TOCSY mixing efficiency

Factors That Affect TOCSY Mixing Efficiency... 

If we used a continuous-wave spin lock (ROESY mixing scheme) for TOCSY mixing, the only way efficient transfer could occur via 7 couplings within a spin system would be if the two protons have opposite resonance offsets (v0 - vr), where vr is the carrier or reference frequency. This is a special case of the Hartmann-Hahn match ... 

Fig. 15. Polarization-transfer efficiency in a spin system consisting of three spins 1/2 under ideal isotropic-mixing conditions (effective ///-TOCSY coupling topology). (A) Damped transfer functions - t U12I) as a function of the relative coupling constant...

The main difference between approaches that use delays during the basis sequence (Methods C and D) or after completed basis sequences (Methods A and B) is the efficiency of Hartmann-Hahn transfer. In Methods A and B, no Hartmann-Hahn transfer occurs during the compensating delays. If Methods A or B are used, the total mixing time (including the compensating delays) must be increased by 50% compared to a partially compensated multiple-pulse TOCSY sequence with roe/4> in order to obtain the same Hartmann-Hahn transfer. 

In general, a given pulse sequence can act as a TOCSY or as a TACSY mixing sequence, depending on the rf amplitude, the irradiation frequency, and the spin system to which is it applied. Therefore, it is important to consider the offset dependence of the coherence-transfer efficiency of a Hartmann-Hahn sequence. In this respect, a rough distinction between highly selective and band-selective Hartmann-Hahn experiments is useful. 

Although in principle the simple scheme presented in Fig. 5.59 should provide TOCSY spectra, its suitability for practical use is limited by the effective bandwidth of the continuous-wave spin-lock. Spins which are off-resonance from the applied low-power pulse experience a reduced rf field causing the Hartmann-Hahn match to breakdown and transfer to cease. This is analogous to the poor performance of an off-resonance 180° pulse (Section 3.2.1). The solution to these problems is to replace the continuous-wave spin-lock with an extended sequence of composite 180 pulses which extend the effective bandwidth without excessive power requirements. Composite pulses themselves are described in Chapter 9 alongside the common mixing schemes employed in TOCSY, so shall not be discussed here. Suffice it to say at this point that these composite pulses act as more efficient broadband 180 pulses within the general scheme of Fig. 5.60. 

Kramer and Glaser analysed the transfer efficiency of cross-relaxation compensated (Clean) TOCSY sequences for applications to residual dipolar couplings. Surprisingly most conventional Clean TOCSY sequences are very inefficient for dipolar transfer. It is shown theoretically, that this is a general property of all phase-alternating mixing sequences, i.e., for such sequences the suppression of cross-relaxation excludes dipolar transfer in the spin-diffusion limit. A new family of clean dipolar TOCSY sequences is derived which provides excellent transfer efficiencies for a broad range of offset frequencies. 

It has been suggested that the relative sign and the magnitude of the scalar and the residual dipolar couplings in homonuclear spin 1/2 systems can be determined based on the sign and the amplitude of the transfer fimctions in a TOCSY experiment. The efficiency of different mixing sequences and different transfer functions has been examined both theoretically and experimentally. The efficiencies of various separated local field (SLF) experiments... 

The duration of the mixing period determines the efficiency of transfer. If a sufficiently long mixing time is chosen, correlations of the whole proton spin systems are found. TOCSY experiments with short mixing times reveal mainly correlations between directly coupled nuclei. 

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