The quadrupolar spectrum of a spin I = 1 in a lipid bilayer in the presence of paramagnetic ions

The NMR signal from selectively deuterated molecules in a lipid bilayer where there are paramagnetic ions present in the aqueous region is influenced by both the nuclear quadrupole interaction and the dipolar interaction between the deuterium nuclei and the surrounding ions. The quadrupolar split powder pattern is no longer symmetric about the center of the spectrum. The spectra which result from the use of the quadrupole echo pulse sequence are quite complex since this sequence does not refocus the dephasing due to the dipolar interaction. A new pulse sequence which refocuses both the dipolar and quadrupolar interactions is suggested. Both ²H and ¹⁴N spectra from lipid molecules with the phosphatidylcholine headgroup obtained with the conventional quadrupole echo sequence and the new sequence are compared with predictions of density matrix theory calculations. There is excellent agreement between the experimental and simulated spectra.