Combined analysis of diffusion and relaxation behavior of water in apple parenchyma cells

With apple parenchymal cells as an example, we demonstrate the expedience of combined analysis of the relaxation and diffusion of water molecules in plant cells by NMR spectroscopy. At small diffusion times, our approach discerns three relaxation components pertaining to water in the vacuole, cytoplasm, and intercellular space. The corresponding self-diffusion coefficients are determined. At long diffusion times, it is possible to distinguish two components. For the slow-relaxing component (vacuolar water) we observe the mode of restricted diffusion. For the fast-relaxing components, the diffusion coefficient anomalously increases with time.     

PFG-NMR measurements of the self-diffusion coefficients of water in equilibrium poly(HEMA-co-THFMA) hydrogels

The self-diffusion coefficients for water in a series of copolymers of 2-hydroxyethyl methacrylate, HEMA, and tetrahydrofurfuryl methacrylate, THFMA, swollen with water to their equilibrium states have been studied at 310 K using PFG-NMR. The self-diffusion coefficients calculated from the Stejskal-Tanner equation, D(obs), for all of the hydrated polymers were found to be dependent on the NMR storage time, as a result of spin exchange between the proton reservoirs of the water and the polymers, reaching an equilibrium plateau value at long storage times. The true values of the diffusion coefficients were calculated from the values of D(obs) in the plateau regions by applying a correction for the fraction of water protons present, obtained from the equilibrium water contents of the gels. The true self-diffusion coefficient for water in polyHEMA obtained at 310 K by this method was 5.5 x 10(-10) m(2)s-1. For the copolymers containing 20% HEMA or more a single value of the self-diffusion coefficient was found, which was somewhat larger than the corresponding values obtained for the macroscopic diffusion coefficient from sorption measurements. For polyTHFMA and copolymers containing less than 20% HEMA, the PFG-NMR stimulated echo attenuation decay curves and the log-attenuation plots were characteristic of the presence of two diffusing water species. The self-diffusion coefficients of water in the equilibrium-hydrated copolymers were found to be dependent on the copolymer composition, decreasing with increasing THFMA content.     

Permeability of lysozyme tetragonal crystals to water

Diffusion of water within cross-linked tetragonal crystals of hen egg-white lysozyme has been measured and simulated on a computer using the X-ray structure of water-filled channels within the crystal lattice. Relative to the self-diffusion coefficient of bulk water molecules, the experimental diffusion coefficient of water within the crystal was found to be 13 times reduced in the (001) crystallographic plane and 5 times reduced in the [001] direction. Comparison of the experimental and computer simulated diffusion coefficients shows that steric limitations for water diffusion are mostly responsible for this reduction of the water diffusion in the crystal, with the self-diffusion coefficient of intracrystalline water reduced by no more than 30–40% as compared to that of bulk water.     

Transport properties of polymer solutions. A comparative approach

A variety of transport properties have been measured for solutions of the water soluble polymer poly(ethylene oxide)(PEO) with molecular weights ranging from 200 to 14,000, and volume fractions ranging from 0-80%. The transport properties are thermal conductivity, electrical conductivity at audio frequencies (in solutions containing dilute electrolyte), and water self-diffusion. These data, together with dielectric relaxation data previously reported, are amenable to analysis by the same mixture theory. The ionic conductivity and water self-diffusion coefficient, but not the thermal conductivity, are substantially smaller than predicted by the Maxwell and Hanai mixture relations, calculated using the known transport properties of pure liquid water. A 25% (by volume) solution of PEO exhibits an average dielectric relaxation frequency of the suspending water of one half that of pure water, with clear evidence of a distribution of relaxation times present. The limits of the cumulative distribution of dielectric relaxation times that are consistent with the data are obtained using a linear programming technique. The application of simple mixture theory, under appropriate limiting conditions, yields hydration values for the more dilute polymer solutions that are somewhat larger than values obtained from thermodynamic measurements.     

Lateral diffusion of cholesterol and dimyristoylphosphatidylcholine in a lipid bilayer measured by pulsed field gradient NMR spectroscopy

The pulsed field gradient NMR method for measuring self-diffusion has been used for a direct determination of the lateral diffusion coefficient of cholesterol, fluorine labeled at the 6-position, for an oriented lamellar liquid-crystalline phase of dimyristoylphosphatidylcholine (DMPC)/cholesterol/water. It is found that the diffusion coefficients of DMPC and cholesterol are equal over a large temperature interval. The apparent energy of activation for the diffusion process (58 kJ/mol) is about the same as for a lamellar phase of DMPC/water, whereas the phospholipid lateral diffusion coefficient is approximately four times smaller in the presence of cholesterol.     

Relationship between Recrystallization Rate of Ice Crystals in Sugar Solutions and Water Mobility in Freeze-Concentrated Matrix

To better understand the relation between recrystallization rate and water mobility in freeze-concentrated matrix, isothermal ice recrystallization rates in several sugar aqueous solutions and self-diffusion coefficients of water component in corresponding freeze-concentrated matrix were measured. The sugars used were fructose, glucose, maltose, and sucrose. The sugar concentrations and temperature were varied so that ice contents for all samples were almost equal. Neither recrystallization rates nor diffusion coefficients depended uniformly on temperature. The recrystallization rates increased with increasing the diffusion coefficients, and a direct relationship was found between recrystallization rate and diffusion coefficient. This indicated that self-diffusion coefficient of water component in freeze-concentrated matrix is a useful parameter for predicting and controlling recrystallization rate in sugar solutions relevant to frozen desserts.     

The self-diffusion of water in Artemia cysts

The self-diffusion coefficient of water molecules has been measured by nuclear magnetic resonance in cysts of Artemia over a wide range of hydration. Compared to the value for bulk water, the diffusion coefficient is reduced by a factor of 7 at the highest hydration and by approximately 100 at the lowest hydration. The results are used to evaluate various models that have been proposed to account for the reduction of water self-diffusion coefficients in complex systems.     

Water uptake by growing cells: an assessment of the controlling roles of wall relaxation, solute uptake, and hydraulic conductance

Growing plant cells increase in volume principally by water uptake into the vacuole. There are only three general mechanisms by which a cell can modulate the process of water uptake: (a) by relaxing wall stress to reduce cell turgor pressure (thereby reducing cell water potential), (b) by modifying the solute content of the cell or its surroundings (likewise affecting water potential), and (c) by changing the hydraulic conductance of the water uptake pathway (this works only for cells remote from water potential equilibrium). Recent studies supporting each of these potential mechanisms are reviewed and critically assessed. The importance of solute uptake and hydraulic conductance is advocated by some recent studies, but the evidence is indirect and conclusions remain controversial. For most growing plant cells with substantial turgor pressure, it appears that reduction in cell turgor pressure, as a consequence of wall relaxation, serves as the major initiator and control point for plant cell enlargement. Two views of wall relaxation as a viscoelastic or a chemorheological process are compared and distinguished.     

Intracellular diffusion of water

Self-diffusion of cell water has been measured at diffusion times ranging from 0.3 ms to 1.0 s for human red cells, yeast, and brine shrimp using various pulsed gradient NMR methods. Intracellular diffusion coefficients and membrane permeabilities are calculated from these data with the aid of previous theoretical results for regularly spaced permeable planar barriers. The intracellular diffusion coefficients of water range from 1.2 X 10(-6) to 6 X 10(-6) cm2/s for the various samples. Outer-membrane permeabilities to water range from 0.0001 to 0.01 cm/s. The self-diffusion coefficient of lipid in a sample of human breast adipose tissue was found to be 1.5 X 10(-7) cm2/s.     

1H-nmr study of water dynamics in hydrated collagen: transverse relaxation-time and diffusion analysis

Water proton transverse relaxation times (T2) and self-diffusion coefficients (D) were measured in randomly oriented hydrated collagen fibers. Three T2 relaxation times were discerned indicating the presence of at least three water fractions in the collagen sample. The D values associated with each water fraction were determined. The diffusion time dependence of D suggests water motion is restricted by macromolecular structure. The experimental results are discussed with reference to the structural properties of hydrated collagen fibers.     

Cation diffusion in cartilage measured by pulsed field gradient NMR

In this study, the pulsed field gradient (PFG) nuclear magnetic resonance (NMR) technique was used for the investigation of (1) concentration and compression effects on cation self-diffusion, and (2) restricted diffusion of cations in cartilage. Since physiologically relevant cations like Na+ are difficult to investigate owing to their very short relaxation times, the cations tetramethylammonium (TMA) and tetraethylammonium (TEA) were employed for diffusion studies in samples of explanted cartilage. Results indicated that the diffusion of monovalent cations shows strong similarities to observations already made in studies of the diffusion of water in cartilage: with increasing compression, i.e. decreasing water content, the diffusion coefficient of the cation decreases concomitantly. The diffusion coefficients also showed a decrease with increasing cation concentrations, basically reflecting the corresponding decrease in the water content. Both results could be explained by the well-established model of Mackie and Meares. This, together with the similarity of the diffusion coefficient D in cartilage relative to free solution (about 50%) for both cations, is consistent with the view that the water content and not the charge is the most important determinant of the intratissue diffusivity of monovalent cations. Diffusion studies with increasing observation times showed strong evidence of restricted diffusion, allowing the estimation of the geometry of barriers within cartilage.     

Diffusional properties of methanogenic granular sludge: 1H NMR characterization

The diffusive properties of anaerobic methanogenic and sulfidogenic aggregates present in wastewater treatment bioreactors were studied using diffusion analysis by relaxation time-separated pulsed-field gradient nuclear magnetic resonance (NMR) spectroscopy and NMR imaging. NMR spectroscopy measurements were performed at 22 degrees C with 10 ml of granular sludge at a magnetic field strength of 0.5 T (20 MHz resonance frequency for protons). Self-diffusion coefficients of H(2)O in the investigated series of mesophilic aggregates were found to be 51 to 78% lower than the self-diffusion coefficient of free water. Interestingly, self-diffusion coefficients of H(2)O were independent of the aggregate size for the size fractions investigated. Diffusional transport occurred faster in aggregates growing under nutrient-rich conditions (e.g., the bottom of a reactor) or at high (55 degrees C) temperatures than in aggregates cultivated in nutrient-poor conditions or at low (10 degrees C) temperatures. Exposure of aggregates to 2.5% glutaraldehyde or heat (70 or 90 degrees C for 30 min) modified the diffusional transport up to 20%. In contrast, deactivation of aggregates by HgCl(2) did not affect the H(2)O self-diffusion coefficient in aggregates. Analysis of NMR images of a single aggregate shows that methanogenic aggregates possess a spin-spin relaxation time and self-diffusion coefficient distribution, which are due to both physical (porosity) and chemical (metal sulfide precipitates) factors.     

An NMR study of the temperature dependence of the coefficient of water self-diffusion through lipid bilayer membranes

The temperature dependence of the coefficient of water self-diffusion through plane-parallel lipid multilayers of the phospholipid dioleoylphosphatidylcholine oriented on a glass support has been studied in the temperature range of 20-60 degrees C by the method of NMR with magnetic field pulse gradient. The values of the coefficients of transbilayer water diffusion are by four orders of magnitude less than for bulky water and ten times less than the coefficients of lateral diffusion of the lipid under the same conditions. The temperature dependence of the coefficient of water diffusion is described by the Arrhenius law with an apparent activation energy of about 41 kJ/mol, which far exceeds the activation energy for the diffusion of bulky water (18 kJ/mol). The experimental data were analyzed using a "dissolving-diffusion" model, by simulating the passage of water through membrane channels, and by analyzing the exchange of water molecules in states with different modes of translation mobility, including pore channels and bilayer "defects". Each of the approaches used made it possible to take the significance of bilayer permeability for the apparent energy of activation of water diffusion into account and estimate the energies of activation of water diffusion in the hydrophobic moiety of the bilayer, which were found to be close to the values for bulky water. The coefficients of water diffusion in the system under examination and the coefficients of permeation of water through the bilayer were estimated, and the effect of bilayer "defects" on the coefficients of water diffusion along and across bilayers was studied.     

NMR-investigation of restricted self-diffusion of oil in rape seeds

The self-diffusion of oil and water in rape seeds (Brassica napus L.) was measured with the NMR pulsed field gradient technique. The self-diffusion of oil was found to be completely restricted for diffusion times > 30 ms. The experiments could be explained in terms of the model of diffusion within spherical droplets and a Gaussian mass distribution of the droplet radii. The mean droplet radius was found to be about 0.7 m; this value decreased somewhat with increasing moisture content of the seeds. The experiments could also be explained with a Gaussian number distribution of droplet radii and a fraction of immobile protons in the NMR signal of 5 ... 10%, possibly arising from lipid protons. Though the transverse nuclear magnetic relaxation decay of the oil protons is not a single exponential we observe one uniform diffusive mobility for the oil molecules. The water self-diffusion coefficient at maximum moisture content of about 40% was determined to be 4.2 · 10^-10 m² s^-1 which is typical for swollen polymer-solvent systems at such a concentration. Offprint requests to: P. W. Kuchel     

Diffusional Properties of Methanogenic Granular Sludge: 1H NMR Characterization

The diffusive properties of anaerobic methanogenic and sulfidogenic aggregates present in wastewater treatment bioreactors were studied using diffusion analysis by relaxation time-separated pulsed-field gradient nuclear magnetic resonance (NMR) spectroscopy and NMR imaging. NMR spectroscopy measurements were performed at 22°C with 10 ml of granular sludge at a magnetic field strength of 0.5 T (20 MHz resonance frequency for protons). Self-diffusion coefficients of H₂O in the investigated series of mesophilic aggregates were found to be 51 to 78% lower than the self-diffusion coefficient of free water. Interestingly, self-diffusion coefficients of H₂O were independent of the aggregate size for the size fractions investigated. Diffusional transport occurred faster in aggregates growing under nutrient-rich conditions (e.g., the bottom of a reactor) or at high (55°C) temperatures than in aggregates cultivated in nutrient-poor conditions or at low (10°C) temperatures. Exposure of aggregates to 2.5% glutaraldehyde or heat (70 or 90°C for 30 min) modified the diffusional transport up to 20%. In contrast, deactivation of aggregates by HgCl₂ did not affect the H₂O self-diffusion coefficient in aggregates. Analysis of NMR images of a single aggregate shows that methanogenic aggregates possess a spin-spin relaxation time and self-diffusion coefficient distribution, which are due to both physical (porosity) and chemical (metal sulfide precipitates) factors.     

A model for water motion in crystals of lysozyme based on an incoherent quasielastic neutron-scattering study

This paper reports an incoherent quasielastic neutron scattering study of the single particle, diffusive motions of water molecules surrounding a globular protein, the hen egg-white lysozyme. For the first time such an analysis has been done on protein crystals. It can thus be directly related and compared with a recent structural study of the same sample. The measurement temperature ranged from 100 to 300 K, but focus was on the room temperature analysis. The very good agreement between the structural and dynamical studies suggested a model for the dynamics of water in triclinic crystals of lysozyme in the time range approximately 330 ps and at 300 K. Herein, the dynamics of all water molecules is affected by the presence of the protein, and the water molecules can be divided into two populations. The first mainly corresponds to the first hydration shell, in which water molecules reorient themselves fivefold to 10-fold slower than in bulk solvent, and diffuse by jumps from hydration site to hydration site. The long-range diffusion coefficient is five to sixfold less than for bulk solvent. The second group corresponds to water molecules further away from the surface of the protein, in a second incomplete hydration layer, confined between hydrated macromolecules. Within the time scale probed they undergo a translational diffusion with a self-diffusion coefficient reduced approximately 50-fold compared with bulk solvent. As protein crystals have a highly crowded arrangement close to the packing of macromolecules in cells, our conclusion can be discussed with respect to solvent behavior in intracellular media: as the mobility is highest next to the surface, it suggests that under some crowding conditions, a two-dimensional motion for the transport of metabolites can be dominant.     

Molecular dynamics simulation of liquid water under the influence of an external electric field

Molecular dynamics simulations of liquid water were performed at 258K and a density of 1.0?g/cm³ under various applied external electric field, ranging 0～10¹⁰?V/m. The influence of external field on structural and dynamical properties of water was investigated. The simple point charge (SPC) model is used for water molecules. An enhancement of the water hydrogen bond structure with increasing strength of the electric field has been deduced from the radial distribution functions and the analysis of hydrogen bonds structure. With increasing field strength, water system has a more perfect structure, which is similar to ice structure. However, the electrofreezing phenomenon of liquid water has not been detected since the self-diffusion coefficient was very large. The self-diffusion coefficient decreases remarkably with increasing strength of electric field and the self-diffusion coefficient is anisotropic.     

Water state and diffusion through lipid bilayers: Effect of hydration degree

The dependences of adsorbed water state (obtained from the variations in ¹H NMR spectra with the angle between the bilayer normal and magnetic field direction) and water diffusion along the bilayer normal (measured using pulsed field gradient ¹H NMR) on hydration degree have been studied in macroscopically oriented bilayers of dioleoylphosphatidylcholine. The angle dependences of the shape of NMR spectrum are qualitatively different only for water concentrations higher and lower than that achieved by hydration from saturated vapors (χ_eq, about 23%). At concentrations lower than χ_eq, all water in the sample either makes the hydration shells of the lipid polar heads or is in fast exchange with the shell water, so the spin-echo signal from water is detected only within a narrow range of angles close to the magic angle, 54.7°. At concentration exceeding χ_eq, the spin-echo signal from water is retained at all orientations, suggesting that a portion of water between bilayers (quasi-free water) slowly exchanges with water bound to the polar heads. There is an inverse dependence of the coefficient of water self-diffusion through the bilayer system on the hydration degree, which is described in the Tanner model with account of water self-diffusion in the hydrophobic part of the bilayer. Bilayer permeability, distribution coefficient of molecules between aqueous and lipid phases, and water self-diffusion coefficient in the hydrophobic region of the bilayer are estimated.     

Concerning Measurements of the Self-Diffusion Coefficient of Water Molecules and Time of Proton Spin-Lattice Relaxation in Plant Tissues

Measurements of the coefficient of water molecules self-diffusion (D) and the time of spin-lattice relaxation (T ₁) in prosenchyme (elongated) plant cells, whose length significantly exceeding their transverse size, show that the orientation of plant tissues in the H ₀field significantly affects the measured parameters. We conclude that this effect should be taken into account in experiments on the measurement of self-diffusion coefficients and time of proton spin-lattice relaxation in plant tissues containing prosenchyme cells.