Pietro Cicuta Mechanics, Dynamics and Thermodynamics of phospholipid membranes Cavendish Laboratory,...
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Pietro Cicuta Mechanics, Dynamics and Thermodynamics of phospholipid membranes Cavendish Laboratory, University of Cambridge z Institute, Leiden. 26 - 30 September 2011
Pietro Cicuta Mechanics, Dynamics and Thermodynamics of phospholipid membranes Cavendish Laboratory, University of Cambridge Lorentz Institute, Leiden
Pietro Cicuta Mechanics, Dynamics and Thermodynamics of
phospholipid membranes Cavendish Laboratory, University of
Cambridge Lorentz Institute, Leiden. 26 - 30 September 2011
Slide 2
Prof Sarah Veatch, Michigan Univ. and Prof Sarah Keller, Univ.
Washington, Seattle. Background: Phase behavior of phospholipid
membranes Lipid rafts, signalling and transport in cells
Slide 3
Electro-Formation of Giant Uni-Lamellar Vesicles (GUV) ITO
Glass plates, 45 oven, AC field 1V, 10Hz Two ITO coated slides form
a capacitor. GUV grow over a few hours when an AC electric field is
applied.
Slide 4
Image analysis and feature tracking through a movie 1:1
DOPC:DPPC + 30% cholesterol 10 m
Slide 5
Image analysis and feature tracking through a movie 1:1
DOPC:DPPC + 30% cholesterol 10C 20C 40 m How can we relate the mean
square displacement to the membrane (2D) viscostity ?
Stokes-Einstein makes it trivial in 3D, by D(r)= kT/(6 r). =2 D(r)
t P. Cicuta, S.L. Keller and S.L. Veatch, J. Phys. Chem. B 111
(2007) 3328-3331
Slide 6
3D sphere: D(r)= kT/(6 r) . But a 2D domain in a membrane is
clearly not Stokes flow of a sphere. . Neither is it just membrane
flow around a cylinder. x y z h membrane above and below there is
water w Saffman and Delbruck in 1975 calculated the flow for this
case: Note the very weak dependence on r
Slide 7
D(r) dependence on size large r (or low viscosity) Hughes limit
D0D0
Slide 8
D 0 dependence on temperature P. Cicuta, S.L. Keller and S.L.
Veatch, J. Phys. Chem. B 111 (2007) 3328-3331
Slide 9
Slide 10
Line tension of domains near critical point
Slide 11
Capillary spectrum of fluctuations = 0 [ (T c -T) / T ] With =1
as in the 2d Ising model
Slide 12
Ising critical behavior also from above T c Biophysical Journal
95, 236 (2008)
Slide 13
Ising critical behavior also from above T c TcTc T Rafts ??
Biophysical Journal 95, 236 (2008)
Slide 14
Same critical behavior also in cell blebs ACS CHEMICAL BIOLOGY
3, 287 (2008) Vesicles isolated from the plasma membranes of living
rat basophilic leukemia (RBL-2H3) mast cells and other cell types
also display critical behavior.
Slide 15
Fundamental interest In lipid vesicles, fluctuations are huge!
Can be observed by light microscopy within 0.5C of Tc.
Extrapolating from our data we expect fluctuations with correlation
lengths of 50 nm to occur between 2C8C above their critical
temperature. In plasma membranes of unstimulated cells, no
micrometer-scale domains are observed by fluorescence microscopy at
the cells growth temperature. Therefore, domains or composition
fluctuations must be submicrometer in dimension if they are
present. Submicrometer differences in membrane composition may
confer advantages for cell processes. Dynamic, small-scale membrane
heterogeneities could result from critical fluctuations near a
critical temperature, rather than small domains far below Tc that
are prevented from coalescing. Here we have shown that it is
possible to tune domain size (and line tension) by changing the
membranes proximity to a miscibility critical point. Relevance to
Biology
Slide 16
Slide 17
Julicher and Lipowsky (1992, 1996) = 0 is a sphere. For x 0.5:
formation of bud around = 3.1, and budding off at = 4.4 Reduced
line tension Area fraction The (strange) vesicle shape This
calculation is with the assumption of free volume. + line tension
shown before All vesicles would bud if volume could equilibrate.
J.Phys.Cond.Mat 22, 062101 (2010) See also:Semrau S, Idema T,
Holtzer L, Schmidt T and Storm C Phys. Rev. Lett. 100 088101
(2008)
Slide 18
Optical Tweezers (1/3) choice of fast CMOS or sensitive CCD
camera mirror tube lens dichroic X and Y axis AOD beamsplitter
monitor power 1064nm Yitterbium fiber laser fiber white light lamp
condenser Sample cell Motorised sample stage Motorised z-focus
Bright LED Custom electronics Custom software 60x water immersion
objective mirror U(x)=1/2 k trap x 2 x U Typical k trap = 5 pN/
m
Slide 19
Acousto Optical Deflectors Inverted microscope (x63 Water
immersion) CCD Camera CMOS Camera 1064nm 1.1W Laser Tweezers
controller Optical Tweezers (2/3)
Slide 20
Optical Tweezers (3/3)
Slide 21
Mechanical Properties of Red Blood Cells Soft Matter 7, 2042
(2011) Medical and Biological Engineering and Computing 48,
1055-1063 (2010) Optics Express 18, 7076 (2010) Biophysical Journal
97, 16061615 (2009) Physical Biology 5, 036007 (2008)
Slide 22
Actively deforming a giant vesicle Driving mode 2, and
observing its amplitude Active rheology of phospholipid vesicles
Phys. Rev. E 84, 021930 (2011)
Slide 23
What are the fits ? First the parameters and are tted to the
phase, and then the stiffness is determined from the amplitude.
Fitting gives: = 1.2 10 8 N m 1 = 19 k B T. The value of varies
with mode number High frequency 1/f asymptotic modes 2,3,4 mode 2
Response, and mechanical properties
Slide 24
Theoretical framework of membrane mechanics Helfrich (1972):
For small deviations around a sphere: Where U lm is the
displacement, decomposed onto spherical harmonics Y l m Applying
equipartition theorem, and projecting on equator plane, gives the
mean amplitude of fluctuations for each equatorial mode: Where h m
is the F.T. of the equatorial displacement h( )
Slide 25
Extending the theory to actively driven modes Eq. of motion of
an eigenmode: Trap pos.: Gives force: Combining the above, and in
frequency domain: The response function: a fancy driven damped
harmonic oscillator M. A. Peterson, Mol. Cryst. Liq. Cryst. 127,
257 (1985)
Slide 26
Why drive a system actively? The intrinsic spectrum of
fluctuations contains thermal and any non- thermal motion; The
response to external drive isolates the material properties. Allows
to verify presence of non-thermal sources of fluctuation (e.g. ion
pumps molecular motors, chemical energy in general)
Slide 27
Thank you In Washington and Michigan Universities Prof Sarah
Veatch, Prof Sarah Keller and Dr Aurelia Honerkamp Smith In
Cambridge University Experiments: Dr Aidan Brown and Dr Young Zoon
Yoon Optical Trap: Dr Jurij Kotar Funding: EPSRC, KAIST-Cavendish
programmes (MoST and KICOS), Nanotechnology IRC, Oppenheimer Fund,
Royal Society, MRC, HFSP. Acknowledgements