Coupling Amplification in 2D MAS NMR and Its Application to Torsion Angle Determination in Peptides

A technique for amplifying the apparent magnitudes of 13 C‐ 1 H v r /2 pud d/ 3, where dd is the dipolar coupling strength, the and 15 N‐ 1 H dipolar interactions in magic-angle spinning experi- dipolar spectrum is dominated by an uninformative cenments is described. By inserting rotor-synchronized 1807 pulses in terband. For a one-bond N ‐ H dipolar coupling measured the evolution period of a 2D dipolar-chemical shift experiment, with MREV-8 homonuclear decoupling, dd E 4.5 kHz and heteronuclear dipolar couplings are doubled or quadrupled rela- the centerband dominates for spinning speeds as low as vr / tive to the spinning speed. The increased number of dipolar side- 2pE 1.5 kHz. While low spinning speeds are desirable for bands is desirable for retaining structural information in the indi- extracting structural information, the necessity of resolving rectly detected dipolar dimension while resolving inequivalent sites different chemical moieties through isotropic chemical shifts in the isotropic chemical shift dimension at relatively high spinresults in an opposing constraint. Higher spinning speeds ning speeds. This coupling amplification method is incorporated into an experiment that determines the peptide torsion angle f average the chemical shift anisotropy (dCSA) more comthrough the relative orientation of the C a ‐H a and N‐H N bonds. pletely and thus yield better-resolved spectra. This conflict It is shown both experimentally and theoretically that the angular is exacerbated at higher magnetic field strengths, since shift resolution of the measurement is enhanced significantly by the anisotropies scale linearly with the field strength while the selective doubling of the N‐H N coupling. q 1997 Academic Press dipolar couplings are field-independent. Thus, the desirable