Abstract
We describe a magic-angle spinning NMR experiment for selective 13C−15N distance measurements in uniformly 13C,15N-labeled solids, where multiple 13C−15N and 13C−13C interactions complicate the accurate measurement of structurally interesting, weak 13C−15N 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 13C−15N dipolar interaction in a multiple spin system. Concurrently the remaining 13C−15N dipolar couplings and all 13C−13C scalar couplings to the selected 13C are suppressed. The 13C−15N coupling of interest is extracted by a least-squares fit of the experimentally observed modulation of the 13C 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 13C,15N-labeled model systems: asparagine, N-acetyl-l-Val-l-Leu and N-formyl-l-Met-l-Leu-l-Phe; in N-formyl-[U-13C,15N]l-Met-l-Leu-l-Phe we have determined a total of 16 internuclear distances in the 2.5−6 Å range.