title: Conformational constraints on the headgroup and sn-2 chain of bilayer DMPC from NMR dipolar couplings.
abstract: This paper presents new NMR constraints on the conformation of the headgroup, glycerol backbone, and sn-2 chain of 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) in the liquid-crystalline bilayer. Using two-dimensional 13C-1H chemical shift correlation spectroscopy, we find significant dipolar couplings between the carboxyl carbon CO2 and the headgroup protons. This indicates that a conformation in which the DMPC headgroup and the beginning of the sn-2 chain bend toward each other is significantly populated in the fluid bilayer. The predominance of this headgroup orientation can be further confirmed by 31P-13C dipolar couplings from the literature, which constrain the glycerol G2-G3 torsion angle to be close to trans, excluding a significant presence of one of the two conformations found in the DMPC crystal. Combining and reexamining 20 known NMR couplings for the glycerol backbone and its adjacent segments of L alpha-DMPC, we find that several torsion angles and bond orientations in the core of the DMPC molecule are constrained severely and must differ from those in the crystal structure. We propose a consistent molecular model for phosphocholine lipids in the liquid-crystalline phase, with a rigid backbone in the core of the molecule, a bent-back headgroup, and increasing mobility toward the ends of the acyl chains and the headgroup.
authors.affiliation: Materials Sciences Division, Lawrence Berkeley National Laboratory Berkeley, California 94720, USA.
authors.forename: M
authors.initials: M
authors.lastname: Hong
authors.forename: K
authors.initials: K
authors.lastname: Schmidt-Rohr
authors.forename: H
authors.initials: H
authors.lastname: Zimmermann
mesh_terms: D004134:Dimyristoylphosphatidylcholine / Q000737:chemistry*; D008051:Lipid Bilayers / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D008968:Molecular Conformation
chemical_list: D008051:Lipid Bilayers; D004134:Dimyristoylphosphatidylcholine