Pulse sequences, developments

The initial relaxation results on PP over a temperature range from 24°C to -195°C are summarized in Figure 13. The T3 data were collected using a pulse sequence developed by Torchia (44) which allows cross-polarization enhancement of the signals. The T] p data were determined at 58 kHz using Tj p methodology of Schaefer et al. (1). As indicated in the figure, each of the carbons display individual relaxation rates in both types of experiments. The CH and CH2 carbons have a T minimum at ca. -110°C, nearly the same temperature as that reported by McBrierty et al.(45) for the proton Ti minimum in isotactic PP. 

Figure 3 shows a typical pulse sequence developed for DQ excitation. A systematic variation in the DQ excitation and reconversion periods can be used to probe the van Vleck s second moment of the coupled spins,57 whose magnitude depends on both the number of interacting spins and the internuclear distances ... 

Case 4—Real-world pulse sequence development by... 

Some of the motivations to pursue spectral simulation in a clinical MRS setting include providing metabolite prior information for use in parametric spectral analysis procedures, pulse sequence parameter optimization for observation of specific metabolite structures and shortening times for pulse sequence development. This section will describe, in some detail, examples of each with particular regard for the design and level of prior information inclusion of each simulation and the clinical use of the results. 

Since there are no aliphatic protons present in perdeuterated proteins, new strategies must be employed and new pulse sequences developed for the NMR assignment and structure determination. 

Spin-Lattice Relaxation. In order to determine whether each resonance line comprises a single component, we first measured the spin-lattice relaxation time Tic by the pulse sequence developed by Torchia [53] or by the standard saturation-recovery pulse sequence. The Tic values thus obtained were 2560,263 and 1.7 s for resonance line I and 0.37 s for line II. As reported by several investigators, the line at 33 ppm is associated with three different Tic values [ 17,54,55]. This means that this line is contributed to by three components with different molecular mobilities. However, since each component was represented by a single Lorentzian line shape at 33 ppm, they are all assignable to methylene groups in the orthorhombic crystalline form or in the trans-trans conformation. The component with a Tic of s can be assigned to methylene groups with a some-... 

Much of the technical development of NMR over the past half century has focused on improving sensitivity. The fundamental problem is the low starting Boltzmann polarization that arises from the low energies of nuclear spin transitions. Several methods have been developed to improve the sensitivity or S/N in NMR. One major approach is through pulse sequence development to optimize the efficiency and information content of NMR spectra through manipulating the spin physics some of the more important experiments for small molecules were described above. 

Molecular diffusion coefficients are parameters that are not related directly to NMR spectral intensities under normal conditions. However, molecular diffusion can cause NMR signal intensity changes when pulsed field gradients are applied during the FT NMR experiment. A number of pulse sequence developments, particularly the LED sequence, have meant that measurement of diffusion coefficients is relatively routine. The editing of H NMR spectra of biofluids based on diffusion alone or on a combination of spin relaxation and diffusion has been demonstrated. This has been termed the Diffusion and Relaxation Editing (DIRE) pulse sequence. This approach is... 

This requires the use of a multiple-pulse scheme with a nonvanishing effective r.f. field. Such a nonvanishing effective field can be added to a sequence without an effective field by either adding an additional small-angle pulse after a full cycle of the sequence, or by increasing the power of one of the pulse phases by a small amount. There have been quite a number of multiple-pulse sequences developed for the liquid-state TOCSY experiment [3, 22], which provide offset compensation at low r.f. powers. However, the requirements in the present context are different from the ones in TOCSY due to the fact that the dipolar coupling is not isotropic like the J-coupling and is not, therefore, invariant under nonselective pulses. 

Theoretically all the pulse sequences developed for IR observation should also be applicable for based experiments provided that the larger chemical shift range... 

Since the publication of the first edition of this book, there have been many important developments in the field of NMR spectroscopy. These developments have included the award of two Nobel prizes in 2002 to Kurt Wiithrich for his major contributions to biomolecular NMR spectroscopy and in 2003 to Paul C. Lauterbur and Sir Peter Mansfield for their work on MRI, both awards confirming the scientific importance of the general method and its wide application. Consequently, this second edition has been extended to incorporate a number of these pulse sequence developments. Nevertheless, to understand these sophisticated methods it is still necessary that students and newcomers start with the basic experiments and proceed on a step-by-step basis. In this context NMR-SIM is an outstanding, user-friendly simulation program that may be used by both the novice and the expert as an efficient training tool. Therefore, it is no surprise that BRUKER have included NMR-SIM in their latest spectrometer software package TOPSPIN. 

WAHUHA multiple-pulse sequence developed by Waugh, Huber, and Haeberlen... 

An additional option is the use of relaxation-compensating pulse sequences. This is the domain of the pulse-sequence developers. At the moment, there are two approaches available. One uses heteronuclear multiple-quantum coherences, in which the dipolar relaxation is removed [17]. The achieved gain in intensity is shown on the example of an RNA. 

Fig. 4. Accordion HMQC pulse sequence developed by Zangger and Armitage. - This experiment was developed to facilitate the study of metallothioneins in which cadmium has been inserted as an isomorphic replacement for the zinc atoms normally bound to metallothioneins. By using the accordion HMQC experiment, Zangger and Armitage were able to observe correlation responses to the seven cadmium atoms in a single experiment rather than having to perform several conventional experiments with varied optimization.

Fig. 14. IMPEACH-MBC (IMproved PErformance ACcordion-optimized Heteionucicar multiple bond correlation) pulse sequence developed by Martin and co-workers. The experiment is a further modification of the ACCORD-HMBC experiment that utilizes a constant time variable delay in lieu of a simple variable delay. The con.stant time variable delay introduces the interval, D/2 — 180° "C — D/2, which precedes the variable delay interval, vd. As the evolution lime ti is incremented, the interval vd is decremented in the usual fashion. However, at the same time, the Dll (ct A) intervals are incremented in a manner to keep the overall duration of the period D + vd a constant time interval. Hence, homonuclear modulation, which plagues ACCORD-HMBC experiments, is suppressed by the constant time of the intcrv al D + vd. In contrast, evolving heteronuclear couplings arc refocused at time D by the 180° - C pulse located at DU. These couplings then evolve during the variable interval vd to be sampled in the usual, accordion manner. By using this approach, the constant time variable delay pulse sequence element is of constant duration for homonuclear components of magnetization while serving as a variable delay for heteronuclear components.

INADEQUATE spectrum and a ID proton-decoupled spectrum in a single experiment, elaborately termed PANACEA Parallel Acquisition NMR, an All-in-one Combination of Experimental Applications [134]. The addition of a third receiver allows for the simultaneous acquisition of a N spectrum also Clearly with these methods there exists enormous future potential, and a whole new area of exploration for those engaged in pulse sequence development. 

A large array of heteronuclear H-X dipolar recoupling pulse sequences, developed mainly for X= C and N," combine information on H-chemical shift and H-X dipole-dipole coupling. 

The copolymerizations of VDF with a variety of monomers, as well as their applications, were reviewed in Ameduri s paper [37], The structural variations present in these polymers include monomer inversions, monomer/stereo-sequence effects (in copolymers containing monomers with branch-forming stmcture elements), chain ends, and short/LCBs. The characterization of these polymers in early studies predominantly depended on ID-NMR and empirical chemical shift additivity rules [38,39]. With the improvement in both NMR hardware and pulse sequence development, 2D-NMR characterization of these polymers has become more popular. 

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