Practical Aspects of Carbon-13 Double Quantum NMR
We want to address an erroneous view that has recently been advanced on 13C double quantum NMR. A recent publication (I) introduced a modified version (I) of the original double quantum NMR experiments; this new version claims to offer an attractive increase in sensitivity by a factor of three, or a decrease in the size of the required data matrix by a factor of four. To avoid disappointment among spectroscopists we wish to make it clear that this modified version offers no gain in sensitivity whatsoever in comparison with the other double quantum experiments (2-6), provided they are properly optimized. It is shown here that the sensitivity of this modified experiment can be even slightly lower. The claim of a reduction of the size of the required data matrix is also shown to be incorrect. In the original two-dimensional double quantum “C experiments, the acquisition time t2 starts immediately after the final “read pulse” (Fig. 1), and the time during which the double quantum coherence evolves is labeled ti. The detected 13C-13C satellites (7) are modulated in amplitude as a function of tl with the double quantum frequencies (2). In the quadrature-Fi-detection version of this experiment (3), results are combined with those of a second experiment which contains a composite 45: pulse (8) in the evolution period. This allows the sign of the modulation frequency to be determined, which simplifies the interpretation of the results. A simpler and more sensitive alternative to obtain quadrature F, detection has been proposed more recently (6); an increase of the flip angle of the final “read pulse” to 135” provides directly satellite signals which are modulated in phase as a function oft,. In either case, data acquisition is started after the final read pulse and the coherence transfer echo (9) is detected (4, 6). The time domain signal of a carbon A, directly coupled to carbon X, is then given by sA(tl, t2) = c exp[-i(nA + &)t,] exp[i& + d&2]
Improvement of 2D NOE and 2D correlated spectra by triangular multiplication
Triangular multiplication is a computational routine for improvement of two-dimensional nuclear Overhauser enhancement and two-dimensional correlated spectra. The routine is applied after Fourier transformation in both dimensions. Spurious noise peaks can be effectively removed from the spectra and the ratio of signal to random noise can be improved. The practical application of triangular multiplication is illustrated with a two-dimensional NOE spectrum of ferrocytochrome c.
Tradeoffs and limitations in statistically based image reconstruction problems.
TRADEOFFS AND LIMITATIONS IN STATISTICALLY BASED IMAGE RECONSTRUCTION PROBLEMS