With the advent in 1990 of pulsed-field-gradient (PFG)
Here we show several alternatives to obtain phase-sensitive homonuclear 2D spectra in very short times with techniques for coherence selection in high-resolution NMR experiments, the concept of routine in chemical and bio-no phase cycling. Thus, phase-sensitive 2D TOCSY and 2D ROESY spectra can be obtained with a single scan chemical applications has changed. Many new, highly reliable NMR experiments use PFGs instead of the conventional per t 1 increment ( Fig. ) using an approach similar to that described previously for 2D NOESY spectra ( ) . phase cycle procedure to select the desired coherence-transfer pathway (CTP) (1, 2). The main advantages of the gradient-selection method are: (i) its greater speed, particularly for multidimensional determinations, resulting in much shorter spectrometer time requirements; and (ii) the much higher quality of the resulting spectra, always devoid of subtraction errors, thus allowing a much easier analysis of NMR parameters. Both advantages are due to the effective suppression by the gradients of unwanted magnetizations arising from undesired CTPs. Outstanding examples are the perfect suppression of 1 H-12 C (or 1 H-14 N) magnetization effortlessly achieved in inverse experiments, or the frequency-independent suppression of the water signal from aqueous solutions. Another interesting advantage of PFG selection is that quadrature detection can be readily obtained in the indirect dimensions of nD experiments using a single scan per increment (3).
However, some precautions must be taken into account when designing novel PFG-based NMR methods aimed at obtaining spectra with maximum sensitivity and with signals in pure absorption lineshape (4). Thus, a PFG inserted into the variable evolution period in a multidimensional experiment selects only one of the two desired CTPs (P-or Ntype selection) in each scan. Therefore, the resulting spectra are usually presented in magnitude mode, thus overcoming the undesirable mixed lineshapes of P or N singly selected signals (5-7). In order to obtain phase-sensitive spectra, pulse sequences must be modified so that no PFGs are applied during the variable evolution period. This option gener-