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Biological Membranes
薛雅薇中央大學物理系
Cell Membrane
Two primary building blocks : - Protein - Lipid, or fat . Lipids form a bilayer.
The glycocalyx carbohydrate network on the outside surface: - responsible for cell–cell recognition and adhesion to other cells.
Membrane Structure (I)
1. Supporting and retaining the cytoplasm (inner cell)
2. Being a selective barrier: Get nutrients in and waste products out. non-polar molecules and some small polar molecules can cross. Most polar compounds, e.g. amino acids, must be specifically
transported across the membrane by proteins (Transport).
Functions of cell membranes
4. Recognition
3. Communication (signaling, via receptors)
Information Flow Through the Plasma Membrane via a Membrane Receptor
Structure and function are related.
Structure of Lipid MoleculeLipid molecule contains both hydrophilic
and hydrophobic regions.Polar headgroup : hydrophilic (i.e. water-loving)
Tail (or acyl chain): hydrophobic (i.e. water-fearing)
polar
non-polarH2C
H2C
H2C
H2C
CH2
CH2
CH2
CH2
CH2
CH2
CH2
CH3
H2C
H2C
H2C
H2C
H2C
H2C
H2C
O
O = C
H
C
O
H C
HO
CH2
O¯ P = O
O – CH2 – CH2 – N+(CH3)3
CH2
CH2
CH2
CH3
H2C
H2C
H2C
H2C
C = O
CH2
CH2
CH2
CH
CH
60:40 DPPC/Chol bilayers
Lipid motion: - Lateral - Rotational - nodding - …
In liquid crystalline phase
Lipid rafts are postulated to be very important in signal transduction in cells (as signaling platforms, for example).
Consists of specialized domains of different compositions and properties.
These specialized domains (called lipid rafts) are enriched in cholesterol (膽固醇 ) and sphingolipids (high melting lipids)
Size of domain: 0 – 10m
Membrane Structure (II)
It has found that many diseases, such as Alzheimer's, Huntington's, Parkinson's disease, AIDS..etc are related to lipid rafts.
Fluorescence image(1:1 brainPC/brainSPM)+25mol%Chol
Crane & Tamm 2004
Kruijff et al. 2001
AFM images(1:1 DOPC/SPM)+25mol%Chol
Two bilayer thicknesses
Nuclear magnetic resonance (NMR, 核磁共振 )
Differential scanning calorimetry (DSC)Fluorescence microscopyAtomic force microscopy (AFM, 原子力顯微鏡 )
Probing Membrane’s structure & dynamics
Use model membranes
Principles of NMR (Nuclear Magnetic Resonance)
H0 : Static Magnetic Field
H1(t): Alternating Field with frequency
For a bare spin in the presence of H0:
Hzeeman = nH0• I Em = nH0m
For 2H nuclear spin, I = 1
m = 1
0
1
L
= E, resonance occurs (absorption of energy)
L
m = 1
0
1
L
For a spin in the membranes
info: local environments, mol. dynamics
H = H zeeman + HQ
Solid-state NMR spectrometer (固態核磁共振儀 )
- Probing specific site by choosing proper L
- Sample in any form- Non-invasive
H 2 C
H 2 C
H 2 C
H 2 C
C D 2
C D 2
C D 2
C D 2
C D 2
C D 2
C D 2
C D 3
D 2 C
D 2 C
D 2 C
D 2 C
D 2 C
D 2 C
D 2 C
O
O = C
H
C
O
HC
H
O
CH 2
O ¯ P = O
O – CH 2 – CH 2 – N + (CH 3 ) 3
CH 2
CH 2
CH 2
CH 3
H 2C
H 2C
H 2C
H 2C
C = O
CH 2
CH 2
CH 2
CH
CH
Multi-component lipid mixtures, with one lipid labeled
x10
-3
-100 -50 0 50 100
kHz
ld
lo
lo
Characteristic 2H NMR spectra for liquid phases
Liquid-disordered(disordered chain)
Liquid-ordered(highly ordered chain)
Cholesterol-associated
Chemical composition, fraction of each domain, lipid-lipid & lipid-protein interactions, dynamics, mechanical properties …
-100 -50 0 50 100
kHz
17 oC
22 oC
25 oC
Inte
nsi
ty
gel + ld
gel
ld
Pure POPE-d31
Probing phase transition
Lattice melting & chain melting
Thank You
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