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