Frequency selective heteronuclear dipolar recoupling in rotating solids:: Accurate 13C-15N distance measurements in uniformly 13C,15N-labeled peptides

We describe a magic-angle spinning NMR experiment for selective C-13-N-15 distance measurements in uniformly C-13,N-15-labeled solids, where multiple C-13-N-15 and C-13-C-13 interactions complicate the accurate measurement of structurally interesting, weak C-13-N-15 dipolar couplings. The new experiment, termed FSR (frequency selective REDOR), combines the REDOR pulse sequence with a frequency selective spin-echo to recouple a single C-13-N-15 dipolar interaction in a multiple spin system. Concurrently the remaining C-13-N-15 dipolar couplings and all C-13-C-13 scalar couplings to the selected C-13 are suppressed. The C-13-N-15 coupling of interest is extracted by a least-squares fit of the experimentally observed modulation of the C-13 spin-echo intensity to the analytical expression describing the dipolar dephasing in an isolated heteronuclear spin pair under conventional REDOR. The experiment is demonstrated in three uniformly C-13,N-15-labeled model systems: asparagine, N-acetyl-L-Val-L-Leu acid N-formyl-L-Met-L-Leu-L-Phe; in N-formyl-[U-C-13,N-15]L-Met-L-Leu-L-Phe we have determined a total of 16 internuclear distances in the 2.5-6 Angstrom range.