TY - JOUR
T1 - Cholesterol supports headgroup superlattice domain formation in fluid phospholipid/cholesterol bilayers
AU - Cannon, Brian
AU - Lewis, Anthony
AU - Metze, Jennifer
AU - Thiagarajan, Visveswaran
AU - Vaughn, Mark W.
AU - Somerharju, Pentti
AU - Virtanen, Jorma
AU - Huang, Juyang
AU - Cheng, Kwan Hon
PY - 2006/3/30
Y1 - 2006/3/30
N2 - Fluorescence and Fourier transform infrared (FTIR) spectroscopic techniques were used to explore the effect of added cholesterol on the composition-dependent formation of putative phospholipid headgroup superlattices in fluid 1-palmitoyl-2-oleoyl-phosphatidylethanolamine/1-palmitoyl-2-oleoyl- phosphatidylcholine/cholesterol (POPE/POPC/CHOL) bilayers. Steady-state fluorescence anisotropy measurements of diphenylhexatriene (DPH) chain-labeled phosphatidylcholine (DPH-PC) revealed significant dips at several POPE-to-phospholipid mole fractions (XPE'S) when the cholesterol-to-lipid mole fraction (XCHOL) was fixed at 0.00, 0.35, 0.40, and 0.50. Most of the observed dips occur at or close to critical X PE's predicted by the Headgroup Superlattice (SL) model, suggesting that phospholipid headgroups of different structures tend to adopt regular distributions even in the presence of cholesterol. Time-resolved fluorescence anisotropy measurements revealed that DPH-PC senses a disordered and highly mobile microenvironment in the POPE/POPC/CHOL bilayers at those critical X PE's, indicating that this probe may partition to defect regions in the bilayers. The presence of coexisting packing defect regions and regularly distributed SL domains is a key feature predicted by the Headgroup SL model. Importantly, probe-free FTIR measurements of acyl chain C-H, interfacial carbonyl, and headgroup phosphate stretching peak frequencies revealed the presence of abrupt changes at XPE's close to those observed in the fluorescence data. When XPE was varied from 0.60 to 0.72 and X CHOL from 0.34 to 0.46, a clear dip at the lipid composition coordinates (XPE, XCHOL) ≈ (0.68, 0.40) was observed in the three-dimensional surface plots of DPH-PC anisotropy as well as the carbonyl and phosphate stretching frequencies. The critical XCHOL at 0.40 agrees with the Cholesterol SL model, which assumes that cholesterol and phospholipid form SL domains at the lipid acyl chain level. In conclusion, this study provides evidence that cholesterol supports formation of phospholipid headgroup SLs in fluid state ternary lipid bilayers. The feasibility of the parallel existence of SLs at the lipid headgroup and acyl chain levels supports the relevance of the lipid SL model for the membranes of eukaryotic cells that typically contain significant amounts of cholesterol. We speculate that lipid SL formation may play a central role in the regulation of membrane lipid compositions, maintenance of organelle boundaries, and other crucial phenomena in those cells.
AB - Fluorescence and Fourier transform infrared (FTIR) spectroscopic techniques were used to explore the effect of added cholesterol on the composition-dependent formation of putative phospholipid headgroup superlattices in fluid 1-palmitoyl-2-oleoyl-phosphatidylethanolamine/1-palmitoyl-2-oleoyl- phosphatidylcholine/cholesterol (POPE/POPC/CHOL) bilayers. Steady-state fluorescence anisotropy measurements of diphenylhexatriene (DPH) chain-labeled phosphatidylcholine (DPH-PC) revealed significant dips at several POPE-to-phospholipid mole fractions (XPE'S) when the cholesterol-to-lipid mole fraction (XCHOL) was fixed at 0.00, 0.35, 0.40, and 0.50. Most of the observed dips occur at or close to critical X PE's predicted by the Headgroup Superlattice (SL) model, suggesting that phospholipid headgroups of different structures tend to adopt regular distributions even in the presence of cholesterol. Time-resolved fluorescence anisotropy measurements revealed that DPH-PC senses a disordered and highly mobile microenvironment in the POPE/POPC/CHOL bilayers at those critical X PE's, indicating that this probe may partition to defect regions in the bilayers. The presence of coexisting packing defect regions and regularly distributed SL domains is a key feature predicted by the Headgroup SL model. Importantly, probe-free FTIR measurements of acyl chain C-H, interfacial carbonyl, and headgroup phosphate stretching peak frequencies revealed the presence of abrupt changes at XPE's close to those observed in the fluorescence data. When XPE was varied from 0.60 to 0.72 and X CHOL from 0.34 to 0.46, a clear dip at the lipid composition coordinates (XPE, XCHOL) ≈ (0.68, 0.40) was observed in the three-dimensional surface plots of DPH-PC anisotropy as well as the carbonyl and phosphate stretching frequencies. The critical XCHOL at 0.40 agrees with the Cholesterol SL model, which assumes that cholesterol and phospholipid form SL domains at the lipid acyl chain level. In conclusion, this study provides evidence that cholesterol supports formation of phospholipid headgroup SLs in fluid state ternary lipid bilayers. The feasibility of the parallel existence of SLs at the lipid headgroup and acyl chain levels supports the relevance of the lipid SL model for the membranes of eukaryotic cells that typically contain significant amounts of cholesterol. We speculate that lipid SL formation may play a central role in the regulation of membrane lipid compositions, maintenance of organelle boundaries, and other crucial phenomena in those cells.
UR - http://www.scopus.com/inward/record.url?scp=33645811118&partnerID=8YFLogxK
U2 - 10.1021/jp0558371
DO - 10.1021/jp0558371
M3 - Article
C2 - 16553452
AN - SCOPUS:33645811118
SN - 1520-6106
VL - 110
SP - 6339
EP - 6350
JO - Journal of Physical Chemistry B
JF - Journal of Physical Chemistry B
IS - 12
ER -