Journal of Magnetic Resonance, Series B最新文献

A 3DFT gradient-echo technique has been developed which, in conjunction with series-resonant gradient-coil circuits, can produce three-dimensional NMR images with an echo time of less than 100 μs. The method involves a read-gradient waveform composed of two sinusoids of different frequencies. This is an improvement on previous imaging sequences using a single sinusoid where only half ofkspace was sampled and where the second half was calculated using conjugate symmetry. The inaccuracies involved in the necessary “cut and paste” ofkspace inevitably lead to artifacts in the final image. The important features of the new method are that with suitable phase encoding all octants ofkspace are sampled, the RF pulse is applied when the gradients are all zero, and the echo forms when the gradient is essentially constant. This method will allow more extensive application of solid imaging techniques to biological samplesin vivo.

The localized¹H MR spectrum of human muscle has recently been reported to feature unassigned, orientation-dependent resonance lines. For their characterizationin vivo,various NMR techniques were combined with 3D spatial localization: 2D-J spectroscopy, zero-quantum- and Zeeman-order-filtering, double-quantum-filtering, 2D-constant-time COSY, dipolar-order filtering, and 2D-longitudinal-order separated spectroscopy. The successful implementation of these methods on a whole-body MR system and their application to study human subjects is described.¹H MR spectra of human muscle were found to feature residual dipolar couplings and anisotropic susceptibilities which render resonance frequencies, phases, and—with some sequences—signal intensities orientation dependent. Two of the unidentified resonances unequivocally form a dipolar doublet of two equivalent protons, centered at 3.93 ppm. All unknown as well as previously assigned peaks in the range between 2.7 and 3.6 ppm are either subject to dipolar coupling themselves or overlap with spectral contributions of metabolites involved in dipolar coupling. The methyl protons of creatine are likely to be subject to residual dipolar coupling and do therefore form a dipolar triplet and not a singlet as previously assumed. Finally, X3, a further unidentified peak at 3.5 ppm, appears to be part of a multiplet with its center at 3.3 ppm and overlapping the trimethylammonium resonance.

The temperature dependence of the spin–lattice relaxation of denatured nitrosyl hemoglobin (HbNO), nitrosyl myoglobin, powdered HbNO, and hematin-NO was studied between 4 and 70 K. The results were fitted with bothTⁿande^−Δ/Tmodels. In the first model, the relaxation is mediated by tunneling modes of a two-level system. A correlation between thenvalues and the functional state of the protein was observed. The striking coincidence of the range of the low-lying energy level and the temperature range where EPR spectra change suggests the existence of two conformations of the bound heme. The importance of the presence and structure of the globin is revealed in the difference between relaxation parameters of native proteins, denatured proteins, and hematin.

The intensities of saturation-transfer EPR (ST-EPR) spectra from nitroxyl spin labels have proved a sensitive means for studying slow exchange processes (both Heisenberg spin exchange and physical/chemical exchange) and weak interactions with paramagnetic ions, via the dependence on the effective spin–lattice relaxation rate (D. Marsh,Appl. Magn. Reson.3, 53, 1992). The dependences of the second-harmonic EPR absorption intensities detected in phase quadrature with the field modulation (V^′₂) on the microwaveH₁field, and on the effective relaxation times, were studied both theoretically and experimentally. Power-saturation curves and normalized integrated intensities (I_ST) of theV^′₂spectra were determined as a function of the concentration of a spin-labeled phospholipid in lipid membranes and of the concentration of paramagnetic Ni²⁺ions in the aqueous phase as a means of varying the effective relaxation times. The results were correlated with progressive-saturation measurements of the double-integrated intensities of the conventional EPR spectra. Intensities of theV^′₂spectra were calculated from the Bloch equations incorporating the modulation and microwave fields (K. Halbach,Helv. Phys. Acta27, 259, 1954), and the results were fitted to the experimental data. The ST-EPR intensities depend approximately linearly on the effectiveT₁, but with a nonzero intercept. On the basis of the theoretical calculations and experimental correlations, relations betweenI_STandT₁are suggested that may improve precision in the application of this alternative form of ST-EPR spectroscopy to biological systems.