Sign determination of dipolar couplings in field-oriented bicelles by variable angle sample spinning (VASS)

Residual dipolar couplings are being increasingly used as structural constr aints for NMR studies of biomolecules. A problem arises when dipolar coupli ng contributions are larger than scalar contributions for a given spin pair , as is commonly observed in solid state NMR studies, in that signs of dipo lar couplings cannot easily be determined. Here the sign ambiguities of dip olar couplings in field-oriented bicelles are resolved by variable angle sa mple spinning (VASS) techniques. The director behavior of field-oriented bi celles (DMPC/DHPC, DMPC/CHAPSO) in VASS is studied by P-31 NMR. A stable co nfiguration occurs when the spinning angle is smaller than the magic angle, 54.7 degrees, and the director (or bicelle normal) of the disks is mainly distributed in a plane perpendicular to the rotation axis. Since the dipola r couplings depend on how the bicelles are oriented with respect to the mag netic field, it is shown that the dipolar interaction can be scaled to the same order as the J-coupling by moving the spinning axis from 0 degrees tow ard 54.7 degrees. Thus the relative sign of dipolar and scalar couplings ca n be determined.