COMPARISON OF ROTATIONAL-ECHO DOUBLE-RESONANCE AND DOUBLE-CROSS-POLARIZATION NMR FOR DETECTION OF WEAK HETERONUCLEAR DIPOLAR COUPLING IN SOLIDS

Insect exoskeleton, multiply labeled with C-13 and N-15, was examined by rotational-echo double-resonance (REDOR) and double-cross-polarization (DCP) magic-angle spinning C-13 NMR. Low levels of incorporation of label make the analysis of these samples a practical test of the relative advantages of REDOR and DCP for the detection of weak, heteronuclear dipolar coupling between rare spins in solids. The sensitivity of REDOR for the detection of directly bonded C-13-N-15 pairs is an order of magnitude greater than that of DCP when neither label is involved in homonuclear dipolar coupling of strength comparable to the spinning frequency. However, if either of the C-13 or N-15 labels undergoes homonuclear spin flips, DCP gains in relative sensitivity and is easier to use for spin counting than REDOR.