Transfer time, spin-lock cross-polarization

The initial building-up of the carbon magnetization via polarization transfer is characterized with the spin-lock cross-polarization time Tcir It is sensitive to the static interactions, i.e. transfer of the polarization via static dipolar interactions (proportional to e J). Hence, the shortest TCH will have carbons a) in more rigid systems and b) with more directly attached protons, particularly the CH2 (or... 

Figure 4. Determination of the C-N matched spin-lock cross-polarization transfer time constant for two solid double-labeled amino acids using double cross polarization. T > is the time constant for the transfer from N to an aliphatic C, and T. is for the transfer from N to an amide C. The enrichment for both materials was 90 atom %. Key , NHi CH,COOH (t - 0.9) where Ti, = 10.93 ms and O, NH, COCH,CH(NHi)COOH (t = 0.9) where Ta = 3.64 ms.

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.

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