perfusion, oxidative stress, and inflammation, all of which are implicated in its pathogenesis. A dyslipidemia characterized by low plasma levels of high-density lipoproteins (HDL) and elevated triglycerides has been described in preeclampsia. Apolipoprotein A-I (apoA-I), a constituent of HDL is an anti-inflammatory agent. This study investigated whether apoA-I protects against hypertension and adverse placental changes in a proinflammatory cytokine (TNF-α)-induced model of preeclampsia. Further, this study investigated whether apoA-I protects against the inhibitory effect of TNF-α in a human in vitro model of trophoblast invasion. Administration of apoA-I to pregnant mice before infusion with TNF-α resulted in a significant reduction in the cytokine-induced increase in systolic blood pressure. MRI measurement of T2 relaxation, a parameter that is tissue specific and sensitive to physiological changes within tissues, showed a reversal of TNF-α-induced placental changes. Preincubation of endothelial cells with apoA-I protected against the TNF-α-induced inhibition of HTR-8/SVneo (trophoblast) cell integration into endothelial (UtMVEC) networks. These data suggest that a healthy lipid profile may affect pregnancy outcomes by priming endothelial cells in preparation for trophoblast invasion.