Multiexponential proton relaxation processes of compartmentalized water in gels

Changes in Natural Silk Fibres by Hydration, Tensile Loading and Heating as Studied by 1H NMR: Anisotropy in NMR Relaxation Times

B. mori silkworm natural silk is a fibrous biopolymer with a block copolymer design containing both hydrophobic and hydrophilic regions. Using ¹H NMR relaxation, this work studied B. mori natural silk fibres oriented at 0° and 90° to the static magnetic field B₀ to clarify how measured NMR parameters reflect the structure and anisotropic properties of hydrated silk fibres. The FTIR method was applied to monitor the changes in the silk I and β-sheet conformations. Unloaded B. mori silk fibres at different hydration levels (HL), the silk threads before and after tensile loading in water, and fibres after a stepped increase in temperature have been explored. NMR data discovered two components in T₁ and T₂ relaxations for both orientations of silk fibres (0° and 90°). For the slower T₂ component, the results showed an obvious anisotropic effect with higher relaxation times for the silk fibres oriented at 90° to B₀. The T₁ component (water protons, HL = 0.11) was sequentially decreased over a range of fibres: 0° oriented, randomly oriented, silk B. mori cocoon, 90° oriented. The degree of anisotropy in T₂ relaxation was decreasing with increasing HL. The T₂ in silk threads oriented at 0° and 90° also showed anisotropy in increased HL (to 0.42 g H₂O/g dry matter), at tensile loading, and at an increasing temperature towards 320 K. The changes in NMR parameters and different relaxation mechanisms affecting water molecular interactions and silk properties have been discussed. The findings provide new insights relating to the water anisotropy in hydrated Bombyx mori silk fibres at tensile loading and under a changing HL and temperature.

Case Studies of Physical and Chemical Gels Based on Microbial Polysaccharides

Microbial anionic polysaccharides, although structurally closely related, exhibit strikingly different gelling properties in aqueous media (physical gels formation). Experimental observations are traced to differences, minor differences in some instances, in primary structures. Chemical gels have been prepared using deacylated gellan by means of an interchain partial esterification procedure, and a sample subjected to preliminary characterization in terms of swelling properties. NMR water proton relaxation data show that such amples display a remarkable water retention capability.

Monitoring tissue development in acellular matrix-based regeneration for bladder tissue engineering: multiexponential diffusion and T2* for improved specificity

Cell-seeded acellular matrices (ACMs) are a promising approach for the tissue engineering of soft tissues and organs, such as the urinary bladder. ACM contains site-preferred structural and functional molecules, and degradation products derived from ACM play important roles in tissue remodeling. Regeneration proceeds along concurrent trajectories of cell growth and matrix degradation, characterized by evolving biophysical and biochemical properties. The assessment of tissue development through a noninvasive imaging technique, such as MRI, must therefore be capable of distinguishing these concurrent biophysical and biochemical changes. However, although MRI provides exquisite sensitivity to tissue microstructure, composition and function, specificity remains limited. In this study, multiexponential diffusion and the effective transverse relaxation time T(2)* were investigated for their ability to assess cell growth and tissue composition, respectively. Bladder ACMs prepared with and without hyaluronic acid, and ACMs seeded with smooth muscle cells, were assessed on MRI. The slow diffusion fraction from multiexponential diffusion analysis demonstrated the best correlation with cellularity, with minimal influence from underlying matrix degradation. T(2)* measurements were sensitive to macromolecular content, specifically the presence of hyaluronic acid, without confounding influence from tissue hydration. T(2)* also appeared to be sensitive to cell filling of the matrix pore space. Compared with these metrics, commonly used MRI parameters, such as T(1), T(2) and single diffusion coefficients, were more limited in specificity. The use of T(2) to measure tissue structure and composition is limited by its large dependence on water content, and single diffusion can only reflect the overall characteristics of the extra- and intracellular environment. These findings are important for further development of more specific MRI methods for the monitoring of regeneration in tissue-engineered systems.

Characterization of engineered cartilage constructs using multiexponential T₂ relaxation analysis and support vector regression

Increased sensitivity in the characterization of cartilage matrix status by magnetic resonance (MR) imaging, through the identification of surrogate markers for tissue quality, would be of great use in the noninvasive evaluation of engineered cartilage. Recent advances in MR evaluation of cartilage include multiexponential and multiparametric analysis, which we now extend to engineered cartilage. We studied constructs which developed from chondrocytes seeded in collagen hydrogels. MR measurements of transverse relaxation times were performed on samples after 1, 2, 3, and 4 weeks of development. Corresponding biochemical measurements of sulfated glycosaminoglycan (sGAG) were also performed. sGAG per wet weight increased from 7.74±1.34 μg/mg in week 1 to 21.06±4.14 μg/mg in week 4. Using multiexponential T₂ analysis, we detected at least three distinct water compartments, with T₂ values and weight fractions of (45 ms, 3%), (200 ms, 4%), and (500 ms, 97%), respectively. These values are consistent with known properties of engineered cartilage and previous studies of native cartilage. Correlations between sGAG and MR measurements were examined using conventional univariate analysis with T₂ data from monoexponential fits with individual multiexponential compartment fractions and sums of these fractions, through multiple linear regression based on linear combinations of fractions, and, finally, with multivariate analysis using the support vector regression (SVR) formalism. The phenomenological relationship between T₂ from monoexponential fitting and sGAG exhibited a correlation coefficient of r²=0.56, comparable to the more physically motivated correlations between individual fractions or sums of fractions and sGAG; the correlation based on the sum of the two proteoglycan-associated fractions was r²=0.58. Correlations between measured sGAG and those calculated using standard linear regression were more modest, with r² in the range 0.43-0.54. However, correlations using SVR exhibited r² values in the range 0.68-0.93. These results indicate that the SVR-based multivariate approach was able to determine tissue sGAG with substantially higher accuracy than conventional monoexponential T₂ measurements or conventional regression modeling based on water fractions. This combined technique, in which the results of multiexponential analysis are examined with multivariate statistical techniques, holds the potential to greatly improve the accuracy of cartilage matrix characterization in engineered constructs using noninvasive MR data.

1H-NMR analysis of water mobility in cultured phototrophic biofilms

The present work reports on the first attempt to study water mobility in phototrophic biofilms, applying the (1)H-NMR relaxometry technique to closely monitored microbial communities grown in a microcosm under controlled ambient conditions. Longitudinal water proton relaxation times exhibited a bi-exponential behavior in all biofilm samples, indicating two types of water molecules with diverging dynamic properties, confined to different compartments of the biofilm. The fast-relaxing component can be attributed to water molecules tightly bound to the intracellular matrix, while the slow-relaxing component could reflect the behavior of water embedded in the biopolymer matrix, confined into matrix pores and channels. The results are discussed with respect to a possible key role of exopolysaccharides and uronic acids in water binding in phototrophic biofilms.

Effects of frozen storage and sample temperature on water compartmentation and multiexponential transverse relaxation in cartilage

Multiexponential transverse relaxation in tissue has been interpreted as a marker of water compartmentation. Articular cartilage has been reported to exhibit such relaxation in several studies, with the relative contributions of tissue heterogeneity and tissue microstructure remaining unspecified. In bovine nasal cartilage, conflicting data regarding the existence of multiexponential relaxation have been reported. Imaging and analysis artifacts as well as rapid chemical exchange between tissue compartments have been identified as potential causes for this discrepancy. Here, we find that disruption of cartilage microstructure by freeze-thawing can greatly alter the character of transverse relaxation in this tissue. We conclude that fresh cartilage exhibits multiexponential relaxation based upon its microstructural water compartments, but that multiexponentiality can be lost or rendered undetectable by freeze-thawing. In addition, we find that increasing chemical exchange by raising sample temperature from 4°C to 37°C does not substantially limit the ability to detect multiexponential relaxation.

The acellular matrix (ACM) for bladder tissue engineering: A quantitative magnetic resonance imaging study

Bladder acellular matrices (ACMs) derived from natural tissue are gaining increasing attention for their role in tissue engineering and regeneration. Unlike conventional scaffolds based on biodegradable polymers or gels, ACMs possess native biomechanical and many acquired biologic properties. Efforts to optimize ACM-based scaffolds are ongoing and would be greatly assisted by a noninvasive means to characterize scaffold properties and monitor interaction with cells. MRI is well suited to this role, but research with MRI for scaffold characterization has been limited. This study presents initial results from quantitative MRI measurements for bladder ACM characterization and investigates the effects of incorporating hyaluronic acid, a natural biomaterial useful in tissue-engineering and regeneration. Measured MR relaxation times (T(1), T(2)) and diffusion coefficient were consistent with increased water uptake and glycosaminoglycan content observed on biochemistry in hyaluronic acid ACMs. Multicomponent MRI provided greater specificity, with diffusion data showing an acellular environment and T(2) components distinguishing the separate effects of increased glycosaminoglycans and hydration. These results suggest that quantitative MRI may provide useful information on matrix composition and structure, which is valuable in guiding further development using bladder ACMs for organ regeneration and in strategies involving the use of hyaluronic acid.

Multi-components of T2 relaxation in ex vivo cartilage and tendon

The multi-components of T2 relaxation in cartilage and tendon were investigated by microscopic MRI (microMRI) at 13 and 26 microm transverse resolutions. Two imaging protocols were used to quantify T2 relaxation in the specimens, a 5-point sampling and a 60-point sampling. Both multi-exponential and non-negative-least-square (NNLS) fitting methods were used to analyze the microMRI signal. When the imaging voxel size was 6.76 x 10(-4)mm3 and within the limit of practical signal-to-noise ratio (SNR) in microscopic imaging experiments, we found that (1) canine tendon has multiple T2 components; (2) bovine nasal cartilage has a single T2 component; and (3) canine articular cartilage has a single T2 component. The T2 profiles from both 5-point and 60-point methods were found to be consistent in articular cartilage. In addition, the depletion of the glycosaminoglycan component in cartilage by the trypsin digestion method was found to result in a 9.81-20.52% increase in T2 relaxation in articular cartilage, depending upon the angle at which the tissue specimen was oriented in the magnetic field.

Aqueous urea as a model system for bi-exponential relaxation

OBJECT

To evaluate the utility of aqueous urea, doped inner- and outer-sphere relaxation agents, as an adjustable two-component model system.

MATERIALS AND METHODS

T2 was measured from 12 molal urea mixtures at pH 7.0 with varying amounts of the MnCl2 and FeO(1.44) (Feridex, Berlex Inc, Montville, NJ).

RESULTS

Bi-exponential relaxation was observed, with rates that were bilinearly related to [MnCl2] and [FeO(1.44)]. FeO(1.44) had comparable relaxivities on both urea and water, while MnCl2 relaxivity was > 15x larger for water than for urea.

CONCLUSION

Aqueous urea, doped with inner- and outer-sphere contrast agents, is a two-compartment model system, which can be exhibit a wide range of different T2s and signal fractions.

Brain tumor evaluation and segmentation by in vivo proton spectroscopy and relaxometry

A new methodology has been developed for the evaluation and segmentation of brain tumors using information obtained by different magnetic resonance techniques such as in vivo proton magnetic resonance spectroscopy (1HMRS) and relaxometry. In vivo 1HMRS may be used as a preoperative technique that allows noninvasive monitoring of metabolites to identify the different tissue types present in the lesion (active tumor, necrotic tissue, edema, and normal or non-affected tissue). Spatial resolution for treatment consideration may be improved by using 1HMRS combined or fused with images obtained by relaxometry which exhibit excellent spatial resolution. Some segmentation schemes are presented and discussed. The results show that segmentation performed in this way efficiently determines the spatial localization of the tumor both qualitatively and quantitatively. It provides appropriate information for therapy planning and application of therapies such as radiosurgery or radiotherapy and future control of patient evolution.

Proton fluctuations and water diffusion in dextran chemical hydrogels studied by incoherent elastic and quasielastic neutron scattering

Proton fluctuations reporting local motions of the glycosidic linkages of chemically crosslinked dextran hydrogels with well defined pore-size distributions are studied by static and dynamic neutron-scattering approaches. The dependence of the dynamic behaviour of water on the pore sizes is also discussed.

Signal characteristics of FLAIR related to water content: comparison with conventional spin echo imaging in infarcted rat brain

To examine the correlation between tissue water content and signal intensity on fluid-attenuated inversion recovery (FLAIR) images, we analyzed infarcted rat brain, verified the results by theoretical simulation, and compared them with conventional spin-echo images. We produced brain infarction with cavitation in five rats by middle cerebral artery occlusion. After in vivo MRI, histologic sections of the MRI plane were obtained. We measured the signal intensity of regions on FLAIR and spin-echo images, and measured the area of cavitation on histologic sections. We plotted curves of cavity percentage to signal intensity. Theoretical values were calculated using a two-compartment model. On the curve of cavity area to signal intensity, the signal on FLAIR images peaked in tissues with 20% to 30% area of cavitation. On the theoretical curve, the signal on FLAIR images peaked at 90% tissue water content. These results seem to be characteristic of FLAIR.

The gel-forming behaviour of dextran in the presence of KCl: a quantitative 13C and pulsed field gradient (PFG) NMR study

Although the gel forming ability of certain polysaccharides in the presence of ions is a well-known phenomenon, detailed physicochemical mechanisms of such processes are still unknown. In this investigation high resolution 13C NMR as well as 1H pulsed field gradient (PFG) NMR were used to investigate the mobility of dextran in the sol and in the gel state. Gel-formation of dextran can be easily induced by the addition of large amounts of potassium chloride. No major differences in the T(1) relaxation times of dextran in the sol and in the gel state could be observed. Accordingly, the analysis of the 13C NMR spectroscopic data did not provide any indication of an observable line-broadening upon gel-formation. However, a KCl concentration dependent decrease of signal intensity in comparison to an internal standard was detected. On the other hand, the PFG NMR studies clearly indicated a gradual diminution of the self-diffusion coefficient of the dextran with increasing molecular weight as well as in the presence of potassium chloride. These measurements revealed in agreement with spectroscopic data that at least one potassium ion per monomer subunit (i.e. one glycopyranose residue) is necessary for gel formation.

Spin-spin relaxation times in myocardial hypertrophy induced by endocrine agents in rat

Magnetic resonance techniques afford a significant advantage for noninvasive diagnosis of cardiovascular pathology. The purpose of our present study was to assay the proton nuclear magnetic resonance (1H-NMR) sensitivity in the differential diagnosis of certain endocrine cardiovascular complications. In this context, we investigated the water state and content in the hypertrophied myocardium. Male and female Wistar rats were treated with different hormones (hydrocortisone acetate, testosterone, estradiol, thyroid hormones) in combination with isoproterenol (a synthetic catecholamine that induces myocardial ischemia and hypertrophy). The animals were sacrificed after 20 days of treatment and samples of integral myocardium and left ventricular myocardium were analyzed on a 1H-NMR AREMI spectrometer (0.6 T; proton resonance at 25 MHz). The estimation of T2 was made by Carr Purcell-Meiboom-Gill pulse sequence. The data were fitted to a bi-exponential curve, yielding short (T21) values for bound water and long (T22) values for free water. In order to evaluate the myocardial hypertrophy, the following ratios were calculated: integral myocardium to body weight; left ventricle to body weight; left ventricle to integral myocardium. The first two ratios were also calculated for dried tissue, in order to estimate its contribution to myocardial hypertrophy. Our findings demonstrate that myocardial hypertrophy is associated with a decrease of T22, as a consequence of the increase in the dried component (i.e. proteins) of the tissue, while the total tissue water (H2Ot%), measured by gravimetry) was not significantly modified. Nevertheless, it is reasonable that the increase in the protein content would be proportional with the increase in H2Ot%. The decrease of T21 seems to be proportional with the level of left ventricle hypertrophy in female groups. The 1H-NMR measurements were much sensitive for the differential diagnosis of myocardial hypertrophy in the case of left ventricle.

Optimal time spacings for T2 measurements: monoexponential and biexponential systems

We have developed an optimal design strategy, i.e. a choice of times at which the magnetization should be measured, in spin-echo measurements, when the number of measurements is fixed in advance. Results are given for samples whose relaxation is described by either an exponential or biexponential decay curve. The analysis is based on having an initial estimate of the ranges in which the relaxation times are likely to lie. The optimal design consists of a set of easily parameterized non-uniformly spaced measurement times, as opposed to present implementation of spin-echo experiments. Analysis of the biexponential case shows that an order of magnitude greater signal-to-noise is required to achieve T2 estimates of comparable precision to monoexponential measurements with the same number of data points. The optimal designs lead to an improved ability to discriminate between two relatively similar relaxation times.

1H NMR relaxation study of a chitosan-cyclodextrin network

The results of measurements of longitudinal and transverse proton relaxation times for a chemical network obtained by reacting chitosan with oxidized beta-cyclodextrin (beta-cyclodextrin polyaldehyde) are presented. The network was characterized by a 'two-component' transverse relaxation mechanism relative to structurally different environments experienced by water molecules. Different environments were also indicated by the temperature of the spin-spin relaxation times (T2) studied in the range 4-50 degrees C. Between 4 and 18 degrees C, proton exchange between the matrix and water prevails on the inter- and intra-molecular dipolar interactions of the water confined in the meshes of the network, resulting in a marked change in the slope of T2 with temperature. Stiffness of the matrix and reduced mobility of water in the gel meshes are prerequisites for observing such relaxation phenomena. Possible mechanisms contributing to the activation energy in the case of chitosan-cyclodextrin networks are discussed. The behaviour of the chitosan-cyclodextrin hydrogel is compared with that of a gellan gel.

Noninvasive in vivo monitoring of drug release and polymer erosion from biodegradable polymers by EPR spectroscopy and NMR imaging

Biodegradable polymers have attracted much attention as implantable drug delivery systems. Uncertainty in extrapolating in vitro results to in vivo systems due to the difficulties of appropriate characterization in vivo, however, is a significant issue in the development of these systems. To circumvent this limitation, noninvasive magnetic resonance techniques, electron paramagnetic resonance (EPR) and magnetic resonance imaging (MRI), were applied to characterize drug release and polymer degradation in vitro and in vivo. MRI makes it possible to monitor water content, tablet shape, and response of the biological system such as edema and encapsulation. The results of the MRI experiments give the first direct proof in vivo of postulated mechanisms of polymer erosion. Using nitroxide radicals as model drug releasing compounds, information on the mechanism of drug release and microviscosity inside the implant can be obtained by means of 1.2 GHz EPR spectroscopy. To be able to attribute nitroxide mobility to a particular layer of the implant, sandwich-like tablets were manufactured, taking advantage of the distinct spectral features of nitroxides containing different isotopes of nitrogen (15N vs 14N). The use of both noninvasive methods to monitor processes in vivo leads to new insights in understanding the mechanisms of drug release and polymer degradation.

NMR studies on water and polymer diffusion in dextran gels. Influence of potassium ions on microstructure formation and gelation mechanism

At room temperature aqueous solutions of dextrans with concentrations > 25% (w/w) exhibit a sol-gel transition in the presence of > 1.0 M potassium chloride. In dextrans the gelation was unexpected due to missing anionic groups that usually provide the binding sites for cations. The quantitative investigation of the gel formation is based on changes of the diffusibility of water and dextran chains. The apparent diffusion coefficients of bulk water (in the order of 10(-6) cm2/s) and of water trapped in the junction zones as well as of polymer chains (in the order of 10(-7) to 10(-8) cm2/s) are determined by employing pulsed field gradient stimulated echo (PFGSTE) NMR. The restricted diffusion of bulk water in viscous sols and in soft and rigid gels has been quantitatively analyzed providing data for interbarrier distances (pore size), permeabilities of the diffusion barriers (density of junction zones) and interbarrier diffusion coefficients of water. Based on already published x-ray structure data and in accordance with the diffusion data presented in this paper "potassium-bonding" is assumed to be the most important interaction for the formation of a microstructure and for the stabilization of cross-links. The ionic radius of the potassium ion perfectly fits to the cage established by six oxygen atoms of glucose units of three polymer chains. Other cations, such as Li+, Na+, Rb+ and Cs+, according to their nonfitting ionic radii, do not provoke dextran gelation under these conditions. The mechanism of the transitions from sol to soft gel and further to rigid gel is discussed on the basis of restricted diffusion and x-ray structure data.

Effects of dextran on hippocampal brain slice water, extracellular space, calcium kinetics and histology

Hippocampal brain slices are valuable models for studying brain function but are compromised by several artifacts, including significant water gain and histologic injury, which occur under certain incubation conditions. Addition of colloid to Krebs-Ringer buffer (K-R) has been shown to eliminate water gain but has not achieved widespread acceptance. We confirm prior observations that dextran and PEG lessen the increase in slice mass during incubation in a dose-dependent manner with no water gain occurring at 4% concentrations. However, we also observe that addition of colloid to standard K-R induces severe neuronal pyknosis. Fortunately, the pyknosis can be eliminated by reduction in buffer osmolarity through adjustment of NaCl, producing markedly improved slice histology in dextran buffer, especially in the CA3 and CA4 regions of the hippocampus which are severely injured when incubated submerged in K-R at 37 degrees C. Extracellular space markers are not affected by either colloid. The volume of distribution for 45Ca is much larger in dextran buffers than in K-R and variability of 45Ca kinetics is also reduced. In the presence of dextran, hypoxia induces significant slice water gain, a relatively selective histologic injury and an alteration of tissue Ca2+ kinetics. Use of dextran buffers may eliminate many troubling brain slice artifacts.

Action of compression and cations on the proton and deuterium relaxation in cartilage

In this paper, investigations are described on the influence of osmotic pressures and of varying cation concentrations on water relaxation times in cartilage (pig articular cartilage and bovine nasal cartilage). Both water content and relaxation times decrease strongly with increasing osmotic pressure. This relaxation behavior can be explained in terms of a fast chemical exchange between unbound and bound water. Na+ does not influence water content or relaxation times, whereas Ca2+ causes a small reduction in these parameters.

Estimation of water content and water mobility in the nucleus and cytoplasm of Xenopus laevis oocytes by NMR microscopy

NMR microscopy is a noninvasive approach for studying cell structure and properties. Spatially resolved measurements of the relaxation times T1 and T2 provided information on the water proton spin density and water mobility in different parts of Xenopus laevis oocytes. The spin-lattice relaxation time T1 was determined using a saturation-recovery sequence and the common spin-echo sequence with increasing repetition times, while the transverse relaxation time T2 was measured by means of the spin-echo sequence with varying echo times. From the relaxation times, the mole fractions of possible reorientational correlation times tau c for different types of intracellular water were calculated according to a simple two-phase model. The values for T1, T2, and proton spin density (i.e., water content) are: nucleus >> animal cytoplasm > vegetal cytoplasm. Based on the estimation of tau c, nearly 90% of the nuclear water and 74.4% of the water of the animal pole was considered as free mobile water, whereas 55.5% of the water of the vegetal pole appeared as bound water.