Pulsed Nmr study of water in agar gels

Woessner, D. E.; Snowden, B. S.

doi:10.1016/0021-9797(70)90181-5

Cited by 121 publications

(61 citation statements)

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“…The dramatic broadening of the water-1 H resonance upon gelation of agarose was originally attributed to extensive perturbations of bulk water (14), and similar ideas regarding longrange ordering and slowing down of water motions have been invoked to explain water-1 H and 2 H relaxation enhancements in biological tissue (15,16). Subsequent more systematic studies attributed the relaxation enhancement to a small fraction of "bound" water molecules or exchanging hydroxyl protons (17,18). These studies were restricted to relatively high frequencies (Ͼ2 MHz) and could therefore not fully elucidate the molecular origin of the observed relaxation effects.…”

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confidence: 87%

Molecular basis of water proton relaxation in gels and tissue

Chávez

Halle

2006

Magnetic Resonance in Med

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An extensive set of water-1 H magnetic relaxation dispersion (MRD) data are presented for aqueous agarose and gelatin gels. It is demonstrated that the EMOR model, which was developed in a companion paper to this study (see Halle, this issue), accounts for the dependence of the water-1 H spin-lattice relaxation rate on resonance frequency over more than four decades and on pH. The parameter values deduced from analysis of the 1 H MRD data are consistent with values derived from 2 H MRD profiles from the same gels and with small-molecule reference data. This agreement indicates that the water-1 H relaxation dispersion in aqueous biopolymer gels is produced directly by exchange-mediated orientational randomization of internal water molecules or labile biopolymer protons, with little or no role played by collective biopolymer vibrations or coherent spin diffusion. This ubiquitous mechanism is proposed to be the principal source of water-1 H spin-lattice relaxation at low magnetic fields in all aqueous systems with rotationally immobile biopolymers, including biological tissue. The same mechanism also contributes to transverse and rotating-frame relaxation and magnetization transfer at high fields. Intrinsic contrast in magnetic resonance images of soft tissue is generated largely by spatial variations in spin relaxation rates. Whereas the phenomenology of water-1 H relaxation in tissue is well documented (1-3), the underlying molecular mechanism remains controversial (4 -9). The task of elucidating the relaxation mechanism is complicated by incomplete knowledge about the chemical composition and supramolecular structure of most tissues. In addition, it is usually not possible to vary biopolymer composition, pH, or temperature in a controlled way while maintaining the integrity of the tissue. For these reasons, most mechanistic studies have been carried out on model systems, such as aqueous biopolymer gels, with relaxation characteristics similar to those of tissue. Here we report an extensive set of water-1 H relaxation data from two widely used tissue models: aqueous gels of agarose and gelatin.Detailed information about relaxation mechanisms in complex systems can be obtained from the magnetic relaxation dispersion (MRD) profile, that is, the field/frequency dependence of the spin-lattice relaxation rate, R 1 ϭ 1/T 1 . Ideally, the dispersion profile should be recorded from typical MRI frequencies of order 100 MHz down to the kHz range. The water-1 H MRD profiles reported here cover more than four frequency decades. The strong frequency dependence of R 1 at low fields, which is the focus of the present study, can be used to enhance image contrast in prepolarized MRI experiments (10). The low-field R 1 dispersion also yields information about the zero-frequency dipolar contributions to transverse relaxation and steadystate magnetization transfer and to the low-frequency dipolar contribution to rotating-frame spin-lattice relaxation.Agarose is a linear polysaccharide with a disaccharide repeat (agarobiose) composed...

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confidence: 87%

Molecular basis of water proton relaxation in gels and tissue

Chávez

Halle

2006

Magnetic Resonance in Med

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“…It was pointed out (1-3) that the IBW model does not hold for protein solutions (9) and agar gels (10). We believe that these two systems may be sufficiently different from muscle tissue that the same theoretical model should not be required to explain the NMR properties of all of them.…”

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confidence: 89%

Rate-Dependency Hypothesis

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changes in these properties on freezing.We do not know how the NMR relaxation data in frozen tissue relate to NMR relaxation in unfrozen tissue; we do not know the mechanism of freezing or the alterations in structure and segregation caused by freezing. Thus, we feel neither competent nor obliged to respond to points 2, 3, and 4 above.In their fifth criticism, Chang and Woessner correctly point out (i) that our model neglected the effect of dipolar interaction of the protons in the irrotationally bound water molecules with the protons in the macromolecular structure (8), and (ii) that inclusion of this effect might bring the model into better agreement with the data in the isotope dilution experiment (2). This has also been pointed out to us by Edzes and Samulski (5), who included this intermolecular contribution to T2 for water molecules in the bound state in fitting our proton T2 data for barnacle muscle, extracted its value, and found that it almost dominates the dependence on deuterium concentration in a manner consistent with rapid exchange. They pointed out that it is only the closeness of the deuterium and proton T2's which requires the introduction of the effects of Tb and allows determination of the free parameters in the IBW model. Inclusion of the intermolecular contribution does not disturb the validity of the "incipient motional narrowing" consistency check (3) referred to above.Edzes and Samulski reworked the barnacle T2 data because they were able to derive, from cross-relaxation effects in the relaxation time T1 (albeit of water in chicken muscle), an independent estimate of the intermolecular contribution to T2 in the bound state; the latter agrees very well with that found from the isotope dilution measurements in barnacle muscle and suggests that the model they (and Chang and Woessner) propose for isotopic dilution is valid. The cross-relaxation effects also require (5) that water molecules be bound for times Tb greater than a Larmor period (that is, Tb > 10-8 second); the parameters obtained from barnacle muscle (2,3,5) are consistent with this requirement (Tb -10-5 second). Inclusion of intermolecular contributions to T2 for the protons of water molecules in the bound state does not therefore vitiate the IBW model, but rather sustains it.It was pointed out (1-3) that the IBW model does not hold for protein solutions (9) and agar gels (10). We believe that these two systems may be sufficiently different from muscle tissue that the same theoretical model should not be required to explain the NMR properties of 1182 all of them. The rigid substrate (rigid for, say, tens of microseconds) required for the IBW model may be present in muscle and not in agar gels or protein solutions; it is, in fact, the purpose of the NMR experiments to ascertain these things. With regard to the T1 dispersion data of Held et al. (11), we find support rather than contradiction of the model in question. As far as T10 and T1 dispersion effects are concerned, the muscle systems are, according to the model, effectively ...

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“…For this simpler version of the EMOR model, the exact solution has been obtained from an analytical solution of the stochastic Liouville equation (Halle and Nilsson, unpublished results). The exact result is somewhat lengthy, but it deviates only slightly (and then only when both 0 I and ⍀ D I are large) from the simple expression: [35] where the dipole frequency is now defined as ⍀Ј D ϭ (3/ 2) D WW S WW . This result should be compared with Eq.…”

Section: Beyond the Motional-narrowing Regimementioning

confidence: 89%

“…In the EMOR model this observation is readily explained by the acid-and base-catalyzed labile-proton exchange (Vaca Chávez and Halle, this issue). The fracton/ two-pool theory also cannot explain the R 1 maximum observed as a function of temperature for agarose gels (11,35). At frequencies above ϳ100 kHz, this theory predicts that R 1 ϳ r P ϳ T. On the low-frequency plateau, where R 1 ϭ r W3 P , the identification (10) r W3 P ϭ f I /(T 2,solid ϩ I ) with T 2,solid Ϸ 10 s taken to be governed by temperatureindependent spin diffusion (10), it is clear that R 1 cannot decrease with temperature.…”

Section: Other Modelsmentioning

confidence: 99%

Molecular theory of field‐dependent proton spin‐lattice relaxation in tissue

Halle

2006

Magnetic Resonance in Med

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A molecular theory is presented for the field-dependent spinlattice relaxation time of water in tissue. The theory attributes the large relaxation enhancement observed at low frequencies to intermediary protons in labile groups or internal water molecules that act as relaxation sinks for the bulk water protons. Exchange of intermediary protons not only transfers magnetization to bulk water protons, it also drives relaxation by a mechanism of exchange-mediated orientational randomization (EMOR). An analytical expression for T 1 is derived that remains valid outside the motional-narrowing regime. Cross-relaxation between intermediary protons and polymer protons plays an important role, whereas spin diffusion among polymer protons can be neglected. For sufficiently slow exchange, the dispersion midpoint is determined by the local dipolar field rather than by molecular motions, which makes the dispersion frequency insensitive to temperature and system composition. The EMOR model differs fundamentally from previous models that identify collective polymer vibrations or hydration water dynamics as the molecular motion responsible for spin relaxation. Unlike previous models, the EMOR model accounts quantitatively The spin-lattice relaxation time, T 1 , of tissue water is one of the principal contrast modalities in clinical MRI. Although the phenomenology of water-1 H relaxation in soft tissue is well documented (1-3), the molecular determinants of natural and pathological T 1 variations have not been established. Because of the molecular-level complexity of biological tissue, studies aimed at unraveling the relaxation mechanism have often employed aqueous gels as tissue models. The magnetic relaxation dispersion (MRD), that is, the variation of R 1 ϭ 1/T 1 with magnetic field strength (or Larmor frequency) is particularly informative about the molecular motions that induce spin relaxation. However, even though numerous MRD studies of tissue models have been conducted, we still cannot predict the MRD profile from known system properties. There is even disagreement about the nature of the molecular motions that are responsible for the observed relaxation dispersion. For example, some authors invoke slow water dynamics in the hydration layer at the biopolymer surface (4 -6), while others focus on collective biopolymer vibrations (7-10).We recently reported an extensive set of water-2 H MRD data from polysaccharide and polypeptide gels (11) (Vaca Chávez et al., submitted). Since cross-relaxation and spin diffusion can be neglected for the quadrupolar 2 H nuclide, the analysis of 2 H MRD data is relatively straightforward. The 2 H MRD profiles can be accounted for quantitatively by a model that attributes the relaxation dispersion to a small deuteron population in intimate and long-lived association with the biopolymer. These intermediary deuterons either reside in labile groups (such as hydroxyl or amino groups) or belong to water molecules that are temporarily trapped within the biopolymer. According to this model, spin re...

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Pulsed Nmr study of water in agar gels

Cited by 121 publications

References 17 publications

Molecular basis of water proton relaxation in gels and tissue

Molecular basis of water proton relaxation in gels and tissue

Rate-Dependency Hypothesis

Molecular theory of field‐dependent proton spin‐lattice relaxation in tissue

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