Determination of a Precise Interatomic Distance in a Helical Peptide by REDOR NMR

A new spectroscopic technique, rotational-echo double-resonance (REDOR) NMR, for solids utilizes magic-angle spinning and measures directly the dipolar coupling between stable-isotope-labeled nuclei and, thus, interatomic distances. REDOR has been used to measure the ¹³C-¹⁵N interatomic distance in a nine-residue fragment, Ac-Phe-MeA(1-¹³C)- MeA(d₆)-MeA-Val-Gly(¹⁵N)-Leu-MeA-MeA-OBzl (MeA = α-methylalanine or aminoisobutyric acid (Aib)), of the peptide antibiotic emerimicin. The crystal structure of the peptide emerimicin 1-9 benzyl ester was determined previously, and the measurement by REDOR of a known interatomic distance allows both validation and a practical demonstration of the precision of REDOR. The ability to map precisely intermolecular distances suggests applications of REDOR in the solid, or aggregated state, for determinations of the conformations of ligand molecules in drug-receptor, inhibitor-enzyme, and antigen-antibody complexes.