In the past decade, homonuclear coherence-transfer exper-The direct benefits of the DPFGSE pulse sequence are the elimination of unwanted magnetization, the optimization of iments (COSY, RELAY-COSY, TOCSY) (1-4) have emerged as indispensable methods for obtaining geminal, out-of-band suppression, and the maintenance of constant phase (11)(12)(13). This sequence has been demonstrated to be useful vicinal, and relayed scalar spin assignments in organic molecules, peptides, and proteins. At present, there exist numernot only for solvent suppression (11) and homonuclear NOE ous implementations of these homonuclear experiments; magnetization-transfer experiments ( ), but also for heterosome examples include one-dimensional experiments innuclear polarization transfer (13). volving selective excitation (5-7), nonselective two-dimen-We adapted the DPFGSE scheme to create selective-excisional experiments (1-4, 8, 9), and selective-excitation tation 1D DPFGSE-SELCOSY, -SELRELAY and -SELexperiments in two-dimensional form (10). Of these, selec-TOCSY pulse sequences (Figs. 1A, 1B) using selective recttive-excitation one-dimensional homonuclear coherenceangular ''soft'' pulses for selective spin inversion. Note that transfer experiments (1-D COSY, 1-D RELAY-COSY, 1-D the rectangular soft pulses can be replaced by any suitably TOCSY) have emerged as important tools for examining shaped pulse, e.g., Gaussian, sine, or hyperbolic secant (12). specific coherence-transfer processes in organic molecules As shown in Fig. , the double-gradient spin echo forms the and biopolymers (5-7). In comparison to 2D experiments, common starting point for all of our 1D selective experione-dimensional coherence-transfer experiments feature imments: a nonselective 90Њ ''hard'' pulse followed by two proved spectral resolution and shorter experiment times. soft rectangular 180Њ inversion pulses, with positive Z gradi-These advantages make the one-dimensional experiment atents of different magnitude (G 1 , G 2 ) flanking each of the tractive as a ''testbed'' for optimizing and designing two-180Њ inversion pulses. Thus, during the spin-echo period, or higher-dimensional coherence-transfer experiments, or for nonselected transverse magnetization is dephased by each studying spin-magnetization-transfer processes where rapid gradient sequence and purged from the resulting spectrum. sampling and higher resolution are desired.
Moreover, the inverse soft pulses in the DPFGSE facilitate In this Note, we present novel selective one-dimensional the application of Exorcycle phase cycling ( ), which we homonuclear COSY (SELCOSY, SELRELAY) and TOCSY find is very powerful in canceling out unwanted signals.
(SELTOCSY) pulse sequences for the study of coherence-To create the SELCOSY and SELRELAY experiment, transfer processes in biopolymers and other molecules. These we add on a G 3 -[D-180Њ-D-90Њ] n -G 3 PFG-hard experiments combine the standard COSY, RELAY-COSY, and pulse sequence immediately following the double-gradient TOCSY pulse sequences with the ''excitation-sculpting'' douspin-echo sequence (Fig. ). Here, the hard loop sequence ble-pulse field-gradient spin-echo technique (DPFGSE) determines the type of coherence transfer: for SELCOSY, n (11, 12). The versatility of the DPFGSE excitation-sculpting Å 1, for SELRELAY, n is the order of the relay (2 Å ''1sequence -G 1 -S-G 1 -G 2 -S-G 2 -is such that any step,'' 3 Å ''2-step,'' etc.). The 180Њ and 90Њ nonselective pulse sequence, S, can be incorporated into the gradient (G) pulses are along the y axis, with the 180Њ pulse employed echo pulse scheme, with the result being a pure-phase excitation in the middle of the delay to refocus the evolution of the sequence (11)(12)(13). This type of experiment allows selected chemical shifts (7). The delay, D, is normally set to 1 2 J for transverse magnetization to reequilibrate to its starting position the spins of interest. Note that the G 3 gradient scheme serves without affecting its phase or permitting component mixing. two purposes: first, the G 3 gradients can be used to select P-or N-type coherence pathways, depending on the sign of the last G 3 gradient (Fig. ) (15).