Approaches to the complete determination of the rotation axes and flexibility in the methylene segments of phospholipids: deuterium, carbon-13, and proton NMR studies of dipalmitoyl phosphatidyl choline.

Deuterium NMR has been widely used to investigate flexibility in phospholipids. Usually, the measured deuterium quadrupolar couplings are directly related to segmental motion via a single Saupe order parameter. A more complete description avoiding assumptions concerning orientation and modes of motion can be obtained from five order parameters for a particular segment. Employing this approach, we present experimental NMR measurements using isotopic isomers of dipalmitoyl phosphatidyl choline in bilayer dispersions for the two stereospecific deuterium quadrupole couplings, the carbon-proton dipole couplings, and, preliminarily, the proton-proton dipole couplings, for the C-2 methylene segment of the sn-2 chain of dipalmitoyl phosphatidyl choline. In addition, we discuss the relation of these couplings to the significant order parameters, and calculate tentative orientations of the sn-2 alpha methylene segment.