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

Abstract

Solid-state phosphorus ( 31P) and deuterium ( 2H) nuclear magnetic resonance (NMR) spectroscopy over the temperature range of 25–50°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 31P-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 31P-NMR resonance signals. The 2H-NMR spectra demonstrate that the quadrupolar splittings (Δν) 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 α-carbon are opposite in sense to those for deuterons on the β-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.