Laura Toppozini CAP Congress 2014 June 19, 2014 Cholesterol Structure in Lipid Rafts.
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Transcript of Laura Toppozini CAP Congress 2014 June 19, 2014 Cholesterol Structure in Lipid Rafts.
Laura ToppoziniCAP Congress 2014June 19, 2014
Cholesterol Structure in Lipid Rafts
Cholesterol in Lipid Membranes
Singer and Nicholson, 1972
Simons, 1997
Mosaic model:Lipids, cholesterol, and
proteins homogeneously mixed
Lipid raft model:Cholesterol-rich domains
What is cholesterol’s role in rafts?
Project Details
20,000- molecule simulations
• What is cholesterol’s role in lipid membranes?
• Can we see raft formation in a binary system?
• What can binary system rafts elucidate about rafts in biological membranes?
Neutron experiment at Canadian Neutron
Beam Centre
Coarse-grained MD simulations from collaborators at University of Mainz
Neutron diffraction with stacked,
oriented bilayers
L Toppozini, S Meinhardt, CL Armstrong, Z Yamani, N Kucerka, F Schmid, MC Rheinstadter. The Structure of Cholesterol in Lipid Rafts. Submitted, under review.
~40Å
20 mol%
~100Å
Liquid-ordered phase
Simulation: CG Molecular Dynamics
60 mol%
Liquid-ordered phase
Experiment: Neutron Scattering
supe
rres
olut
ion
{nm
ξ= λ2
∆ λ
Larger λ
Smaller λ
Cholesterol in Lipid Membranes
32.5% cholesterol Liquid–ordered phase
Selective Deuteration
Selectively highlight molecules
Cholesterol Structure
Cholesterol Structure
Liquid-DisorderedCholesterol-lipid
complexes
Umbrella model
Huang & Feigenson. Biophys J. 1999 76(4):2142-57
Cholesterol Structure
Cholesterol Plaques-Cholesterol bilayers/plaques
-Triclinic unit cella=b=12.8 Åα=95˚β=91.9˚γ=98.1˚
Ordered-Complexes-Ordered lipid-cholesterol complexes-Monoclinic unit cella=b= 11Åα= 131˚
Cholesterol Structures in Rafts
The liquid-ordered phase of a binary system is heterogeneous
Domains in binary systems may be nuclei which lead to rafts in biological membranes.
Small, (~10nm) transient (~100ns) structures are observed in lipid-cholesterol system
Observation of three coexisting cholesterol
structures in the lo phase:
Liquid disordered
Ordered lipid/cholesterol
Cholesterol plaques
Acknowledgements
Maikel C.RheinstadterClare ArmstrongNorbert KucerkaZahra YamaniSebastian MeinhardtFriederike Schmid
Lipid Structure
Liquid-disorderedfluid-like,
hexagonal lipid structure
Liquid-Ordered Rafts
Gel-like monoclinic lipid structure of
lipids in the rafts.
alipid-raft
blipid-raft
Simulations: changing chemical potential
Lipid/cholesterol phase diagram
Membranes, Rafts, and Scattering
~3000 bilayers/wafer give larger signals
Hydrated lipid bilayershave diffusive motion: signals are weak
• Typical neutron scattering setupgives large spatial averaging
• Gives bulk structure
• Are small structures being formed but averaged over?
Measurement of Rafts
neutron
Long coherence length
ξ
Short coherence length
ξ
ξ= λ2
∆ λ∝ √𝐸∆𝐸
wave package
ξξ
Condensation and the Umbrella Model
0Phospholipid head-group acts as umbrella, shields hydrophobic regions of cholesterol
0Cholesterol proximity suppresses tail fluctuations
0These complexes may form domains in lo phase at moderate cholesterol concentrations
Experiment: Neutron Scattering
0Change coherence length in-situ by changing wavelength
0Large ξ setup: averaging over large areas.
0Smaller ξ setup: integrates more efficiently over smaller structures