Surface charge response of the phosphatidylcholine head group in bilayered micelles from phosphorus and deuterium nuclear magnetic resonance

Solid-state phosphorus (P-31) and deuterium (H-2) nuclear magnetic resonance (NMR) spectroscopy over the temperature range of 25-50 degrees C were used to investigate bilayered micelles (bicelles) composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) in the presence of either the anionic lipid 1,2-dimyristoyl-sn-3-phosphoglycerol (DMPG) or the cationic lipid 1,2-dimyristoyl-3-trimethylammonium-propane (DMTAP). The P-31-NMR spectra demonstrate that bicellar structures form with DMPG/DMPC ratios ranging from 0 to 50/50 and with DMTAP/DMPC ratios from 0 to 40/60, while the overall concentration of DHPC remains constant. The formation of bicelles containing charged amphiphiles is contingent upon the presence of NaCl, with 50 mM NaCl being sufficient for bicelle formation at all concentrations of charged amphiphile investigated, while 150 mM NaCl affords better resolution of the various P-31-NMR resonance signals. The H-2-NMR spectra demonstrate that the quadrupolar splittings (Delta v) of head group-deuterated DMPC change inversely as a function of the amount of negative versus positive charge present, and that the changes for deuterons on the ex-carbon are opposite in sense to those for deuterons on the beta-carbon. This indicates that head group-deuterated phosphatidylcholine functions as a molecular voltmeter in bicelles in much the same fashion as it does in spherical vesicles. (C) 1999 Elsevier Science B.V. All rights reserved.