Double TEDOR for the direct detection of stable-isotope triads

An extension of transferred-echo double-resonance (TEDOR) NMR is presented for the detection of three coupled spin-1/2 nuclei: I-1-S-I-2, where I-1 and I-2 are a homonuclear pair with an isotropic chemical-shift difference, By performing two coherence transfers (in combination with a DANTE preparation sequence)? it is possible to monitor coherence transferred sequentially from I-1 to S to I-2 (or the reverse), Initial experiments demonstrating the feasibility of this approach were performed on both mixed and recrystallized C-13- and N-15-labeled alanines, and the results are found to be in good agreement with predictions based on a density matrix formalism. This technique has also been applied to characterize the covalent cross-linking of the bacterial cell wall of a triple-labeled Bacillus subtilis by demonstrating the connectivity of C-13-N-15-C-13 triads, These triads occur only in the cross-link bond. (C) 1996 Academic Press. Inc.