Magnetization, spin locked

Any modification of the magnetization thus arises from relaxation phenomena. The transverse magnetization spin-locked along must end up at its thermal equilibrium value, that is zero. The corresponding evolution is exponential with a time constant denoted by Tip (relaxation time in the rotating frame), very close (if not identical) to T. In practice, the signal is measured (and subsequently Fourier transformed) for a set of x values, in successive experiments, and obeys the equation... 

Clearly, in order to correctly apply CP pulse sequences for quantitative analysis (or even qualitative analysis), many relaxation processes (Tic, T pH, Till. TCp) must be considered and spectral acquisition parameters appropriately set. While a CP spectrum may be obtained when Tic > 7) 11 3TlpH Tqp, a quantitative CP spectrum requires that the recycle delay is sufficient (on the order of Tm) for the protons to be uniformly relaxed at the beginning of the contact time,25 all proton magnetization spin locked in the rotating frame decays at the same rate (T ph), and the contact time is sufficient to allow complete cross polarization (at least 5 times the longest TCp)26 Except when relative peak intensities are constant and appear to be correct, single contact time measurements should be avoided. Instead, 13C spectra and relaxation times should be measured and complete magnetization curves analyzed. 

Parhcular care has to be taken when implementing ROESY experiments. The spin-lock, which holds the spins along a defined axis perpendicular to the stahc magnetic field, can be realized in many different ways and is shU an achve field of research [18, 20]. In most spin-lock sequences the conditions for undesired TOCSY transfer are parhally fulfilled and especially cross-peaks close to the diagonal or anhdiagonal might not be accurately interpretable. Since in most cases the effechveness of the spin-lock also depends on the chemical shift offset, an offset-dependent correction has to be applied to the measured cross-peak intensities [20]. 

In addition, the technique of cross polarization introduced and developed by Pines, Gibby and Waugh (9) is used to increase the signal-to-noise ratio of the spectrum. The proton magnetization is spin-locked along the y axis with a spin-locking field % and the carbons subjected to an RF pulse chosen such that the two fields fulfill the Hartmann-Hahn condition (10), equation [3] (Figure 2). 

Under MAS the quadrupole splitting becomes time dependent, Qg = Qg (f) (see Sect. 2.3.4). This influences both the spin-locking behavior [223] and the polarization transfer [224], with the latter being further affected by the periodic modulation of the IS dipolar interaction. The effect of MAS on spin-locking of the S magnetization depends on the magnitude of the so-called adiabaticity parameter ... 

Fig. 10.23. Cross-polarization pulse sequence. The high abundance nuclei, such as protons, are first irradiated with a standard 90° pulse to create the initial magnetization. A special pair of spin-locking pulses is applied during a period called the contact time in order to transfer the magnetization from the protons to the low abundance nuclei, such as carbons. Protons are then decoupled from carbons during the acquisition of the carbon signal. In the case of protons and carbons, cross-polarization can enhance the observed carbon signal by as much as four-fold.

The simplest double tuned filter can be constructed by a concatenation of two X-half filters and removal of redundant 180° pulse pairs (Fig. 17.4d) [22]. Alternatively, it can also be realized by keeping the 180° pulse pairs and adding short spin-lock periods (to dephase the 1H-13C magnetization which is orthogonal to the spin-lock axis, Fig. 17.4e) [23], or it is based on the gradient-purging scheme of Fig. 17.4b, resulting in the double filter shown in Fig. 17.4f [18]. 

In a ID TOCSY-NOESY experiment [39], the proton magnetization is aligned along the spin-lock axis after the initial selective TOCSY step. The... 

Concatenation of two TOCSY steps in a ID TOCSY-TOCSY experiment [72] is a straightforward matter (fig. 10(a)). After the initial TOCSY transfer, the magnetization is returned to the 2 axis by a nonselective 90° pulse applied perpendicularly to the spin-lock axis. The carrier frequency is changed and the second 90° selective pulse applied to a different proton followed by the second TOCSY spin-lock period. 

Doubly selective ID-TOCSY experiments have been proposed to specifically transfer in-phase magnetization from two designated spins [57, 58]. This transfer will only take place if the two spins are connected by a scalar coupling. This method is achieved by using a double-selective spin-lock after the selective excitation of transverse magnetization of a desired spin. The doubly selective spin-lock can be obtained by using cosine-modulated... 

A single spin-lock purge pulse inserted into the HSQC pulse sequence defocuses most of the undesired proton magnetization while maintaining the proton magnetization of the protons bound to C or [8]. 

The magnetization from C-bound protons is suppressed by the spin-lock purge pulse. Denoting the operators of a proton 2-spin system as and Hb, the product operator calculation yields... 

Since water protons are not bound to or nuclei, the water signal is also suppressed by the spin-lock purge pulse. In practice, the suppression of the water signal is sufficient to record HSQC spectra of protein samples dissolved in mixtures of 95% H20/5% D2O without any further water suppression scheme [12]. For optimum water suppression the carrier frequency must be at the frequency of the water resonance. On resonance, the phase of the water magnetization is not affected by imperfections of the first 180°(ff) pulse, so that no solvent magnetization ends up along the axis of the spin-lock purge pulse. 

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