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The combination of transferred-echo double resonance (TEDOR) with rotational-echo double resonance (REDOR) has been used to measure an 8-Á fluorine-carbon internuclear distance in a nine-residue fragment of the peptide antibiotic emerimicin. The fragment is 19FCH2C0-Phe-MeA-MeA-[l-13C]MeA-[15N]Val-Gly-Leu-MeA-MeA-0Bzl (MeA = a-methylalanine or aminoisobutyric acid). The TEDOR part of this magic-angle-spinning, solid-state NMR experiment selects the 13C label by its dipolar coupling to 15N and suppresses the natural-abundance carbon background. The REDOR part of the experiment measures dipolar coupling of the selected carbon to 19F. The TEDOR-REDOR combined experiment works with a variety of spin */2 nuclei and can be used to characterize intemuclear distances and geometry in macromolecular aggregates that do not crystallize.Recent advances in X-ray crystallography and solution-state NMR spectroscopy have yielded three-dimensional structures of many soluble proteins and peptides.1"3 Unfortunately, the same sort of information, which is essential in elucidating mechanisms of action, is largely unavailable for molecules which function within biological membranes. The problem lies primarily in the inability to prepare samples in a crystalline form suitable for X-ray analysis or to interpret NMR spectra displaying the broad lines of membrane-bound species with solid-like properties.We have previously demonstrated4 the utility of solid-state, rotational-echo double-resonance (REDOR) NMR spectroscopy5,6 in providing accurate structural information for biological solids. REDOR involves the dephasing of transverse, S-spin magnetization by rotor-synchronized I-spin pulses; I and S refer to different types of dipolar coupled rare spins. When the interactions with all other spins can be ignored or suppressed, the comparison of S-spin echo intensities with (S) and without (S0) I-spin dephasing pulses leads directly to the strength of the I-S dipolar coupling and hence I-S intemuclear distances.4The natural-abundance S-spin background complicates the interpretation of REDOR experiments by a contribution to S0 that is unrelated to I-S dipolar coupling. Sometimes this contribution can be measured in separate experiments on unlabeled samples.7 This approach is generally successful if the concentration of label is high or the I-S dipolar coupling is strong. When these conditions are not met, we propose selecting the dipolar coupled spins from among the background of uncoupled spins by a coherence transfer from I to S. Dephasing of the selected S-signal by rotor-synchronized ir pulses applied to a third rare spin (X) can now be used to measure S-X dipolar coupling and intemuclear distances with no background interferences.In this paper, we report the results of background-free distance measurements on a triple-labeled emerimicin 1-9. This peptide sequence is a fluorinated analogue of the N-terminal portion of the emerimicins III and IV, which are members of the peptaibol family of antibiotics known to function as ion channels in ...

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