Seminar - Nanomechanics Lipid Bilayers AFM 2013-03-08
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Transcript of Seminar - Nanomechanics Lipid Bilayers AFM 2013-03-08
Nathaly Marín Medina03/08/13
Biophysics Seminar
Lipid bilayers
Membrane bilayer5 - 8 nm thick
Modified from: bioquimicafosfo.blogspot.com
ErythrocytePrinciples of Biochemistry - Lehninger
Lipid
Lipids
LIPIDS IN BIOMEMBRANE
• Semi-permeable barrier
• Fission/Fusion
• Tuning proteins’ function
• Gating of channels
• Chemical environment• Adjusting membrane
curvature
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
Atomic Force Microscope
Laser
Mirror
Cantilever
Quad PD
Atomic Force Microscope
Laser
Mirror
Cantilever
Quad PD
Atomic Force Microscope
Light source
Focusing optics
Piezo
PhotoDetector
Cantilever
Sample
Force spectroscopy with AFM
Indentation on a lipid bilayer
Interaction forces tip-phospholipids
Effect of ionic strength
DMPC bilayer
(a) Without NaCl + MgCL2
(b) With NaCl + MgCL2
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
Effect of length of tails
The longer the apolar chain, the higher the force required to indent the membrane
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
LA COPA DE VINO NOS ESPERA…
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
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.
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