Adiabatic-passage cross polarization in N-15 NMR spectroscopy of peptides weakly associated to phospholipids: Determination of large RDC

Structural information can be extracted from one-bond residual dipolar couplings (RDC) measured in NMR spectra of systems in field-ordered media. RDC can be on the order of J-couplings if the anisotropy of alignment is ∼ 10-2, 10-fold stronger than that typically used for structural studies of water-soluble proteins. In such systems the performance of 1H→15N polarization transfer methods of the INEPT type is not satisfactory. In this study we show the effectiveness of adiabatic-passage cross-polarization (APCP) in transferring the 1H→15N polarization in the bicelle-associated peptide Leucine Enkephalin (Lenk). APCP is efficient both in static samples and in samples spun at the magic angle (MAS) or any other angle of the spinning axis to the magnetic field (variable-angle spinning, VAS). The anisotropic spectrum of an aligned static sample and the isotropic spectrum of the sample under MAS provide a set of possible values for the 1H-15N RDC of phospholipid-associated Lenk. The unambiguous determination of the 1H-15N RDC was accomplished by means of VAS experiments