During this past year, frequency-selective versions of RFDR, as well as precise measurements of weak homonuclear couplings, have been demonstrated. These efforts were designed as a continuation of last year's work, when we explored in greater detail the underlying spin physics of RFDR, in order to facilitate the accurate measurement of weak dipole-dipole couplings between rare spins and to understand the behavior of dipolar trajectories in multiple spin environments. RFDR has frequently been used as a tool for homonuclear """"""""C-""""""""C (and """"""""N-""""""""N) chemical shift correlation spectroscopy. We have presented in more detail several aspects of homonuclear recoupling and longitudinal exchange using rotor-synchronized spin echo sequences in solid state magic angle spinning (MAS) experiments. These experiments include in one limit (where N=1 and n=O, one 7E pulse per rotor period) the standard version of RFDR, and in other cases (e.g., N=4, n=2) frequency-selective versions. The recoupling effect can be quenched in a manner similar to R2, so that only particular chemical shift differences are recoupled; in addition, selective RFDR permits recoupling of spins for which the chemical shift differences are not a small integer multiple of the MAS rate. This frequency- selective approach has possible utility in examining weak dipole-dipole couplings in the presence of strong interactions. To describe dipolar trajectories accurately, we have adopted an approach to the simulation of these experiments that includes finite pulses and the influence of coherence decay. The latter effect becomes competitive with the strength of weak couplings in many experiments, and a simple empirical approach has been outlined for the selection of decay parameters. In multi-spin systems, dipolar trajectories are shown to be dominated by th e largest couplings. This has been demonstrated in two- and three-spin model compounds, as well as numerous U-""""""""C,""""""""N-labeled peptides, nucleoside monophosphates, and amino acids. For example, two-dimensional correlation spectroscopy based on dipolar exchange among proximate nuclei has been demonstrated in a uniformly """"""""N,""""""""C-labeled sample of the tetrapeptide achatin-H (Gly-L-Phe-L-Ala-L-Asp).
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