The NMR studies of water in biological systems

Deciphering adiabatic rotating frame relaxometry in biological tissues

PURPOSE

This work aims to unravel the intricacies of adiabatic rotating frame relaxometry in biological tissues.

THEORY AND METHODS

The classical formalisms of dipolar relaxationR 1 ρ $$ {R}_{1\rho } $$ andR 2 ρ $$ {R}_{2\rho } $$ were systematically analyzed for water molecules reorienting on "fast" and "slow" timescales. These two timescales are, respectively, responsible for the absence and presence ofR 1 ρ $$ {R}_{1\rho } $$ dispersion. A time-averagedR 1 ρ $$ {R}_{1\rho } $$ orR 2 ρ $$ {R}_{2\rho } $$ over an adiabatic pulse duration was recast into a sum ofR 1 $$ {R}_1 $$ andR 2 $$ {R}_2 $$ , but with different weightings. These weightings depend on the specific modulations of adiabatic pulse waveforms. In this context, stretched hyperbolic secant (HSn $$ HSn $$ ) pulses were characterized. Previously publishedH S 1 $$ HS1 $$ R 1 ρ $$ {R}_{1\rho } $$ , continuous-wave (CW)R 1 ρ $$ {R}_{1\rho } $$ , andR 1 $$ {R}_1 $$ measures from 12 agarose phantoms were used to validate the theoretical predictions. A similar validation was also performed on previously publishedHSn $$ HSn $$ R 1 ρ $$ {R}_{1\rho } $$ (n $$ n $$ =1, 4, 8) andHS 1 $$ HS1 $$ R 2 ρ $$ {R}_{2\rho } $$ from bovine cartilage specimens.

RESULTS

Longitudinal relaxation weighting decreases forHSn $$ HSn $$ pulses asn $$ n $$ increases. Predicted CWR 1 ρ cal $$ {R}_{1\rho}^{cal} $$ values from agarose phantoms align well with the measured CWR 1 ρ exp $$ {R}_{1\rho}^{exp} $$ values, as indicated by a linear regression function:R 1 ρ cal = 1.04 * R 1 ρ exp - 1.96 $$ {R}_{1\rho}^{cal}={1.04}^{\ast }{R}_{1\rho}^{exp}-1.96 $$ . The predicted adiabaticR 1 ρ $$ {R}_{1\rho } $$ andR 2 ρ $$ {R}_{2\rho } $$ from cartilage specimens are consistent with those previously measured, as quantified by:R 1 ρ , 2 ρ cal = 1.10 * R 1 ρ , 2 ρ exp - 0.41 $$ {R}_{1\rho, 2\rho}^{cal}={1.10}^{\ast }{R}_{1\rho, 2\rho}^{exp}-0.41 $$ .

CONCLUSION

This work has theoretically and experimentally demonstrated that adiabaticR 1 ρ $$ {R}_{1\rho } $$ andR 2 ρ $$ {R}_{2\rho } $$ can be recast into a sum ofR 1 $$ {R}_1 $$ andR 2 $$ {R}_2 $$ , with varying weightings. Therefore, any suggestions that adiabatic rotating frame relaxometry in biological tissues could provide more information than the standardR 1 $$ {R}_1 $$ andR 2 $$ {R}_2 $$ warrant closer scrutiny.

Bone marrow adiposity modulation after long duration spaceflight in astronauts

Space travel requires metabolic adaptations from multiple systems. While vital to bone and blood production, human bone marrow adipose (BMA) tissue modulation in space is unknown. Here we show significant downregulation of the lumbar vertebrae BMA in 14 astronauts, 41 days after landing from six months' missions on the International Space Station. Spectral analyses indicated depletion of marrow adipose reserves. We then demonstrate enhanced erythropoiesis temporally related to low BMA. Next, we demonstrated systemic and then, local lumbar vertebrae bone anabolism temporally related to low BMA. These support the hypothesis that BMA is a preferential local energy source supplying the hypermetabolic bone marrow postflight, leading to its downregulation. A late postflight upregulation abolished the lower BMA of female astronauts and BMA modulation amplitude was higher in younger astronauts. The study design in the extreme environment of space can limit these conclusions. BMA modulation in astronauts can help explain observations on Earth.

Orientation dependent proton transverse relaxation in the human brain white matter: The magic angle effect on a cylindrical helix

PURPOSE

To overcome some limitations of previous proton orientation-dependent transverse relaxation formalisms in human brain white matter (WM) by a generalized magic angle effect function.

METHODS

A cylindrical helix model was developed embracing anisotropic rotational and translational diffusion of restricted molecules in WM, with the former characterized by an axially symmetric system. Transverse relaxation rates R₂ and R₂^∗ were divided into isotropic R₂ⁱ and anisotropic parts, R₂^a ∗ f(α,Φ - ε₀), with α denoting an open angle and ε₀ an orientation (Φ) offset from DTI-derived primary diffusivity direction. The proposed framework (Fit A) was compared to prior models without ε₀ on previously published water and methylene proton transverse relaxation rates from developing, healthy, and pathological WM at 3 T. Goodness of fit was represented by root-mean-square error (RMSE). F-test and linear correlation were used with statistical significance set to P ≤ 0.05.

RESULTS

Fit A significantly (P < 0.01) outperformed prior models as demonstrated by reduced RMSEs, e.g., 0.349 vs. 0.724 in myelin water. Fitted ε₀ was in good agreement with calculated ε₀ from directional diffusivities. Compared with those from healthy adult, the fitted R₂ⁱ, R₂^a, and α from neonates were substantially reduced but ε₀ increased, consistent with developing myelination. Significant positive (R₂ⁱ) and negative (α and R₂^a) correlations were found with aging (demyelination) in elderly.

CONCLUSION

The developed framework can better characterize orientation dependences from a wide range of proton transverse relaxation measurements in the human brain WM, thus shedding new light on myelin microstructural alterations at the molecular level.

In vivo direct imaging of neuronal activity at high temporospatial resolution

There has been a long-standing demand for noninvasive neuroimaging methods that can detect neuronal activity at both high temporal and high spatial resolution. We present a two-dimensional fast line-scan approach that enables direct imaging of neuronal activity with millisecond precision while retaining the high spatial resolution of magnetic resonance imaging (MRI). This approach was demonstrated through in vivo mouse brain imaging at 9.4 tesla during electrical whisker-pad stimulation. In vivo spike recording and optogenetics confirmed the high correlation of the observed MRI signal with neural activity. It also captured the sequential and laminar-specific propagation of neuronal activity along the thalamocortical pathway. This high-resolution, direct imaging of neuronal activity will open up new avenues in brain science by providing a deeper understanding of the brain's functional organization, including the temporospatial dynamics of neural networks.

In-cell NMR: Why and how?

NMR spectroscopy has been applied to cells and tissues analysis since its beginnings, as early as 1950. We have attempted to gather here in a didactic fashion the broad diversity of data and ideas that emerged from NMR investigations on living cells. Covering a large proportion of the periodic table, NMR spectroscopy permits scrutiny of a great variety of atomic nuclei in all living organisms non-invasively. It has thus provided quantitative information on cellular atoms and their chemical environment, dynamics, or interactions. We will show that NMR studies have generated valuable knowledge on a vast array of cellular molecules and events, from water, salts, metabolites, cell walls, proteins, nucleic acids, drugs and drug targets, to pH, redox equilibria and chemical reactions. The characterization of such a multitude of objects at the atomic scale has thus shaped our mental representation of cellular life at multiple levels, together with major techniques like mass-spectrometry or microscopies. NMR studies on cells has accompanied the developments of MRI and metabolomics, and various subfields have flourished, coined with appealing names: fluxomics, foodomics, MRI and MRS (i.e. imaging and localized spectroscopy of living tissues, respectively), whole-cell NMR, on-cell ligand-based NMR, systems NMR, cellular structural biology, in-cell NMR… All these have not grown separately, but rather by reinforcing each other like a braided trunk. Hence, we try here to provide an analytical account of a large ensemble of intricately linked approaches, whose integration has been and will be key to their success. We present extensive overviews, firstly on the various types of information provided by NMR in a cellular environment (the "why", oriented towards a broad readership), and secondly on the employed NMR techniques and setups (the "how", where we discuss the past, current and future methods). Each subsection is constructed as a historical anthology, showing how the intrinsic properties of NMR spectroscopy and its developments structured the accessible knowledge on cellular phenomena. Using this systematic approach, we sought i) to make this review accessible to the broadest audience and ii) to highlight some early techniques that may find renewed interest. Finally, we present a brief discussion on what may be potential and desirable developments in the context of integrative studies in biology.

An efficient R1ρ dispersion imaging method for human knee cartilage using constant magnetization prepared turbo-FLASH

This work aimed to develop an efficient R_1ρ dispersion imaging method for clinical studies of human knee cartilage at 3 T. Eight constant magnetizations (M_prep ) were prepared by tailoring both the duration and amplitude (ω₁ ) of a fully refocused spin-lock preparation pulse. The limited M_prep dynamic range was expanded by the measure, equivalent to that with ω₁  = ∞, from the magic angle location in the deep femoral cartilage. The developed protocol with M_prep  = 60% was demonstrated on one subject's bilateral and two subjects' unilateral asymptomatic knees. The repeatability of the proposed protocol was estimated by two repeated scans with a three-month gap for the last two subjects. The synthetic R_1ρ and R₂ derived from R_1ρ dispersions were compared with the published references using state-of-the-art R_1ρ and R₂ mapping (MAPSS). The proposed protocol demonstrated good (<5%) repeatability quantified by the intra- and intersubject coefficients of variation in the femoral and tibial cartilage. The synthetic R_1ρ (1/s) and the references were comparable in the femoral (23.0 ± 5.3 versus 24.1 ± 3.8, P = 0.67) and the tibial (29.1 ± 8.8 versus 27.1 ± 5.1, P = 0.62), but not the patellar (16.5 ± 4.9 versus 22.7 ± 1.6, P < 0.01) cartilage. The same trends were also observed for the current and the previous R₂ . In conclusion, the developed R_1ρ dispersion imaging scheme has been revealed to be not only efficient but also robust for clinical studies of human knee cartilage at 3 T.

Machine learning assistive rapid, label-free molecular phenotyping of blood with two-dimensional NMR correlational spectroscopy

Translation of the findings in basic science and clinical research into routine practice is hampered by large variations in human phenotype. Developments in genotyping and phenotyping, such as proteomics and lipidomics, are beginning to address these limitations. In this work, we developed a new methodology for rapid, label-free molecular phenotyping of biological fluids (e.g., blood) by exploiting the recent advances in fast and highly efficient multidimensional inverse Laplace decomposition technique. We demonstrated that using two-dimensional T1-T2 correlational spectroscopy on a single drop of blood (<5 μL), a highly time- and patient-specific 'molecular fingerprint' can be obtained in minutes. Machine learning techniques were introduced to transform the NMR correlational map into user-friendly information for point-of-care disease diagnostic and monitoring. The clinical utilities of this technique were demonstrated through the direct analysis of human whole blood in various physiological (e.g., oxygenated/deoxygenated states) and pathological (e.g., blood oxidation, hemoglobinopathies) conditions.

NMR techniques in studying water in biotechnological systems

Different NMR methodologies have been considered in studying water as a part of the structure of heterogeneous biosystems. The current work mostly describes NMR techniques to investigate slow translational dynamics of molecules affecting anisotropic properties of polymers and biomaterials. With these approaches, information about organized structures and their stability could be obtained in conditions when external factors affect biomolecules. Such changes might include rearrangement of macromolecular conformations at fabrication of nano-scaffolds for tissue engineering applications. The changes in water-fiber interactions could be mirrored by the magnetic resonance methods in various relaxations, double-quantum filtered (DQF), 1D and 2D translational diffusion experiments. These findings effectively demonstrate the current state of NMR studies in applying these experiments to the various systems with the anisotropic properties. For fibrous materials, it is shown how NMR correlation experiments with two gradients (orthogonal or collinear) encode diffusion coefficients in anisotropic materials and how to estimate the permeability of cell walls. It is considered how the DQF NMR technique discovers anisotropic water in natural polymers with various cross-links. The findings clarify hydration sites, dynamic properties, and binding of macromolecules discovering the role of specific states in improving scaffold characteristics in tissue engineering processes. Showing the results in developing these NMR tools, this review focuses on the ways of extracting information about biophysical properties of biomaterials from the NMR data obtained.

Role of Different States of Solubilized Water on Solvation Dynamics and Rotational Relaxation of Coumarin 490 in Reverse Micelles of Gemini Surfactants, Water/12-s-12.2Br- (s = 5, 6, 8)/n-Propanol/Cyclohexane

The present study demonstrates how the different states of solubilized water viz. quaternary ammonium headgroup-bound, bulklike, counterion-bound, and free water in reverse micelles of a series of cationic gemini surfactants, water/12-s-12 (s = 5, 6, 8).2Br^-/n-propanol/cyclohexane, control the solvation dynamics and rotational relaxation of Coumarin 490 (C-490) and microenvironment of the reverse micelles. The relative number of solubilized water molecules of a given state per surfactant molecule decides major and minor components. A rapid increase in the number of bulklike water molecules per surfactant molecule as compared to the slow increase in the number of each of headgroup- and counterion-bound water molecules per surfactant molecule with increasing water content (W _o) in a given reverse micellar system is responsible for the increase in the rate of solvation and rotational relaxation of C-490. The increase in the number of counterion-bound water molecules per surfactant molecule and the concomitant decrease in the number of bulklike water molecules per surfactant molecule with increasing spacer chain length of gemini surfactants at a given W _o are ascribed to the slower rates of both solvation and rotational relaxation. Relative abundances of different states of water have a role on the microenvironment of the reverse micelles as well. Thus, a comprehensive effect of different states of water on dynamics in complex biomimicking systems has been presented here.

An order parameter without magic angle effect (OPTIMA) derived from R 1 ρ dispersion in ordered tissue

PURPOSE

MR R₂ imaging of ordered tissue exhibits the magic angle effect, potentially masking subtle pathological changes in cartilage. This work aimed to develop an orientation-independent order parameter (S) exclusively sensitive to collagen degeneration.

METHODS

A theory was developed based on R 1 ρ dispersion coupled with a simplified molecular motion model in which anisotropic R 2 a ( θ ) became directly proportional to correlation time τ b θ and S could be derived. This new parameter was validated with ex vivo R 1 ρ dispersion reported on orientated (n = 4), enzymatically depleted bovine cartilage (n = 6), and osteoarthritic human knee specimens (n = 14) at 9.4 Tesla, which was further demonstrated on 1 healthy human knee in vivo at 3 Tesla.

RESULTS

τ b θ from orientation-dependent R 1 ρ dispersion revealed a significantly high average correlation (r = 0.89 ± 0.05, P < 0.05) with R 2 a (θ) on cartilage samples and a moderate correlation (r = 0.48, P < 0.001) for the human knee in vivo. The derived S (10^-3 ) significantly decreased in advanced osteoarthritis (1.64 ± 0.03 vs. 2.30 ± 0.11, P < 0.001) and collagen-depleted samples (1.30 ± 0.11 vs. 2.12 ± 0.12, P < 0.001) when compared with early osteoarthritis and the control, respectively.

CONCLUSION

The proposed order parameter could be a potentially useful orientation-independent MR biomarker for collagen alterations in cartilage and other highly structured tissues.

Fluid assessment in dialysis patients by point-of-care magnetic relaxometry

Magnetic resonance imaging (MRI) is a powerful diagnostic tool, but its use is restricted to the scanner suite. Here, we demonstrate that a bedside nuclear magnetic resonance (NMR) sensor can assess fluid status changes in individuals at a fraction of the time and cost compared to MRI. Our study recruited patients with end-stage renal disease (ESRD) who were regularly receiving hemodialysis treatments with intradialytic fluid removal as a model of volume overload and healthy controls as a model of euvolemia. Quantitative T 2 measurements of the lower leg of patients with ESRD immediately before and after dialysis were compared to those of euvolemic healthy controls using both a 0.28-T bedside single-voxel NMR sensor and a 1.5-T clinical MRI scanner. In the MRI data, we found that the first sign of fluid overload was an expanded muscle extracellular fluid (ECF) space, a finding undetectable at this stage using physical exam. A decrease in muscle ECF upon fluid removal was similarly detectable with both the bedside sensor and MRI. Bioimpedance measurements performed comparably to the bedside NMR sensor but were generally worse than MRI. These findings suggest that bedside NMR may be a useful method to identify fluid overload early in patients with ESRD and potentially other hypervolemic patient populations.

Transverse relaxation-based assessment of mammographic density and breast tissue composition by single-sided portable NMR

PURPOSE

Elevated mammographic density (MD) is an independent risk factor for breast cancer (BC) as well as a source of masking in X-ray mammography. High-frequency longitudinal monitoring of MD could also be beneficial in hormonal BC prevention, where early MD changes herald the treatment's success. We present a novel approach to quantification of MD in breast tissue using single-sided portable NMR. Its development was motivated by the low cost of portable-NMR instrumentation, the suitability for measurements in vivo, and the absence of ionizing radiation.

METHODS

Five breast slices were obtained from three patients undergoing prophylactic mastectomy or breast reduction surgery. Carr-Purcell-Meiboom-Gill (CPMG) relaxation curves were measured from (1) regions of high and low MD (HMD and LMD, respectively) in the full breast slices; (2) the same regions excised from the full slices; and (3) excised samples after H₂ O-D₂ O replacement. T₂ distributions were reconstructed from the CPMG decays using inverse Laplace transform.

RESULTS

Two major peaks, identified as fat and water, were consistently observed in the T₂ distributions of HMD regions. The LMD T₂ distributions were dominated by the fat peak. The relative areas of the two peaks exhibited statistically significant (P < .005) differences between HMD and LMD regions, enabling their classification as HMD or LMD. The relative-area distributions exhibited no statistically significant differences between full slices and excised samples.

CONCLUSION

T₂ -based portable-NMR analysis is a novel approach to MD quantification. The ability to quantify tissue composition, combined with the low cost of instrumentation, make this approach promising for clinical applications.

Water immobilization by glass microspheres affects biological activity

We recently reported that the water holding capacity of myofibrillar protein hydrogels could be increased upon addition of small amounts of microparticles, particularly glass microspheres. Glass microspheres were found to decrease the spin-spin relaxation time (T₂) of water protons in the gels, which was interpreted as enhanced water binding by the glass. We were thus interested in determining whether the observed effects on water proton relaxation were a direct consequence of water-glass interactions. Here we show how glass microspheres reduce the mobility of pure water, reflected in large decreases in the T₂ of water protons, decreases in the self-diffusion coefficient of water molecules, a lower water activity, and strengthening of O-H bonds. Even though glass is considered an inert material, glass microspheres were shown to inhibit the growth of human embryonic kidney cells, and stimulate or inhibit the growth of leukemia and monocytic lymphoma cells in vitro, depending on dose and time. The germination of alfalfa seeds and the growth of E.coli cells were also inhibited upon exposure to glass microspheres. This work indicates that the properties and behavior of materials, even ones considered inert, can be affected by their size. These observations suggest possible toxicological consequences of exposure to microparticles, but also open us possibilities to affect cellular/organism function via modulation of macromolecular hydration.

Nonfreezing Water Structuration in Heteroprotein Coacervates

Surface-bound water in protein solutions has been identified with a reduction in its freezing point. We studied the presence of such nonfreezing water (NFW) in various protein-polyelectrolyte, micelle-polyelectrolyte, and protein-protein (heteroprotein) coacervates, along with appropriate concentrated solutions of macromolecules alone, finding up to 15% w/w NFW for the heteroprotein coacervate of lactoferrin (LF) and β-lactoglobulin (BLG). The level of NFW is always higher in coacervates than in the control (single macromolecule) systems, particularly for protein-containing coacervates: a coacervate of bovine serum albumin (BSA) and poly(dimethyldiallylammonium chloride) (PDADMAC) showed a ratio of NFW/protein twice that of BSA alone (0.6 vs 0.3), with a similarly high ratio for LF-BLG coacervate. These results are attributed to the maximization of water-protein contacts, structural features that reflect the mode of sample assembly, as they are not seen in a noncoacervated LF-BLG solution with identical concentrations of all species.

The interactions of water with cellulose- and starch-derived pharmaceutical excipients

Water associated with polymeric pharmaceutical excipients derived from cellulose and starch can have a profound effect on the properties of the excipient and on the other ingredients making up a solid dosage form. Important questions which need to be addressed include How much water will be sorbed or desorbed at various relative humidities and temperatures? and What is the thermodynamic state of water associated with the solid as a function of moisture content? A critical review of the literature is presented to demonstrate the most likely answers to these questions. It appears that water exists in at least three thermodynamic states in starch, cellulose, and their derivatives: (1) water directly and tightly bound, with a stoichiometry of one water molecule per anhydroglucose unit; (2) water in a relatively unrestricted form, approaching the properties of bulk or pure liquid water; and (3) water in an intermediate state or states, with properties reflecting a much higher level of structure than bulk water but less than that of tightly bound water. Some implications of such behavior for pharmaceutical systems are discussed.

Water-protein interactions: the secret of protein dynamics

Water-protein interactions help to maintain flexible conformation conditions which are required for multifunctional protein recognition processes. The intimate relationship between the protein surface and hydration water can be analyzed by studying experimental water properties measured in protein systems in solution. In particular, proteins in solution modify the structure and the dynamics of the bulk water at the solute-solvent interface. The ordering effects of proteins on hydration water are extended for several angstroms. In this paper we propose a method for analyzing the dynamical properties of the water molecules present in the hydration shells of proteins. The approach is based on the analysis of the effects of protein-solvent interactions on water protons NMR relaxation parameters. NMR relaxation parameters, especially the nonselective (R₁^NS ) and selective (R₁^SE ) spin-lattice relaxation rates of water protons, are useful for investigating the solvent dynamics at the macromolecule-solvent interfaces as well as the perturbation effects caused by the water-macromolecule interactions on the solvent dynamical properties. In this paper we demonstrate that Nuclear Magnetic Resonance Spectroscopy can be used to determine the dynamical contributions of proteins to the water molecules belonging to their hydration shells.

Consistency of signal intensity and T2* in frozen ex vivo heart muscle, kidney, and liver tissue

PURPOSE

To investigate tissue dependence of the MRI-based thermometry in frozen tissue by quantification and comparison of signal intensity and T2* of ex vivo frozen tissue of three different types: heart muscle, kidney, and liver.

MATERIALS AND METHODS

Tissue samples were frozen and imaged on a 0.5 Tesla MRI scanner with ultrashort echo time (UTE) sequence. Signal intensity and T2* were determined as the temperature of the tissue samples was decreased from room temperature to approximately -40 degrees C. Statistical analysis was performed for (-20 degrees C, -5 degrees C) temperature interval.

RESULTS

The findings of this study demonstrate that signal intensity and T2* are consistent across three types of tissue for (-20 degrees C, -5 degrees C) temperature interval.

CONCLUSION

Both parameters can be used to calculate a single temperature calibration curve for all three types of tissue and potentially in the future serve as a foundation for tissue-independent MRI-based thermometry.

Mixed lipid aggregates containing gangliosides impose different2H-NMR dynamical parameters on water environment depending on their lipid composition

Water dynamics in samples of ceramide tetrasaccharide (Gg4Cer) vesicles and GM1 ganglioside micelles at 300:1 water/lipid mole ratio were studied by using deuterium nuclear magnetic resonance (2H-NMR). GM1 imposes a different restriction on water dynamics that is insensitive to temperatures either above or below its phase transition temperature or below the freezing point of water. The calculated correlation times are in the range of 10(-10) s, typical of water molecules near to the polar groups. Pure GM1 micelles have two distinct water microenvironments dynamically characterized. Their dynamic parameters remain constant with temperature ranging from -18 to 32 degrees C, but the amount of strongly associated water is modified. By contrast, a mixture of single soluble carbohydrates corresponding to GM1 polar head group does not preserve the dynamic parameters of water hydration when the temperature is varied. Incorporation of cholesterol or lysophosphatidylcholine into GM1 micelles substantially increases the mobility of water molecules compared with that found in pure GM1 micelles. The overall results indicate that both the supramolecular organization and the local surface quality (lipid-lipid interaction) strongly influence the interfacial water mobility and the extent of hydration layers in glycosphingolipid aggregates.

In vivo MR thermometry of frozen tissue using R2* and signal intensity

Cryoablation is one of several minimally invasive treatments that may be suitable for a targeted treatment of prostate cancer. Because efficacy is improved when a sufficiently cold end temperature is reached, the purpose of this work was to demonstrate an image-based thermometry method that could provide temperature maps throughout the frozen tissue. In five in vivo canine prostate cryoablation experiments performed under magnetic resonance imaging guidance, two MR parameters were measured and correlated to temperature: R2* and changes in signal intensity. R2* is elevated approximately linearly as tissue temperature decreases below the freezing point, while the signal intensity decreases exponentially. In vivo temperature maps with isotherms at -5 degrees C, -15 degrees C, and -30 degrees C are demonstrated.

Fiber optic near-infrared Raman spectroscopy for clinical noninvasive determination of water content in diseased skin and assessment of cutaneous edema

Currently, measuring Raman spectra of tissues of living patients online and in real time, collecting the spectra in a very short measurement time, and allowing diagnosis immediately after the spectrum is recorded from any body region, are specific advantages that fiber optic near-infrared Raman spectroscopy (NIR RS) might represent for in vivo clinical applications in dermatology. We discuss various methodological aspects and state of the art of fiber optic NIR RS in clinical and experimental dermatology to outline its present advantages and disadvantages for measuring skin in vivo, particularly its water content. Fiber optic NIR Fourier transform (FT) RS has been introduced to dermatological diagnostics to obtain information regarding the molecular composition of the skin up to several hundred micrometers below the skin surface in a relatively fast nondestructive manner. This has been especially important for probing for in vivo assessment of cutaneous (intradermal) edema in patients patch test reactions. Fiber optic NIR FT Raman spectrometers still require further technological developments and optimization, extremely accurate water concentration determination and its intensity calculation in skin tissue, and for clinical applications, a reduction of measurement time and their size. Another promising option could be the possibility of applying mobile and compact fiber optic charge-coupled device (CCD)-based equipment in clinical dermatology.

The importance of sample preservation temperature for analysis of the redox state of human serum albumin

BACKGROUND

Human serum albumin (HSA) is a mixture of human mercaptalbumin (HMA, reduced form) and nonmercaptalbumin (HNA, oxidized form).

METHODS

We have developed a convenient high-performance liquid chromatographic (HPLC) system for the separation of HSA into HMA and HNA, and studied the mercapt<==>nonmercapt conversion (i.e., dynamic change in the redox state) of HSA. Examination of long-term sample preservation temperature on the redox state of HSA is of fundamental importance for analysis of defense systems against oxidants in humans.

RESULTS

The HMA fraction of HSA (f(HMA)) was markedly decreased, i.e., the redox state of HSA samples was more oxidized, when they were kept even at -20 degrees C for 170 days. Moreover, the redox states of five commercial HSA products were analyzed and the results were compared with those for normal control subjects.

CONCLUSIONS

Surprisingly, marked decreases in f(HMA) value for all commercial HSA products were observed.

High resolution MRI reveals global changes in brains of Cln3 mutant mice

Batten disease, the juvenile-onset form of neuronal ceroid lipofuscinosis (NCL), is a progressive neurodegenerative disorder of childhood with an age of onset of 5-10 years of age. JNCL is caused by mutations in the CLN3 gene which encodes a membrane protein of unknown function. Magnetic resonance imaging of the brain of juvenile NCL patients has revealed changes in signal intensity and tissue atrophy, predominantly in the cortex and cerebellum. A mouse model for Batten disease was created by targeted disruption of the murine Cln3 gene in order to further understanding of the pathophysiology of Batten disease and to evaluate potential therapeutic approaches. Several features of the disease are displayed by Cln3 mice including accumulation of characteristic storage material in neurons. The aim of this work was to investigate neurodegeneration in the Cln3 mouse model using high resolution magnetic resonance imaging to measure signal intensity ratios in selected regions of interest. Global changes were observed in the brains of 12-month-old mutant mice that mirror those seen in juvenile NCL patients. There is a decrease in signal intensity ratio in grey matter regions including cortex, hippocampus and cerebellum, tissues where neuronal storage accumulation and cell loss have been seen in the mouse model. The alterations seen in Cln3 mutant mice support the validity of further imaging studies and suggest that this method will have application in assessment of therapeutic approaches in the study of mutant mouse models of NCL including the Cln3 mouse.

Dielectric relaxation of water and water-plasticized biomolecules in relation to cellular water organization, cytoplasmic viscosity, and desiccation tolerance in recalcitrant seed tissues

To understand the relationship between the organization of cellular water, molecular interactions, and desiccation tolerance, dielectric behaviors of water and water-plasticized biomolecules in red oak (Quercus rubra) seeds were studied during dehydration. The thermally stimulated current study showed three dielectric dispersions: (a) the relaxation of loosely-bound water and small polar groups, (b) the relaxation of tightly-bound water, carbohydrate chains, large polar groups of macromolecules, and (c) the "freezing in" of molecular mobility (glassy state). Seven discrete hydration levels (water contents of 1.40, 0.55, 0.41, 0.31, 0.21, 0.13, and 0.08 g/g dry weight, corresponding to -1.5, -8, -11, -14, -24, -74, and -195 MPa, respectively) were identified according to the changes in thermodynamic and dielectric properties of water and water-plasticized biomolecules during dehydration. The implications of intracellular water organization for desiccation tolerance were discussed. Cytoplasmic viscosity increased exponentially at water content < 0.40 g/g dry weight, which was correlated with the great relaxation slowdown of water-plasticized biomolecules, supporting a role for viscosity in metabolic shutdown during dehydration.

Investigating the structure and properties of hydrated hydroxypropyl methylcellulose and egg albumin matrices containing carbamazepine: EPR and NMR study

PURPOSE

The present study was conducted in order to investigate the correlation between the hydration properties of HPMC and EA matrices, gel microstructure and mobility, crystalline changes occurring in the gel and CBZ release kinetics. The influence of HPMC and EA erosion modes on CBZ release kinetics was interpreted in terms of gel microstructures.

METHODS

NMR technique was used to determine the T1 and T2 relaxation rates of water in hydrated matrices. PFGSE NMR technique was employed to determine the SDC of water in the gels. EPR technique was used to determine the rotational correlation time of PCA in the hydrated matrices, gel microviscosity, mobile compartment, alpha, beta, gamma parameters and lorentzian/ gaussian ratio. These parameters are indicative of matrix microstructure.

RESULTS

CBZ release mechanism from HPMC and EA matrices was markedly different. This behavior was related to the different structures of the polymer and protein. T2 relaxation studies and SDC measurements by NMR revealed higher chain hydration for HPMC compared to EA. Using the EPR technique it has been shown that the microviscosity and mobile compartment of matrices containing HPMC are lower than matrices containing EA. The microviscosity, mobile compartment and S-parameter values of hydrated matrices containing different EA/CBZ ratios were in correlation with the crystallization properties of CBZ in the gels, matrix erosion properties and CBZ release kinetics from the matrices.

CONCLUSIONS

Characterization of matrix structures using EPR and NMR techniques supported our hypothesis concerning the mechanism involved in HPMC-CBZ interaction. EA/CBZ matrix microstructure features, analyzed by NMR and EPR techniques, were in correlation with the crystalline changes occurring in the gel and drug release kinetics.

In vivo magnetic resonance imaging and relaxometry study of a porous hydrogel implanted in the trapezius muscle of rabbits

In vivo magnetic resonance imaging (MRI) and relaxometry were performed to assess noninvasively the tissue reaction and the biological integration of hydrogels made of poly[N-(2-hydroxypropyl) methacrylamide] (PHPMA) after implantation in the trapezius muscle of rabbits. The benefits of incorporating RGD peptide sequences in the polymer backbone were also investigated. The histological status of each implant was probed by the trend of their transversal relaxation times, T(2), while their biocompatibility was evaluated by analyzing the host tissue response through the evolution of the relaxation times of the adjacent muscle tissue. MR results showed the good acceptability of both hydrogels by the host tissue. The transversal relaxation curves of each implant exhibited two distinct phases as a function of implantation time: (1) a monoexponential phase, dominated by the influx of fluids inside the implants; and (2) a biexponential phase related to the infiltration of cells and the granulation tissue formation within the porous structure of each polymer. These MR findings were correlated with the results of conventional histological analyses. The present study demonstrates the effectiveness of MR methods in noninvasively monitoring the biocompatibility and histological status of implanted porous biomaterials.

Structure and hydration properties of hydroxypropyl methylcellulose matrices containing naproxen and naproxen sodium

The present study was conducted to obtain a deeper insight into the mechanism of drug release from HPMC matrices. The microstructure, mobility, internal pH and the state of water within the gel layer of hydrated HPMC matrices (having different molecular weights) containing naproxen sodium (NS) and naproxen (N) were studied using Electron Paramagnetic Resonance (EPR), Nuclear Magnetic Resonance (NMR) and Differential Scanning Calorimetry (DSC) techniques. The study show that matrices composed of various viscosity grades of HPMC are characterized by similar microviscosity values in spite of the difference in their molecular weight. The NMR and DSC results led to the conclusion that higher molecular weights of HPMC are characterized by higher water absorption capacity and higher swelling. Analysis of non-freezable water in HPMC(K4M)-NS system revealed that addition of NS to solution increased the fraction of water bound to K4M+NS compared with the equivalent solutions without NS. The results suggest that the drug is participating in the crystallization of water and leads to the formation of a three dimensional network structure that decreases the freedom of water in K4M+NS samples. Calculation of the number of hydration shells showed that up to 2.2 layers are involved in HPMC-NS hydration compared to 1.5 layers for HPMC gel without NS. This was explained based on the different water ordering in the gel induced by NS as results of its absorption to polymer surface. Microviscosity values measured by EPR for K4M/N and K4M/NS hydrated matrices were found to be higher for K4M/N matrices, especially at initial stage of hydration. Mobile compartment calculations showed lower values for K4M/N compared with K4M/NS matrices. pH measurements by EPR revealed that incorporation of N to HPMC matrix led to lower internal pH value inside the hydrated tablet compared with NS. This behavior led to lower solubility of N which dictates its surface erosion mechanism, compared with NS matrix that was characterized by higher internal pH value and higher drug solubility. These properties of HPMC/NS increased chain hydration and stability, and led to drug release by the diffusion mechanism.

Correlation between drug release kinetics from proteineous matrix and matrix structure: EPR and NMR study

The present study was conducted in order to probe the microstructure, microviscosity, and hydration properties of matrices containing two model drugs, naproxen sodium (NS) and naproxen (N), and egg albumin (EA) as matrix carrier. The results suggested that N release from EA matrix was controlled by a bulk erosion mechanism in combination with additional processes (crystal dissolution/crystallization rate) compared with NS matrix, which behaved as a non-erodible matrix and drug release occurred by diffusion through the gel. Using EPR technique it has been shown that incorporating NS into EA matrix strongly influences the microstructure of the protein gel, and hence the transport of the penetrant within the matrix, compared with matrices containing N. The presence of NS increased the protein chain mobility and hydration which supports our previous results showing that NS cause unfolding of EA. In contrast, N caused only marginal effect on EA chain mobility. The gel formed in EA/NS matrices was more porous compared with EA/N matrices as revealed by the lower rotational correlation time of PCA (lower microviscosity) in EA/NS matrices compared with EA/N. However, EA/N gelled matrices were more heterogeneous, i.e., containing a higher number of components having different mobility. The T(1) and T(2) relaxation studies by NMR provided an additional support for the higher chain hydration in EA/NS matrices compared with EA/N as indicated by the higher relaxation rates in the gelled matrices. Internal pH measurements by EPR revealed that the micro-pH inside 100% EA and 50/50 EA/N matrices were lower than 50/50 EA/NS matrices and in all cases lower than the penetrating buffer pH. The lower pH compartment formed in N matrices affected N solubility and crystal dissolution rate, which can explain its lower release rate compared with EA, from the same formulation. The EPR and NMR data supports our findings that NS caused unfolding of the protein, affected matrix structure, and converted it to a hydrophobic non-erodible matrix compared with EA/N matrix in which the native properties of EA were mainly retained.

Effect of thermal denaturation on water-collagen interactions: NMR relaxation and differential scanning calorimetry analysis

The dependence of the proton spin-lattice relaxation rate, and of the enthalpy and temperature of denaturation on water content, were studied by nmr and differential scanning calorimetry (DSC) in native and denatured collagen. Collagen was first heated at four different temperatures ranging from 40 to 70 degrees C. The percentage of denatured collagen induced by these preheating treatments was determined from DSC measurements. The DSC results are discussed in terms of heat-induced structural changes. A two-exponential behavior for the spin-lattice relaxation was observed with the appearance of denatured collagen. This was attributed to the presence of a noncollagen protein fraction. The variations in the different longitudinal relaxation rates as a function of the moisture content and of the denatured collagen percentage are described within the multiphase water proton exchange model. This study highlights the complementarity of the information obtained from the two analytical tools used.

1H-NMR spectroscopy of the intracellular water of resting and rigor frog skeletal muscle

The intracellular water of intact/relaxed and skinned/rigor fibers of frog skeletal muscle was studied at slack and stretched lengths by use of 1H-nuclear magnetic resonance (NMR) technique. The angular dependent changes of the NMR spectra of the water proton indicated that part of the intracellular water is aligned along the muscle fiber axis. The longitudinal and transverse proton-spin relaxation processes of the intracellular water were composed of a single- and multi-exponential processes respectively and the rate of both relaxations became slow as the water content of muscle fiber was increased. The longitudinal relaxation process was almost the same at slack and at stretched lengths for both intact/relaxed and skinned/rigor fibers. On the other hand, the transverse relaxation process was slightly but significantly faster at stretched than that at slack length for skinned/rigor fibers while it was almost the same at slack and at stretched lengths for intact/relaxed fibers. These results may be explained as the intracellular water located in the overlap region between actin and myosin filaments is less structured in rigor state than that in relaxed state, or that the rigor formation disrupts the structured water bound to myofilaments.

1H-nuclear magnetic resonance evidence for acto-myosin-dependent structural changes of the intracellular water of frog skeletal muscle fiber

The proton-spin relaxation process of the intracellular water in intact-relaxed and skinned-rigor fibers of frog skeletal muscle was studied for slack and stretched fibers by use of 1H-nuclear magnetic resonance technique. The longitudinal and transverse proton-spin relaxation processes of the intracellular water of intact-relaxed and skinned-rigor fibers were composed of a single- and multi-exponential processes, respectively. The longitudinal relaxation process was almost the same in slack as well as in stretched fibers for both intact-relaxed and skinned-rigor fibers. On the other hand, the transverse relaxation process was slightly but significantly faster in stretched than in slack fibers in the case of skinned-rigor fibers while it was almost the same in slack and in stretched fibers in the case of intact-relaxed fibers. As the overlap between actin and myosin filaments is maximal in slack fibers and minimal in stretched fibers, these results indicate that the intracellular water located in the overlap region is less structured in rigor fibers than that in relaxed fibers. This suggests that the rigor crossbridge formation disrupts the structured water bound to myosin and actin filaments in muscle fiber.

Nuclear magnetic resonance microscopy of single neurons under hypotonic perturbation

Nuclear magnetic resonance (NMR) characteristics of water in perfused single neurons undergoing a 20% hypotonic perturbation were examined quantitatively using NMR microscopy. The transverse relaxation times (T2) in the cytoplasm and nucleus increased by 24.0 +/- 8.5% (average +/- SE, n = 8) and 29.7 +/- 5.3% (n = 6), respectively, whereas the apparent diffusion coefficients (ADC) showed no significant change. These findings are consistent with the behaviors of a perfect osmometer and with accepted molecular relaxation and diffusion models and have significant impacts on current views of properties of cellular water. Furthermore, the results suggest that the increase of tissue intracellular-to-extracellular volume ratio during cell swelling is the predominant mechanism underlying the ADC reduction in acute brain ischemia. These data are the first direct quantitative measurements of the NMR characteristics of water in the cytoplasm and nucleus of single cells undergoing physiological perturbations and may lead to an improved diagnostic capability for NMR imaging in a variety of disease states.

Comparative 1H-NMR studies on the physical state of water in soft contact lens and mouse lens

The physical state of water in mouse lenses (2-, 4- or 8-wk-old) and soft contact lenses (SCLs, water content from 18.4 to 79.2%) were studied by measuring spin-lattice relaxation times (T1) and apparent intermolecular cross-relaxation times (TIS) from irradiated protein or polymer protons to water protons, using 360 MHz 1H-NMR spectrometer at 25 degrees C. (1) 1/T1 values of SCLs increased gradually with increasing dry weight (W(%)). 1/TIS values of SCLs were approximately zero at W of 20.8 and 26.8%, increased gradually from 26.8% and then steeply above approximately 50%. (2) A plot of 1/T1 vs. W(%) of mouse lenses was almost equal to that of SCLs. However, a plot of 1/TIS vs. W(%) was an approximately straight line with the intercept at W of 23% and with the slope which is almost equal to that of SCLs above W of approximately 50%. The plot of 1/TIS vs. W(%) of mouse lenses might indicate the significant change in the physical state of water and/or protein-water interactions above W of 23%.

Age-related change in redox state of human serum albumin

Human serum albumin (HSA) is the mixture of human mercaptalbumin (HMA, reduced form) and human nonmercaptalbumin (HNA, oxidized form). We developed a rapid and concise HPLC system to obtain the clear resolution of HSA into HMA and HNA, using an Asahipak GS-520H column. The mean value of the fraction of HMA (f(HMA)) for healthy young male subjects was 0.76 +/- 0.04 (n = 54). However, the f(HMA, 60-90) value for healthy elderly subjects (where the numbers in brackets indicate the range of ages) was 0.48 +/- 0.06 (n = 183). In healthy elderly subjects, f(HMA) was significantly lower than in healthy young male subjects, indicating that HSA in the elderly becomes more oxidized than in the young subjects. Consequently, we suggest that one of the important functions of serum albumin could be to participate in the maintenance of a constant redox potential in the extracellular fluids, thus securing a certain redox buffer capacity. f(HMA) on HSA might reflect this redox buffer capacity with age.

Longitudinal (T1) and transverse (T2) relaxation times of tissue water protons in mouse developing sarcoma 180-A: effect of hyperthermia

In vitro studies on Swiss mice, inoculated with dead or live sarcoma 180-A ascites cells, were carried out to monitor the changes, if any, in the longitudinal (T1) and transverse (T2) relaxation times of tissue water protons following hyperthermic treatment and subsequent thermotolerance. Both relaxation processes exhibited biexponential relaxation curves. With increasing size of the tumours, the longitudinal relaxation behaviour changed from bi- to mono-exponential. This was not observed for the transverse relaxation phenomenon. Inoculation with either dead or live cells caused an immediate increase in the both relaxation times. In the case of dead cell inoculation, the increase lasted for only 24 h after which the relaxation times became the same as for the uninoculated controls. The transverse relaxation time increased as a result of exposure to hyperthermia. During development of thermotolerance, both the relaxation times decreased.

Nuclear magnetic resonance Hahn spin-echo decay (T2) in live rats with endotoxin lung injury

To determine the possibility of using nuclear magnetic resonance imaging to study experimentally induced lung injury, we measured in the lungs of spontaneously breathing living rats the time course of both the Hahn spin-echo decay (T2) and the proton density after endotoxin injury. In order to minimize artifacts arising from motions of the nearby chest wall and heart, we used a motion-insensitive technique (the interleaved line scan). A typical Hahn T2 measurement was obtained over a region of interest from a series of images each with a different echo time, which ranged from 16 to 110 ms. Lung water content was determined by integrating the proton density over the region of interest. The Hahn T2 and proton density were measured before and at 1, 3, 6, and 9 h after intravenous injection of endotoxin. The effects of the treatment administered before and after endotoxin injection were also evaluated. Endotoxin treatment caused lengthening of both fast (T2f) and slow (T2s) Hahn T2 components but had no significant effect on the proton density, consistent with the notion that endotoxin causes lung injury without significant lung water accumulation in rats. However, the methylprednisolone treatment prevented the lengthening of T2s but did not seem to have a significant effect on T2f. Our results suggest that NMR imaging can be used to detect and monitor experimental lung injury in intact living animals, even in the absence of variations of lung water content.

Proton relaxation studies of water compartmentalization in a model neurological system

Proton relaxation measurements from 18 crayfish abdominal nerve cords (a model of human CNS) are used to demonstrate that the transverse (though not the longitudinal) relaxation can be decomposed into four reproducible components that, in conjunction with optical and electron microscopy of the morphology, can be assigned to three water compartments within the cord and possibly to the mobile lipid protons. The assignments are extraaxonal water protons (32 +/- 9% and mean T2 = 600 +/- 200 ms), axonal water protons (59 +/- 12% and mean T2 = 200 +/- 30 ms), intramyelinic water protons (7 +/- 4% and mean T2 = 50 +/- 20 ms), and finally an unsubstantiated assignment of lipid protons (2.0 +/- 2.0% and mean T2 = 7 +/- 4 ms).

An investigation of the origins of contrast in NMR spin echo images of plant tissue

The effects that the spatial distribution of water protons and their transverse relaxation times have on the image contrast of spin echo images of courgette was investigated. The T2-weighted image of courgette contains the most anatomical information. The image contrast was explained using a phenomenological theory based on the Bloch equations, which gave an insight into the morphology and microdynamics of water in the plant tissue. The perceived contrast in the spin echo images of courgette, glucose and Sephadex bead solutions can be dramatically altered by keeping all the imaging acquisition parameters constant, such as the recycle and echo time, but reducing the interpulse spacing by introducing a CPMG train of 180 degrees pulses into the middle of the sequence. These changes were interpreted by considering the microenvironment of the water. This work demonstrates that the origin of image contrast in T2-weighted images of plant tissue can be understood using the water proton transverse relaxation theory developed by Hills et al.

The dependence of proton longitudinal and transverse relaxation times on cell-cycle phase: mouse MCA-transformed 10T1/2 TCL-15 cells

Attempts to determine proton NMR longitudinal relaxation times (T1) as a function of cell-cycle stage using cells synchronized by chemical methods have yielded conflicting results (P. T. Beall, C. F. Hazlewood, and P. N. Rao, Science 192, 904 (1976); R. N. Muller et al., FEBS Lett. 114, 231 (1980); D. N. Wheatley, et al., J. Cell Sci. 88, 13 (1987]. This has raised the question whether a true dependence of T1 on cell-cycle phase exists. In the present study, the centrifugal elutriation technique was used to obtain relatively pure, synchronized cell populations of TCL-15 cells (a methylcholanthrene-transformed line of mouse 10T1/2 cells) for measurement of proton NMR relaxation rates. This technique provides a means to procure synchronized cell populations without the use of chemical agents as in the above-cited investigations and therefore avoid possible effects caused by the chemical agents of the NMR relaxation processes. Both T1 and the transverse relaxation time, T2, of water protons in synchronized-cell pellets obtained in this study, exhibited a dependence on cell-cycle phase at least for the first half of the cell cycle (G1 to S). Cells in G1 phase exhibited quantitatively higher T1 and T2 relaxation times compared to those measured for cells in mid S phase. Such changes were found to correlate with changes in water content. The distribution of cell-cycle phases of each cell population was determined by the DNA histogram using flow cytometric methods. Possible relaxation mechanisms which may contribute to the cell-cycle-specific phenomena of the intracellular T1 and T2 times are discussed.

Multiexponential proton relaxation in model cellular systems

Water proton relaxation measurements obtained from model cellular systems composed of red blood cell (RBC) ghosts are presented. The purpose of the investigation was to evaluate hypotheses concerning the possible sources of multiple exponential components in similar relaxation measurements made on tissue. Both laboratory frame transverse and longitudinal relaxation rates, as well as rotating frame relaxation rates, were measured in preparations of RBC ghosts and "extracellular fluid" that were, (a) uniformly mixed or (b) compartmentalized by layering, as the concentration of serum albumin was varied in the "extracellular fluid." The data show that although transmembrane exchange is too fast to give rise to multiexponential relaxation, multiple components can result from compartmentalization at the level of the cellular organization and do not necessarily require different tissue types. In addition, the data clearly demonstrate the importance of protein adsorption to cellular membranes as a determinant of the concentration of freely mobile solute protein molecules in tissue fluids.

Application of continuous relaxation time distributions to the fitting of data from model systems and excised tissue

Biological systems exhibit heterogeneity at many different levels, leading to the expectation of multiple relaxation time components for water protons in tissue samples. Traditional methods which fit the relaxation data to an a priori number of discrete components are open to observer bias in their interpretation of this data, and moreover, are intuitively less realistic for heterogeneous systems than methods which produce continuous relaxation time distributions. Previous validations of continuous distribution techniques have been made on simulated data assuming uniform Gaussian noise. In the current work we have investigated the ability of one particular linear inverse theory technique to reproduce known relaxation time distributions from the data on a controllable model system. Furthermore, using the experience gained on the model system, we have applied this same technique to the analysis of in vitro relaxation time measurements on excised brain tissue and found for water protons in white matter, four reproducible components for the transverse relaxation, whereas gray matter gave rise to only two. The longitudinal relaxation displayed only one component in either white matter or gray matter.

Use of NMR and MRI to study water relations in foods

Water is the most important component of a food system, because it influences so many process variables, product characteristics, and stability attributes. Some of the most successful techniques used to probe the behavior of water in food systems are Nuclear Magnetic Resonance (NMR) spectroscopy and, more recently, pulsed-field gradient NMR and Magnetic Resonance Imaging (MRI). The purpose of this chapter is to review the theory underlying these techniques and to present several examples of how they have been applied to study water relations in foods.