Phase separation in binary mixtures of bipolar and monopolar lipid dispersions revealed by 2H NMR spectroscopy, small angle x-ray scattering, and molecular theory.
Binary mixtures of C(20)BAS and POPC membranes were studied by solid-state (2)H NMR spectroscopy and small angle x-ray scattering (SAXS) over a wide range of concentrations and at different temperatures. Three specifically deuterated C(20)BAS derivatives--[1',1',20',20'-(2)H(4)]C(20)BAS, [2',2',19',19'-(2)H(4)]C(20)BAS, and [10',11'-(2)H(2)]C(20)BAS--combined with protiated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), as well as membranes containing POPC-d(31) and fully protiated bolalipid, were used in NMR experiments to obtain structural information for the mixtures. The (2)H NMR spectra of [10',11'-(2)H(2)]C(20)BAS/POPC membrane dispersions reveal that the bolalipid is predominantly in the transmembrane conformation at high bolalipid concentrations (100, 90, and 70 mol %). At < or =50 mol % C(20)BAS, smaller quadrupolar couplings appear in the spectra, indicating the presence of U-shaped conformers. The proportion of U-shaped bolalipids increases as the amount of POPC in the membrane increases; however, the transmembrane component remains the dominant bolalipid conformation in the membrane even at 45 degrees C and 10 mol % C(20)BAS, where it accounts for approximately 50% of the bolalipid population. The large fraction of C(20)BAS transmembrane conformers, regardless of the C(20)BAS/POPC ratio, together with the findings from molecular mean-field theory calculations, suggests the coexistence of phase-separated bolalipid-rich domains and POPC-rich domains. A single lamellar repeat distance was observed in SAXS experiments corresponding to the average repeat spacing expected for C(20)BAS- and POPC-rich domains. These observations are consistent with the presence of microphase-separated domains in the mixed membrane samples that arise from POPC-C(20)BAS hydrophobic mismatch.
Brownholland
David P
DP
Department of Chemistry, Purdue University, West Lafayette, Indiana, USA.
Longo
Gabriel S
GS
Struts
Andrey V
AV
Justice
Matthew J
MJ
Szleifer
Igal
I
Petrache
Horia I
HI
Brown
Michael F
MF
Thompson
David H
DH
D003903:Deuterium; D008051:Lipid Bilayers / Q000737:chemistry; D008956:Models, Chemical; D019906:Nuclear Magnetic Resonance, Biomolecular; D010713:Phosphatidylcholines / Q000737:chemistry*; D010742:Phospholipid Ethers / Q000737:chemistry*; D053838:Scattering, Small Angle; D013696:Temperature; D014961:X-Ray Diffraction
C480343:2,2'-di-O-decyl-3,3'-O-1,20-eicosanyl-bis-rac-glycero-1,1'-diphosphocholine; D008051:Lipid Bilayers; D010713:Phosphatidylcholines; D010742:Phospholipid Ethers; D003903:Deuterium; C028694:1-palmitoyl-2-oleoylphosphatidylcholine