THE ORDER OF PHOSPHATIDYLCHOLINE HEADGROUP IN SONICATED CHOLESTEROL PHOSPHOLIPID-VESICLES AS REVEALED BY 1H-NMR

H-1-NMR spectra of sonicated cholesterol/lysophosphatidylcholine and cholesterol/phosphatidylcholine vesicles have been obtained at 400 MHz. The peaks of the choline N-methyl protons are split at 28-degrees-C, showing a broad and a sharp component, for the vesicles with diameters from 220 angstrom to 320 angstrom. By the usage of NMR shift reagent, the broad and sharp components are assigned, respectively, to the choline N-methyl protons in the inner and outer monolayers of the bilayer. In addition, the linewidth of the broad component varies as a function of vesicle size, but the linewidth of the sharp component remains relatively constant. This new observation together with previous H-1- and F-19-NMR studies on the asymmetric packing arrangement of the hydrocarbon chains in the two monolayers of the bilayer (Schuh et al., 1982, Biochim, Biophys. Acta 687, 219-225; Wu et al., 1985, Biochemistry 24, 7153-7161) provide an explanation for previous controversy about the NMR linewidth of small vesicles. On the other hand, the choline head-group in the inner and outer monolayers of cholesterol/phospholipid small vesicles must exist in markedly different environments as reflected by the different motional states of phospholipids in the two monolayers.