Journal of Magnetic Resonance, Series B最新文献

Crosstalk due to coupling produces noise correlation between receiver coils. It has been stated that this correlation reduces the signal-to-noise ratio obtainable from combining signals from the coils. In this paper, it is shown that the effects of crosstalk on the signal-to-noise ratio may in theory be eliminated by properly combining signals. Equations are derived which show how the signals from two coils should be combined in the presence of crosstalk in order to obtain the same signal-to-noise ratio as in an ideal case of no crosstalk. The deviation from optimum signal-to-noise ratio due to imperfect circuits and amplifiers is discussed. An experimental technique for achieving the proper combination of signals is presented.

The hydroxyl and some alkoxyl spin adducts of 5,5-dimethyl-1-pyrroline 1-oxide (DMPO) are difficult to assign due to the remarkable similarity of their EPR spectra. The utility of resolving superhyperfine (SHF) structure followed by computer simulations has been demonstrated to assist in the assignment of EPR spectra with close values of hyperfine splitting constants, e.g., DMPO/^·OH and DMPO/^·OR. Here,^·OR is the alkoxyl radical derived from thermal decomposition of 2,2′-azobis (2-amidinopropane) hydrochloride (AAPH). In addition, two other spin traps, derivatives of 2H-imidazole 1-oxide, namely, 2,2,4-trimethyl-2H-imidazole 1-oxide (TMIO) and 2,2-dimethyl-4-phenyl-2H-imidazole 1-oxide (DMPIO), have been used in a model study to develop a procedure for distinguishing between oxygen-centered spin adducts. These results are compared with those for DMPO. TMIO and DMPIO spin traps provide more distinguishable individual spectra with^·OH and AAPH-derived^·OR radicals than the DMPO spin trap. The formation of DMPO/^·OR(AAPH) and DMPIO/^·OR(AAPH) spin adducts was confirmed by mass spectrometry. The comparison of spin trapping by DMPO and 2H-imidazole 1-oxides using typical biological sources of other oxygen-centered radicals reveals application limits of these spin traps. For example, 2H-imidazole 1-oxides do not form superoxide spin adducts in the xanthine/xanthine oxidase system. Also, for the first time, experimental evidence is presented for SHF structure in spectra of TMIO and DMPIO spin adducts with^·OH/^·OD and^·CH₃/^·CD₃radical species.

¹⁷O shielding constants have been utilized to investigate solvation differences of the cis/trans isomers ofN-methylformamide (NMF),N-ethylformamide (NEF), andtert-butylformamide (TBF) in a variety of solvents with particular emphasis on aqueous solution. Comparisons are also made with protected peptides of the formulas CH₃CO–YOH, CH₃CO–Y–NHR (Y = Pro, Sar), and CH₃CO–Y–Z–NHR (Y = Pro; Z =D-Ala) selectively enriched in¹⁷O at the acetyl oxygen atom. Hydration at the amide oxygen induces large and specific modifications of the¹⁷O shielding constants, which are practically the same for the cis and trans isomers of NMF, NEF, and the protected peptides. Fortert-butylformamide, the strong deshielding of the trans isomer compared to that of the cis isomer may be attributed to an out-of-plane (torsion-angle) deformation of the amide bond and/or a significant reduction of solvation of the trans isomer due to steric inhibition of the bulkytert-butyl group. Good linear correlation between δ(¹⁷O) of amides and δ(¹⁷O) of acetone was found for different solvents which have varying dielectric constants and solvation abilities. Sum-over-states calculations, within the solvaton model, underestimate effects of the dielectric constant of the medium on¹⁷O shielding, while finite-perturbation-theory calculations give good agreement with the experiment.

Coherent and incoherent flows in fertilized quail and bantam eggs have been studied with the aid of NMR microimaging techniques in the course of incubation until the end of the sixth day. The methods employed were multiplane tagging NMR imaging and a NMR gradient-echo imaging pulse sequence supplemented by bipolar gradient pulses in the coherence-evolution interval. The latter technique is suited for recording of velocity maps as well as for localizing of regions with enhanced echo attenuation by incoherent motions. Slight coherent displacements in the middle of the upper part of the egg white were found after the fourth day of incubation with the aid of both pulse schemes. The maximum velocity was estimated to be 1 mm/s. More pronounced effects revealed themselves in the examination of incoherent motions. After the same time of incubation and in a somewhat more restricted area of the upper part of the egg white, distinct motions could be localized consistently with either technique. It is shown that these motions are directed to and from the yolk. Furthermore, the analysis of the time fluctuations of the local signals with the aid of a Fourier transformation showed that the flow is largely of a pulsating nature. The pulsation frequency was found to be 0.4 Hz.

Water self diffusion in excised rat lungs has been measured using pulsed-field-gradient (PFG) techniques. The apparent diffusion coefficient,D_app, was measured from a plot of the magnetizationMvsg²_ato be 4.0 × 10⁻⁶cm²/s in the limit of small γδg_a, where γ is the gyromagnetic ratio, δ is the duration of the applied gradient pulses, andg_ais the applied gradient strength.D_appis independent of the diffusion time,t, for values oftbetween 18 and 106 ms. For larger values of γδg_a, an additional smaller value of the slope ofMvsg²_awas observed, indicating the existence of other, more slowly dephasing spins. Variation oftrevealed that the relative magnetization associated with the more slowly dephasing spins decreases astis increased. In addition, the relative magnetization of the slowly dephasing spins decreases as the temperature,T, of the excised rat lung is increased. Slow exchange from the compartment of the more rapidly to that of the more slowly dephasing spins may explain some of the observed dependence of the relative magnetizations ontandT. Measurements of water self diffusion in rat lung at various levels of water content indicate a correlation betweenT₂components and diffusion components. A new technique that combines the PFG with the Carr–Purcell–Meiboom–Gill technique is presented. The application of this technique to excised rat lung confirms the correlation betweenT₂and diffusion components.

Employing the perturbation approach directly in the multiply rotating frame, the master equation governing the spin dynamics of scalar-coupled heteronuclear multiple-spin systems has been derived. The resulting equation for the spin-density operator and the matrix form of the relaxation superoperator are relatively simple under the conditions of the “extended” strong-narrowing limit. The extended strong-narrowing limit, which requires that ω^I_iτ_c<< 1 and 2πJ_ijτ_c<< 1 [where I represents one of the spin types in the heteronuclear spin system, ω^I_iis the chemical-shift range of spin type I in radians per second,J_ijis the coupling constantJbetween spiniand spinj(like or unlike) in hertz, and τ_cis the correlation time in seconds per radian], can be applied to heteronuclear spin systems of both small molecules and biopolymers in high-resolution liquid NMR. This newly developed formalism is used to investigate the effect of transverse cross relaxation on the apparent coupling constants in a heteronuclear¹H–¹H–¹³C three-spin system. The calculation shows that, despite the strong dipolar interaction between directly bonded¹H and¹³C, this perturbation on the apparent couplings is trivial. This result is in contrast to the homonuclear proton spin system, where the scalar coupling constants between two protons are significantly modulated if one proton is strongly dipole coupled to another proton. The underlying physical reasons for the different behavior of heteronuclear and homonuclear systems are explained.