Correction of translational motion artifacts in multi-slice spin-echo imaging using self-calibration.

In this paper, we describe a method for detecting and correcting in-plane bulk translational motion in multislice spin-echo imaging using self-calibration and postprocessing. A constant phase encoding offset between slices is used to evenly spread out the low spatial frequency echoes to allow accurate motion detection by self-calibration. Motion detection in both x and y directions is achieved by interchanging the readout and phase encoding directions for the alternate slices. Displacements are determined by cross correlating the modulus of each 1D transformed echo with a reference box car function whose width equals that of the imaged object. In addition, phase errors induced by the velocity in the readout direction are estimated and corrected using the displacement data. The results obtained from knee studies at 0.5 T and 1.5 T show that the artifacts due to translational motions are significantly suppressed upon correction. The method does not require any additional pulses or time, and the data processing can be easily implemented.

Assignment of the 1H chemical shifts of glycogen.

Assignments of nearly all the 1H chemical shifts of glycogen are made by 2-D 1H-1H homonuclear and 13C-1H heteronuclear COSY. We demonstrated that it is possible to obtain well-resolved 2-D n.m.r. spectra for a large molecule like glycogen. The seven nonequivalent protons of the glucose residues in the alpha-(1----4)-linked chains, and of those at the nonreducing ends, were completely assigned. Distinct chemical shifts for H-1 and H-2 immediately adjacent to the alpha-(1----6) bonds at the branch points were also determined. Several modifications of previous 13C chemical shift assignments were made from the heteronuclear 2-D n.m.r. data.

13C NMR relaxation times of hepatic glycogen in vitro and in vivo.

The field dependence of relaxation times of the C-1 carbon of glycogen was studied in vitro by natural-abundance 13C NMR. T1 is strongly field dependent, while T2 does not change significantly with magnetic field. T1 and T2 were also measured for rat hepatic glycogen enriched with [1-13C]glucose in vivo at 4.7 T, and similar relaxation times were observed as those obtained in vitro at the same field. The in vitro values of T1 were 65 +/- 5 ms at 2.1 T, 142 +/- 10 ms at 4.7 T, and 300 +/- 10 ms at 8.4 T, while T2 values were 6.7 +/- 1 ms at 2.1 T, 9.4 +/- 1 ms at 4.7 T, and 9.5 +/- 1 ms at 8.4 T. Calculations based on the rigid-rotor nearest-neighbor model give qualitatively good agreement with the T1 field dependence with a best-fit correlation time of 6.4 X 10(-9) s, which is significantly smaller than tau M, the estimated overall correlation time for the glycogen molecule (ca. 10(-5) s). A more accurate fit of T1 data using a modified Lipari and Szabo approach indicates that internal fast motions dominate the T1 relaxation in glycogen. On the other hand, the T2 relaxation is dominated by the overall correlation time tau M while the internal motions are almost but not completely unrestricted.

1H NMR visibility of mammalian glycogen in solution.

High-resolution 1H NMR spectra of rabbit liver glycogen in 2H2O were obtained at 500 MHz, and several resonances were assigned by comparison with the chemical shifts of alpha-linked diglucose molecules. The NMR relaxation times T1 and T2 of glycogen in 2H2O were determined to be 1.1 and 0.029 s, respectively. The measured natural linewidth of the carbon-1 proton (12 +/- 2 Hz) is in excellent agreement with that calculated from T2. The visibility measurements made by digesting glycogen and comparing glucose and glycogen signal intensities demonstrate that in spite of the very high molecular weight, all of the proton nuclei in glycogen contribute to the NMR spectrum. The result is not unexpected, since 100% NMR visibility was previously observed from the carbon nuclei of glycogen, due to the rapid intramolecular motions.

Perturbation of tryptophan residues by point mutations in bacteriophage T4 lysozyme studied by optical detection of triplet-state magnetic resonance spectroscopy.

We have investigated perturbations of the triplet-state properties of Trp residues in bacteriophage T4 lysozyme caused by point mutations using low-temperature phosphorescence and optical detection of triplet-state magnetic resonance (ODMR) spectroscopy. Five temperature-sensitive mutants have been studied in detail. These include lysozymes with the point mutations Gln-105----Ala, Gln-105----Gly, Gln-105----Glu, Ala-146----Thr, and Trp-126----Gln. Changes in phosphorescence 0,0 band wavelength, intensity, the triplet-state zero-field splitting (ZFS), and the wavelength dependence of the ZFS were detected only from Trp-138 in each mutant. In the case of the Q105A mutation, the perturbations on Trp-138 have been ascribed to the combination of an increase in the polarizability of the environment and to the loss of hydrogen bonding of the enamine nitrogen of indole. For the Q105G mutation, we believe that Q is replaced by a solvent molecule in H bonding, leading to relatively small changes. In the Q105E mutation, the perturbation results largely from the introduction of a charged residue. In the case of the mutation A146T, the perturbation is associated with a local conformational change in which Trp-138 is shifted to a more solvent-exposed location. On the other hand, no significant spectroscopic changes in Trp-126 and Trp-158 were found in any of the mutants, suggesting that the perturbations are probably localized near Trp-138 for the mutations of positions 105 and 146. However, in the mutation W126Q, which occurs approximately 16 A away from Trp-138, significant changes of Trp-138 are detected, suggesting that the effects of this mutation are propagated over large distances.

Optically detected magnetic resonance study of tyrosine residues in point-mutated bacteriophage T4 lysozyme.

Two spectroscopically distinct types of tyrosine (Tyr) residues in triply point mutated bacteriophage T4 lysozyme, which contains no tryptophan (Trp), have been detected by optical detection of triplet-state magnetic resonance (ODMR) spectroscopy. Their triplet states are characterized by similar E but different D values. The Tyr site which exhibits the lower D value and has the red-shifted phosphorescence origin is quenched by energy transfer to Trp and has D and E values comparable to previously studied Tyr residues. The blue-shifted Tyr site, which is not quenched by Trp, exhibits a larger D value that has been found previously. Calculation of energy-transfer efficiencies of Tyr-Trp pairs based on the crystal structure of the native enzyme provides a possible assignment of Tyr sites to the two different spectral types.

Comparative triplet-state properties of the three tryptophan residues in bacteriophage T4 lysozyme and in the enzyme complex with methylmercury(II).

Triplet-state energies, zero-field splittings (ZFS), and total decay rate constants of the individual triplet-state sublevels of the tryptophan (Trp) residues located at positions 126, 138, and 158 in bacteriophage T4 lysozyme have been determined by using low-temperature phosphorescence and optical detection of magnetic resonance spectroscopy in zero applied magnetic field. An investigation of spectral and kinetic properties of individual Trp residues was facilitated by measurements on point-mutated proteins containing two Trp----Tyr substitutions. We find that the phosphorescence lifetime of the buried Trp-138 is considerably shorter than those of the solvent-exposed Trp residues. CH3HgII binding to cysteine residues in T4 lysozyme selectively perturbs the triplet state of Trp-158 by means of an external heavy-atom effect. In contrast with the previous observation of selective x-sublevel perturbation in the Trp-CH3Hg complex, the radiative character of the z sublevel (z is the out-of-plane axis) is selectively enhanced due to the heavy-atom perturbation of Trp-158. The observed pattern of radiative and total sublevel decay constants of the perturbed Trp is attributed to a special orientation of the Hg atom with respect to the indole plane.

Triplet state sublevel kinetics of tryptophan 54 in the complex of Escherichia coli single-stranded DNA binding protein with single-stranded poly(deoxythymidylic) acid.

The individual sublevel kinetics of the lowest triplet state of tryptophan 54 (Trp 54) which is highly perturbed in the complex of Escherichia coli single-stranded DNA binding protein (Eco SSB) with poly(deoxythymidylic) acid (poly[dT]) have been studied by optically detected magnetic resonance (ODMR) spectroscopy. The triplet sublevel decay constants of Trp 54, kx, ky, kz, are 0.99, 0.072, and 0.045 s-1, respectively, in the poly(dT) complex of a point-mutated Eco SSB in which Trp 88 is substituted by phenylalanine. Tx is the only radiative triplet sublevel. Negative polarity of the Tx----Tz and Tx----Ty phosphorescence-detected ODMR signals results from the steady state population pattern, nx greater than ny, nz, and implies that the relations, px greater than or equal to 14py, and px greater than or equal to 22pz exist for the relative populating rates. Spin-orbit coupling between radiative singlet states and the Tx sublevel of the lowest triplet state of Trp 54 is enhanced selectively upon complexing of Eco SSB with poly(dT).