| 1. |
- Bako, [unknown], et al.
(författare)
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Discussion
- 2009
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Ingår i: Faraday Discussions. - 1364-5498. ; 141, s. 443-465
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Tidskriftsartikel (populärvet., debatt m.m.)
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| 2. |
- Carlström, Göran, et al.
(författare)
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Nuclear spin quenching a new probe of exchange kinetics and droplet size in disperse systems
- 1988
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Ingår i: Molecular Physics. - : Informa UK Limited. - 0026-8976 .- 1362-3028. ; 64:4, s. 659-678
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Tidskriftsartikel (refereegranskat)abstract
- A new experimental technique, nuclear spin quenching (NSQ), is introduced, which can be used to measure droplet size and inter-droplet exchange rates of various intrinsic molecular species in disperse systems. The physical basis of the method is the quenching of the fine structure in spin-coupled N.M.R. spectra by intermolecular proton exchange, the rate of which is controlled by droplet exchange. The theoretical framework needed to interpret NSQ experiments is presented as well as an experimental case study on a water-in-oil microemulsion system. Being non-perturbing and having a large dynamic range, the NSQ technique should become a valuable complement to existing techniques for the study of structure and dynamics in disperse fluids.
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| 3. |
- Carlström, Göran, et al.
(författare)
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Shape fluctuations and water diffusion in microemulsion droplets : A nuclear spin relaxation study
- 1989
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Ingår i: Journal of Physical Chemistry. - : American Chemical Society (ACS). - 0022-3654 .- 1541-5740. ; 93:8, s. 3287-3299
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Tidskriftsartikel (refereegranskat)abstract
- Water 2H and 17O spin relaxation data for the microemulsion phase in the AOT/D 2O/isooctane system are reported. The difference between the transverse (R 2) and the longitudinal (R 1) relaxation rates has been measured as function of droplet size, droplet volume fraction, temperature, and resonance frequency. The 2H longitudinal relaxation rate dispersion has been measured over an extensive frequency range, using the field-cycling technique. The focus in the study is on the contribution from slow molecular processes to the quadrupolar relaxation behavior. For the first time in any system, the theoretically predicted relation between the 17O/ 2H ratios of quadrupolar line splittings and of R 2 - R 1 is verified. The extensive experimental data are used to discriminate among three different dynamic models. It is found that water diffusion within the microemulsion droplet cannot account for the experimental data. Instead, a substantial shape polydispersity seems to be required, where at any instant a large fraction of the droplets are nonspherical. However, it is not necessary that the equilibrium shape is nonspherical.
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| 4. |
- Carlström, Göran, et al.
(författare)
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The state of water in non-ionic surfactant solutions and lyotropic phases : Oxygen-17 magnetic relaxation study
- 1989
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Ingår i: Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases. - 0300-9599. ; 85:5, s. 1049-1063
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Tidskriftsartikel (refereegranskat)abstract
- Water 17O longitudinal and transverse relaxation rates have been measured in aqueous solutions and mesophases of the non-ionic alkyl oligo(ethylene oxide) surfactants C12En (n = 4, 5, 8) over wide ranges of temperature and concentration. In addition, two reference systems have been investigated: high molecular weight poly(ethylene oxide) and the dimer 1,2-dimethoxy ethane. The relaxation data are consistent with the following picture of the state of water in the headgroup shell of C12En aggregates. The shell is compact, containing less than 5 and possibly as little as 2-3 water molecules per EO group. The shell exhibits substantial structural integrity; its water content is essentially invariant with respect to changes in concentration. The headgroup shell is highly dynamic, the rate of water rotation in the shell being reduced by at most a factor of 5 (at room temperature) as compared to bulk water.
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| 5. |
- Carlström, Göran, et al.
(författare)
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Water Dynamics in Microemulsion Droplets. A Nuclear Spin Relaxation Study
- 1988
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Ingår i: Langmuir. - : American Chemical Society (ACS). - 0743-7463 .- 1520-5827. ; 4:6, s. 1346-1352
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Tidskriftsartikel (refereegranskat)abstract
- The state of water in aqueous microemulsion droplets in the system AOT/D2O/isooctane has been investigated by 2H and 17O NMR. Longitudinal relaxation rates are reported as a function of droplet size, droplet volume fraction, temperature, and resonance frequency. We conclude that the surface-induced perturbation of water rotation is of short range (limited to the primary hydration of the AOT head groups) and of modest magnitude (less than a factor of 10 slower rotation than in bulk water).
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| 6. |
- Cesare Marincola, Flaminia, et al.
(författare)
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Competitive Na+ and Rb+ binding in the minor groove of DNA
- 2004
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 1520-5126 .- 0002-7863. ; 126:21, s. 6739-6750
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Tidskriftsartikel (refereegranskat)abstract
- Sequence-dependent coordination of alkali ions to the nucleotide bases in the minor groove of AT-tract B-DNA has recently been inferred from X-ray crystallography, solution NMR and computer simulations. Here, we present new Na-23 and Rb-87 magnetic relaxation dispersion (MRD) data that demonstrate competitive and long-lived binding of Na+ and Rb+ ions in the minor groove of the B-DNA duplex [d(CGCGAATTCGCG)](2). The Na+/Rb+ selectivity of the minor groove is found to be weak, consistent with local structural flexibility. The ion occupancies derived from the MRD data are substantially higher than previously reported, suggesting that groove-bound ions significantly influence the energetics and structural polymorphism of DNA in vivo. For example, in the presence of 0.20 M Na+ and 0.56 M Rb+ at 4 degreesC, the ApT site in the minor groove is occupied by a Rb+ ion, a Na+ ion, or a water molecule 40, 10, and 50% of the time, respectively. In the absence of Rb+, the Na+ occupancy increases to 50%. At 4 degreesC, the mean residence time of groove-bound ions is 0.2 +/- 0.1 mus for Rb+ and 10 ns to 100 mus for Na+. A shorter correlation time of 2 ns is attributed to counterions bridging cross-strand phosphate groups.
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| 7. |
- Chang, Zhiwei, et al.
(författare)
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Longitudinal relaxation in dipole-coupled homonuclear three-spin systems: Distinct correlations and odd spectral densities.
- 2015
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 143:23
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Tidskriftsartikel (refereegranskat)abstract
- A system of three dipole-coupled spins exhibits a surprisingly intricate relaxation behavior. Following Hubbard's pioneering 1958 study, many authors have investigated different aspects of this problem. Nevertheless, on revisiting this classic relaxation problem, we obtain several new results, some of which are at variance with conventional wisdom. Most notably from a fundamental point of view, we find that the odd-valued spectral density function influences longitudinal relaxation. We also show that the effective longitudinal relaxation rate for a non-isochronous three-spin system can exhibit an unusual inverted dispersion step. To clarify these and other issues, we present a comprehensive theoretical treatment of longitudinal relaxation in a three-spin system of arbitrary geometry and with arbitrary rotational dynamics. By using the Liouville-space formulation of Bloch-Wangsness-Redfield theory and a basis of irreducible spherical tensor operators, we show that the number of relaxation components in the different cases can be deduced from symmetry arguments. For the isochronous case, we present the relaxation matrix in analytical form, whereas, for the non-isochronous case, we employ a computationally efficient approach based on the stochastic Liouville equation.
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| 8. |
- Chang, Zhiwei, et al.
(författare)
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Nuclear magnetic relaxation by the dipolar EMOR mechanism: General theory with applications to two-spin systems.
- 2016
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 144:8
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Tidskriftsartikel (refereegranskat)abstract
- In aqueous systems with immobilized macromolecules, including biological tissue, the longitudinal spin relaxation of water protons is primarily induced by exchange-mediated orientational randomization (EMOR) of intra- and intermolecular magnetic dipole-dipole couplings. We have embarked on a systematic program to develop, from the stochastic Liouville equation, a general and rigorous theory that can describe relaxation by the dipolar EMOR mechanism over the full range of exchange rates, dipole coupling strengths, and Larmor frequencies. Here, we present a general theoretical framework applicable to spin systems of arbitrary size with symmetric or asymmetric exchange. So far, the dipolar EMOR theory is only available for a two-spin system with symmetric exchange. Asymmetric exchange, when the spin system is fragmented by the exchange, introduces new and unexpected phenomena. Notably, the anisotropic dipole couplings of non-exchanging spins break the axial symmetry in spin Liouville space, thereby opening up new relaxation channels in the locally anisotropic sites, including longitudinal-transverse cross relaxation. Such cross-mode relaxation operates only at low fields; at higher fields it becomes nonsecular, leading to an unusual inverted relaxation dispersion that splits the extreme-narrowing regime into two sub-regimes. The general dipolar EMOR theory is illustrated here by a detailed analysis of the asymmetric two-spin case, for which we present relaxation dispersion profiles over a wide range of conditions as well as analytical results for integral relaxation rates and time-dependent spin modes in the zero-field and motional-narrowing regimes. The general theoretical framework presented here will enable a quantitative analysis of frequency-dependent water-proton longitudinal relaxation in model systems with immobilized macromolecules and, ultimately, will provide a rigorous link between relaxation-based magnetic resonance image contrast and molecular parameters.
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| 9. |
- Chang, Zhiwei, et al.
(författare)
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Nuclear magnetic relaxation by the dipolar EMOR mechanism : Multi-spin systems
- 2017
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 147:8
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Tidskriftsartikel (refereegranskat)abstract
- In aqueous systems with immobilized macromolecules, including biological tissues, the longitudinal spin relaxation of water protons is primarily induced by exchange-mediated orientational randomization (EMOR) of intra- and intermolecular magnetic dipole-dipole couplings. Starting from the stochastic Liouville equation, we have previously developed a rigorous EMOR relaxation theory for dipole-coupled two-spin and three-spin systems. Here, we extend the stochastic Liouville theory to four-spin systems and use these exact results as a guide for constructing an approximate multi-spin theory, valid for spin systems of arbitrary size. This so-called generalized stochastic Redfield equation (GSRE) theory includes the effects of longitudinal-transverse cross-mode relaxation, which gives rise to an inverted step in the relaxation dispersion profile, and coherent spin mode transfer among solid-like spins, which may be regarded as generalized spin diffusion. The GSRE theory is compared to an existing theory, based on the extended Solomon equations, which does not incorporate these phenomena. Relaxation dispersion profiles are computed from the GSRE theory for systems of up to 16 protons, taken from protein crystal structures. These profiles span the range from the motional narrowing limit, where the coherent mode transfer plays a major role, to the ultra-slow motion limit, where the zero-field rate is closely related to the strong-collision limit of the dipolar relaxation rate. Although a quantitative analysis of experimental data is beyond the scope of this work, it is clear from the magnitude of the predicted relaxation rate and the shape of the relaxation dispersion profile that the dipolar EMOR mechanism is the principal cause of water-1H low-field longitudinal relaxation in aqueous systems of immobilized macromolecules, including soft biological tissues. The relaxation theory developed here therefore provides a basis for molecular-level interpretation of endogenous soft-tissue image contrast obtained by the emerging low-field magnetic resonance imaging techniques.
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| 10. |
- Chang, Zhiwei, et al.
(författare)
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Nuclear magnetic relaxation by the dipolar EMOR mechanism : Three-spin systems
- 2016
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Ingår i: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 145:3
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Tidskriftsartikel (refereegranskat)abstract
- In aqueous systems with immobilized macromolecules, including biological tissue, the longitudinal spin relaxation of water protons is primarily induced by exchange-mediated orientational randomization (EMOR) of intra- and intermolecular magnetic dipole-dipole couplings. Starting from the stochastic Liouville equation, we have developed a non-perturbative theory that can describe relaxation by the dipolar EMOR mechanism over the full range of exchange rates, dipole couplings, and Larmor frequencies. Here, we implement the general dipolar EMOR theory for a macromolecule-bound three-spin system, where one, two, or all three spins exchange with the bulk solution phase. In contrast to the previously studied two-spin system with a single dipole coupling, there are now three dipole couplings, so relaxation is affected by distinct correlations as well as by self-correlations. Moreover, relaxation can now couple the magnetizations with three-spin modes and, in the presence of a static dipole coupling, with two-spin modes. As a result of this complexity, three secondary dispersion steps with different physical origins can appear in the longitudinal relaxation dispersion profile, in addition to the primary dispersion step at the Larmor frequency matching the exchange rate. Furthermore, and in contrast to the two-spin system, longitudinal relaxation can be significantly affected by chemical shifts and by the odd-valued ("imaginary") part of the spectral density function. We anticipate that the detailed studies of two-spin and three-spin systems that have now been completed will provide the foundation for developing an approximate multi-spin dipolar EMOR theory sufficiently accurate and computationally efficient to allow quantitative molecular-level interpretation of frequency-dependent water-proton longitudinal relaxation data from biophysical model systems and soft biological tissue.
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