seminar - nanomechanics lipid bilayers afm 2013-03-08
TRANSCRIPT
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Nathaly Marín Medina03/08/13
Biophysics Seminar
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Lipid bilayers
Membrane bilayer5 - 8 nm thick
Modified from: bioquimicafosfo.blogspot.com
ErythrocytePrinciples of Biochemistry - Lehninger
Lipid
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Lipids
LIPIDS IN BIOMEMBRANE
• Semi-permeable barrier
• Fission/Fusion
• Tuning proteins’ function
• Gating of channels
• Chemical environment• Adjusting membrane
curvature
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Studying mechanical properties of lipid bilayers
• Vesicles under stress• Micropipette aspiration• Quantitative study of elastic moduli
• Atomic Force Microscope (AFM)• Topology of lipid bilayers• Force spectroscopy mode
Mesoscopic outlook
Nanometer scale
Rawicz et al. Effect of Chain Length and Unsaturation of Elasticityof Lipid Bilayers. 2000 - Biophysical Journal
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Atomic Force Microscope
Laser
Mirror
Cantilever
Quad PD
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Atomic Force Microscope
Laser
Mirror
Cantilever
Quad PD
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Atomic Force Microscope
Light source
Focusing optics
Piezo
PhotoDetector
Cantilever
Sample
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Force spectroscopy with AFM
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Indentation on a lipid bilayer
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Interaction forces tip-phospholipids
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Effect of ionic strength
DMPC bilayer
(a) Without NaCl + MgCL2
(b) With NaCl + MgCL2
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Effect of temperature
Gel-fluid phase transition in a DMPC bilayer
Gel phase → solid orderedphase
Melting temperature: ~24°C
Fluid phase → Liquiddisordered phase
19°C
27.2°C
30.3°C
31.3°C
37.5°C
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Effect of length of tails
The longer the apolar chain, the higher the force required to indent the membrane
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Conclusions• AFM force spectroscopy
• New tool to explore the mechanical properties of lipid bilayers• Breakthrough force → molecular fingerprint• Nanometer and piconewton resolution• Bridge the gap with MD simulations
• Chemistry of the phospholipid headgroups• Effect on the mechanical stability of the membrane?
• Fingerprint the mechanical stability of a full cellularmembrane (very ambitious)• Complex mixture of phospholipids• Membrane proteins
Role of each individual phospolipidand protein on the mechanicalproperties of the membrane
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LA COPA DE VINO NOS ESPERA…
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Effect of variety of phospholipids in the bilayer
• Chianta et al. (2006)• Phase separation in a raft-exhibiting DOPC/SM/Chol mixture• Force required to indent the bilayer
Liquid ordered phase → 10.2 nNLiquid disordered phase → 6.5 nN
• Sullan et al. (2009)• DOPC/SM/Chol + ceramide → increases its mechanical stability
in both phases
• Picas et al. (2009)• POPE/POPG (3:1) – two different calcium-induced domains• Higher domains → higher mechanical stability (0.92 nN) → gel phase
Lower domains → lower mechanical stability (0.24 nN) → fluid phase
http://www.lanl.gov/science/1663/august2011/story3full.shtml
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Breakthrough force and friction interrelation• Grant and Tiberg (2002)
• Friction properties of DOPC• Resistance to normal loads → Efficient role as a lubricant
• Benz et al. (2004)• Friction properties of DPPE/DLPE• Single defects in lipid bilayers (AFM) → Stability of the bilayer (SFA)
• Trunfio-Sfarghiu et al. (2008)• Bilayers exhibiting a stronger mechanical resistance to indentation →
lower and more stable friction coefficients
• Oncins et al. (2005)• DMPC bilayer in NaCl buffer solution• The presence of Na+ cations induced structural changes in the bilayer• Three different friction regimes as the vertical force increased.
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Models of film rupture in lipid bilayers• Formation of a hole under the tip
• Continuum nucleation model• Distribution of forces to create a hole is connected to line tension• Free energy associated with the unsaturated bonds
• Molecular model• Each molecule has certain energetically favorable binding sites• Film pressed by an AFM tip → forming a hole is energetically ok
• These theories represent well the experimental data