Phosphorus-31 chemical-shift tensors in barium diethyl phosphate and urea-phosphoric acid: model compounds for phospholipid head-group studies

The molecular orientations of the 31P chemical-shift tensors in the phosphodiester barium diethyl phosphate (BDEP) and in the nonesterified urea-H3PO4 (UPA) complex were determined from single-crystal studies. Although the tensors possess qualitatively similar orientations, there are differences between the two tensors of the order of 20 degrees. The principal values of the BDEP tensor are within 5% of those found in powder spectra of phospholipids, and, for this and other reasons, which are discussed in detail, we believe BDEP is a good model chemical-shift tensor for lipid phosphate. With the BDEP tensor alone, one can label certain head-group conformations as improbable, and, in conjunction with oriented bilayer spectra, one can determine the conformation of lipid head groups. In addition, we suggest a technique for orienting crystals based on the transformation properties of shift tensors, and we demonstrate this method by applying it to UPA. Finally, our results allow us to assess the effect of shift anisotropy relaxation on 31P liquid spectra at high fields.