Homonuclear cross-relaxation

The terms pc and py correspond to 1/Tic and 1/Tih, respectively, and CTCH is the cross-relaxation rate. It should be stressed that the simplicity of the above equation is a consequence of the rareness of the I spins and of the dominant strength of the dipolar interaction between directly bonded nuclei. The situation for homonuclear proton spin systems is often more complicated, since the protons usually constitute a much larger spin system, and a separation into distinct two-spin systems may be not valid in this case. The broadband irradiation of the protons yields, in a steady state, Mhz = 0 and M z = Mj (1 rj). The factor 1 + 77 is called, as introduced above, the nuclear Overhauser enhancement factor. The NOE factor is related in a simple way to the equilibrium magnetizations of the I- and S-spins (which are proportional to the magnetogyric ratios 71 and 7s), the cross-relaxation rate and the relaxation rate of the I-spin ... 

Even larger usable bandwidths can be obtained for a given average rf power if clean homonuclear Hartmann-Hahn sequences are optimized from scratch (Briand and Ernst, 1991 Quant, 1992 Kadkhodaei et al., 1993 Mayr et al., 1993), rather than modifying existing uncompensated TOCSY sequences. The clean CITY sequence (see Fig. 26C, Table 3), which was developed by Briand and Ernst (1991), is still one of the most efficient broadband Hartmann-Hahn sequences with cross-relaxation compensation. The sequence is constructed using Method C and is based on the computer-optimized symmetric composite pulse R = SS with S = 48° 138° (see Fig. 22F, sequence 5g). The TOWNY (TOCSY without... 

The combination of HOHAHA transfer with cross-relaxation in the rotating frame leads to TOCSY-ROESY (TORO) or ROESY-TOCSY (ROTO) experiments (Kessler et al., 1988a Williamson et al., 1992). Two-dimensional hetero-TOCSY-NOESY experiments (Kellogg et al., 1992 Kellogg and Schweitzer, 1993), which combine heteronuclear Hartmann-Hahn transfer with a homonuclear NOESY mixing step, have been used for a P-driven assignment strategy of RNA and DNA spectra. 

Figure 8.10. Schematic illustration of the maximum homonuclear steady-state NOE in the presence (solid line) and absence (dotted line) of external relaxation sourees that compete with cross-relaxation.

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