Two-dimensional chemical exchange spectroscopy

Both homonuclear and heteronuclear versions of relayed nOe experiments are known. The homonuclear relayed NOESY experiment involves both an incoherent transfer of magnetization between two spins H and H that are not coupled but close in space, and a coherent transfer of magnetization between two spins H/and H , that are /-coupled together. The magnetization pathway may be depicted as 

In a heteronuclear nOe experiment, the first step may be identical to the homonuclear nOe experiment (i.e., involving an incoherent transfer of coherence from H to H,), while the second step could involve a coherence transfer from H to C nucleus by an INEPT sequence. These methods suffer from poor sensitivity and have therefore not been used extensively. 

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Macura S, Huang Y, Suter D, Ernst RR (1981) Two-Dimensional Chemical Exchange and Cross-Relaxation Spectroscopy of Coupled Nuclear Spins. J Magn Reson 43 259... 

S. Macura, Y. Huang, D. Suter, R.R. Ernst, Two-dimensional chemical exchange and cross-relaxation spectroscopy of coupled nuclear spins, J. Magn. Reson. 43 (1981) 259-281. 

Macura S, Wurthrich K, Ernst R R 1982 Separation and suppression of coherent transfer effects in two-dimensional NOE and chemical exchange spectroscopy. J Magn Res 46 269-282... 

Figure 3.1 The various time periods in a two-dimensional NMR experiment. Nuclei are allowed to approach a state of thermal equilibrium during the preparation period before the first pulse is applied. This pulse disturbs the equilibrium ptolariza-tion state established during the preparation period, and during the subsequent evolution period the nuclei may be subjected to the influence of other, neighboring spins. If the amplitudes of the nuclei are modulated by the chemical shifts of the nuclei to which they are coupled, 2D-shift-correlated spectra are obtained. On the other hand, if their amplitudes are modulated by the coupling frequencies, then 2D /-resolved spectra result. The evolution period may be followed by a mixing period A, as in Nuclear Overhauser Enhancement Spectroscopy (NOESY) or 2D exchange spectra. The mixing period is followed by the second evolution (detection) period) ij.

Exchange spectroscopy (EXSY) is a two-dimensional method that can detect chemical exchange and fluxional processes before line broadeiting occurs (Figure 22)." ... 

Fig. 17. Two-dimensional exchange spectroscopy (2D-EXSY) spectra of an aqueous solution containing TKCNls, T1(CN)4 , and HCN in a concentration ratio 0.021 0.018 0.166 M, enriched to 95% in C. Temperature = 25°C. Reprinted from Batta et al. (168). Copyright 1993 American Chemical Society, (a) C NMR spectrum, recorded at 100 MHz, mixing time = 0.035 s. The spin-spin coupling between C and H is observed because the proton exchange between HCN and bulk water is slow (309a). (b) T1 NMR spectrum, recorded at 231 MHz, mixing time = 0.020 s.

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