Frequency Selective Heteronuclear Dipolar Recoupling in Rotating Solids: Accurate13C−15N Distance Measurements in Uniformly13C,15N-labeled Peptides

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.