NMR of phospholipids and membranebound systems
Michèle Auger
Department of Chemistry
Université Laval
Québec, Canada
USA-Canada Winter School on
Biomolecular Solid State NMR
January 20-25, 2008
Plasma membrane
‹
‹
‹
Site of important biological processes
Fluid mosaic model
More recently: "dynamic, yet structured” cell membrane model
Singer and Nicolson (1972) Science, 175, 720.
Vereb et al. (2003) PNAS, 100, 8053.
2
Structure of membrane proteins
‹
30% of all proteins are membrane proteins
‹
Important functions
‹
‹
–
Signal transduction
–
Stabilisation of membrane potential
–
Etc...
Less than 0.2% of resolved protein structures are
membrane proteins
Important targets for drug development
3
Strategy
‹
Strategy for the determination of protein structure by solid-state NMR
under MAS conditions
4
Luca et al. (2003) Acc. Chem. Res. 36, 858.
Oriented membranes
‹
Lipids mechanically oriented between glass plates
‹
Bicelle structures oriented in the magnetic field
n
B0
n
Eu3+
5
Helix orientation by
15N
NMR
Correlation between δppm and
orientation
6
Bechinger et al. (2003) Concepts Magn. Reson. 18A, 130.
Uniformly labeled samples
PISEMA spectra
Marassi (2002) Concepts Mag. Reson. 14, 212.
Marassi (2001) Biophys. J. 80, 994.
7
Outline
•
Introduction
•
MAS spectra of lipid bilayers
•
Static 2H and
–
–
–
–
–
–
–
–
–
The plasma membrane
Overview of methods to determine the structure of membrane proteins and peptides
Overview, NMR of lipid molecules
Phospholipids and lipid phases
31P, 1H
and 13C MAS spectra
31P
NMR
H NMR
• The quadrupolar interaction
• Effects of axially symmetric motions
• Lipid phases
• Molecular voltmeter
31
P NMR
• Effect of dynamics
• Vesicle morphology
• Order parameters
• Lipid phases
Application to the study of peptide-lipid interactions
Lipid dynamics
• Overview of motional time scales
• Study of lipid domains by 2D exchange 31P NMR
2
8
Outline
•
Oriented membranes
–
Lipids oriented between glass plates
•
•
–
Bicelles
•
•
•
•
•
•
Typical 31P NMR spectra
Effect of peptides on 31P and 2H spectra
Morphologies
Characteristic spectra
Order parameters
Effect of peptides on bicelle orientation
Fast-tumbling bicelles
Conclusions
9
NMR of lipid molecules
‹
MAS spectra
-
‹
Static powder spectra
-
‹
31P, 1H, 13C
Polar head group: 31P NMR
Acyl chains: 2H NMR
Dynamics
Oriented spectra
-
Lipids oriented between glass plates
Bicelles
10
Phospholipid head groups
11
Phospholipid acyl chains
DLPC
DMPC
DPPC
C12
C14
C16
C18:1
DOPC
Tm
-1°C
23°C
41°C
-20°C
12
Lipid phases
Micelles
Cubic phase
Bilayers
Normal or inverted
hexagonal phases
Bicelle
Oriented bilayers
Multilamellar vesicle
13
31P
MAS NMR
B0
54.7°
‹
‹
Sample
Shielding, deshielding effects
Dynamics, FWHM
80% DMPC
20% DMPG
3
2
1
0
-1
-2
Chemical shift (ppm)
-3
14
1H
MAS NMR spectra of lipid
bilayers
Static
MAS
15
Davis et al. (1995) Biophys. J. 69, 1917.
Effect of intermediate time scale
motions
•
Possible to obtain well-resolved 1H spectra from multilamellar lipid
dispersions at spinning speeds of 3 to 4 kHz
–
•
No identifiable peptide signals at spinning speeds of 3 to 4 kHz
–
–
•
Rapid axial diffusion of the phospholipid about its long axis
Peptide reorientation fast enough?
Yes, τc = 7 X 10-9 s for gramidicin A at 34°C
Additional intermediate time scale motions
–
–
–
Wobble of the diffusion axis with respect to the local bilayer normal
τc = 6 X 10-6 s for gramidicin A
Higher spinning speeds required
16
Solid-state 1H NMR spectra
D M PC
9
8
7
6
D M P C + gram icidin A (T F E )
9
10
8
8
7
6
6
4
2
0
C hem ical shift (ppm )
17
Bouchard et al. (1995) Biophys. J. 69, 1933.
13C
MAS NMR
18
Bouchard al.. (1998) Biochim. Biophys. Acta 1415, 181.
13C
‹
MAS NMR
Effect of gramicidin on DMPC dynamics, 1H T1ρ
19
Bouchard al.. (1998) Biochim. Biophys. Acta 1415, 181.
Why static deuterium NMR?
Static 1H spectrum
Multilamellar vesicles
20
Why static deuterium NMR?
1H
spectrum, MAS
Chemical shift (ppm)
13C
spectrum, MAS
Chemical shift (ppm)
Little information on the order of CH2 groups
21
Quadrupolar nuclei
Spin=1 (2H)
H0
H0 + HQ
m=-1
ν0 + Δν/2
ν0
m=0
ν0
H0
ν0 - Δν/2
m=1
Δν
ν0
ν0
22
Quadrupolar coupling
‹
‹
Nuclei with spins greater than 1/2
Interaction between the quadrupolar moment (eQ) and the electric field gradient at the
center of the nucleus
[(
)]
) (
3 e 2 qQ 1
νQ =
3 cos 2 θ − 1 + η sin 2 θ cos 2φ
2 h 2
eQ
1
HQ =
3 I Z2 − I 2 ) ( 3 cos2 θ − 1) + (η sin 2 θ cos 2φ )
(
4 I ( 2 I − 1)
2
[
‹
‹
‹
e2qQ/h= quadrupolar coupling constant = ∼ 167 kHz (∼ 3700 ppm at 7 T)
VZZ, VYY and VXX= principal components of electric field gradient tensor
If Vyy = Vxx
(
)
3 e 2 qQ 1
νQ =
3 cos 2 θ − 1
2 h 2
η =
]
(V
xx
− V yy
)
V ZZ
VZZ
θ
φ
23
Shape of the spectra
24
Acyl chains labeled with 2H
CD2 Plateau region
Terminal CD3
Quadrupolar splitting:
Δν Q =
3e 2 qQ ⎛ 3 cos 2 θ − 1 ⎞ 3 cos 2 β − 1
⎟⎟
⎜
2h ⎜⎝
2
2
⎠
3 cos 2 α − 1
2
↑ ΔνQ = order
↓ ΔνQ = disorder
30
Davis (1983) Biochim. Biophys. Acta 737, 117.
0
Frequency (kHz)
-30
25
Effect of axially symetric motions
n
α β
C- 2
H
Rotation and oscillation
n
β
C-2H
C-2H
100
0
-100 kHz
3e 2 qQ ⎛ 3 cos 2 θ − 1 ⎞ 3 cos 2 β − 1
⎟⎟
⎜
Δν Q =
2h ⎜⎝
2
2
⎠
3 cos 2 α − 1
2
Fast axial rotation
No motion
Angular brackets:
Time average
26
Typical order profile
35
Quadrupolar splitting (kHz)
30
n
25
20
α β
C- 2
H
15
10
■ DMPC
5
DMPC/4-n-hexadecyl CEU
DMPC/4-n-butyl CEU
0
0
2
4
6
8 10 12
Carbon position
14
3 ⎛ e 2 qQ ⎞
⎟ × SCD
Δν Q = ⎜⎜
4 ⎝ h ⎟⎠
16
3cos2 β −1 3cos2 α −1
SCD =
2
2
conformational orientational
27
Hexagonal phase
δ
Lafleur et al. (1990) Biochemistry 29, 8325.
3e 2 qQ ⎛ 3 cos 2 θ − 1 ⎞ 3 cos 2 β − 1
⎜
⎟⎟
Δν Q =
2h ⎜⎝
2
2
⎠
3 cos 2 α − 1
2
⎛ 3 cos 2 δ − 1 ⎞
⎜⎜
⎟⎟
2
⎝
⎠
28
Hexagonal phase
29
Lafleur et al. (1990) Biochemistry 29, 8325.
Gel phase
30
Asymmetry parameter
‹
Effect of the asymmetry parameter on the 2H NMR spectral
lineshape
V
(
η=
xx
− Vyy
VZZ
)
η = 1.0
η = 0.5
η = 0.2
η = 0.0
31
Molecular voltmeter
‹
Information on the conformational order of polar head groups
Bicellar system made with DMPC-d4
β
β
O
D
α
D
α
O ⎯ P ⎯O ⎯ C ⎯ C ⎯ N+(CH3)3
α
β
O-
D
D
4
3
2
1
0
-1
-2
-3
-4
Frequency (kHz)
‹
PC head group sensitive to the surface charge: molecular voltmeter
+ charge (+)
⇒ ↓ Δνα
⇒ ↑ Δνβ
+ charge (-)
⇒ ↑ Δνα
⇒ ↓ Δνβ
32
Seelig et al. (1987) Biochemistry, 26, 7535.
Molecular voltmeter
+ charge (+)
+ charge (-)
⇒ ↓ Δνα
⇒ ↑ Δνβ
⇒ ↑ Δνα
⇒ ↓ Δνβ
33
Marassi and Macdonald (1992) Biochemistry 31, 10031.
Phospholipids and
Natural abundance,
31P:
31P
NMR
100%
Spin: ½
DMPC
σ11 (80 ppm)
O
σ22 (25 ppm)
O - Polar head group
P
O
O - Glycerol
σ33 (-110 ppm)
34
31P
NMR
σ11 (80 ppm)
O
σ22 (25 ppm)
O- Polar head group
P
O
σ22
σ11
100
1
1 3
σ = (σ 11 + σ 22 + σ 33 ) + ∑ (3 cos 2 θ j − 1)σ jj
3
3 j =1
Isotropic
O- Glycerol
σ33 (-110 ppm)
σ33
0
σ⊥
σ//
Static
σ’⊥
σ’//
0
100
-100
Anisotropic
-50
Axial rotation
50
0
-50
Axial rotation / Oscillation
σ // = σ 11
σ⊥ =
(σ 22 + σ 33 )
2
35
31P
‹
NMR order parameters
Spectral width: dynamics / orientation of the polar head group
δ
S2 =
δ ref
Picard et al. (1999) Biophys. J. 77, 888.
Picard et al. (2000) Can. J. Anal. Sci. Spectrosc. 45, 72.
S2 = 1 → No change
S2 = 0 → Isotropic system
36
31P
NMR, vesicle morphology
a
c
c
r=
a
37
Picard et al. (1999) Biophys. J. 77, 888.
31P
‹
NMR order parameters
Morphology: degree of orientation of the system
S1 =
M 1 − δ iso
δ
Picard et al. (1999) Biophys. J. 77, 888.
Picard et al. (2000) Can. J. Anal. Sci. Spectrosc. 45, 72.
S1 = 1 → Lipid main axis // B0
S1 = - 0.5 → Lipid main axis ⊥ B0
38
S1 as a function of r
39
Picard et al. (1999) Biophys. J. 77, 888.
31P
‹
NMR, vesicle morphology
DMPC spectra
a
r=
c
c
a
40
Picard et al. (1999) Biophys. J. 77, 888.
31P
NMR, lipid phases
n
n
n
41
Novel peptides
O
O
O
BOC
H
N
O
N
H
H
N
O
O
O
O
O
N
H
H
N
O
O
N
H
Boc-(L-EC21-7-LLL-EC21-7-L)n-COOH
O
O
O
O
O
O
O
• Helicoidal structure
• Amphiphilic
• Neutral
O
H
N
O
OH
O
n
14-mer peptide
21-mer peptide
• ↑ vesicle permeability
• Monomolecular ion channel
• Monoprotected peptide (Boc, COOH)
slightly more active
• No effect on vesicle permeability
• Erythrocyte lysis
• Length: 31.5 Å
• Length: 21 Å
Biron et al. (2004) Bioorg. Med. Chem. 12, 1279.
Vandenburg et al. (2002) Chem. Comm. 16,1694.
Meillon et al. (1997) Angew. Chem. Int. Ed. Engl. 36, 967.
Biron et al. (2001) Biopolymers 55, 364.
42
2H
NMR results
—— Pure lipid
—— 60:1
—— 20:1
14-mer
SCD(P) (SCD(M))
Pure
+14-mer
+21-mer
+14-mer
+21-mer
(0.03) 0.21 (0.03)
0.20 (0.03) 0.20
0.20 (0.03) 0.20 (0.03)
21-mer
40
30
20
10
0
-10
Frequency (kHz)
Ouellet et al. (2006) Biophys. J. 90, 4071.
-20
-30
-40
43
31P
NMR results
—— Pure lipid
—— 60:1
—— 20:1
14-mer
21-mer
40
30
20
10
0
-10
Chemical shift (ppm)
S1
Pure
-0.17
-20
S2
+14-mer
+21-mer
-0.02
-0.06
-30
-0.01
-0.05
+14-mer
+21-mer
0.91
0.98
0.86
0.94
44
NMR and dynamics
45
NMR and dynamics
Rotational diffusion (10-8 s)
Vertical motion (10-9 s)
Trans-gauche
isomerizations (10-10 s)
Flip-flop (10-3-104 s)
Oscillation (10-12 s)
Lateral diffusion (10-7 s)
Ondulations (10-6-1 s)
46
Gawrisch (2005) in The Structure of Biological Membranes, CRC Press LLC, pp.147-171
Lateral heterogeneities or domains
Lipid-lipid interactions
‹
Lipid-protein interactions
Protein-protein interactions
Biological roles
­
­
­
Passive transport of small molecules across the membrane
Modulation of the activity of certain membrane proteins
Vesicle fusion
Kleinfeld (1987) Current Topics in Membranes and Transport, 29, 1.
Mouristen and Jorgensen (1995) Mol. Membr. Biol., 12, 15.
47
Lipid rafts
http://publications.nigms.nih.gov/insidethecell/images/ch2_lipidraft.jpg
48
Lateral heterogeneities or domains
‹
Mixture of two phospholipids: DMPC/DSPC
DMPC
n=14
Tm=23oC
DSPC
n=18
Tm=55oC
49
Phase diagram
‹
Phase diagram of the DMPC/DSPC mixture
Temperature (
o
C)
60
50
Fluid
→
40
Fluid+Gel
30
Gel
Partially miscible:
coexistence of gel and fluid
phases
20
0
0.2
0.4
0.6
0.8
1
XDSPC
Tm
Slow lateral diffusion
Fast lateral diffusion
50
Shimshick and McConnell (1973) Biochemistry, 12, 2351.
Adsorbed bilayer
‹
DMPC/DSPC (1:1) bilayer adsorbed on silica beads of well
defined radius (320 nm)
→
→
→
Well defined radius
Reduced membrane
deformations
No tumbling
51
Arnold et al. (2004) Biophys. J., 87, 2456.
2H
NMR spectra
DMPC
DSPC
7
1
55 °C
45 °C
8
5
6
4
3
4
2
2
M2 DSPCd70 (x108 s-2)
M2 DMPCd54 (x108 s-2)
6
1
0
0
Total gel fraction
37 °C
25 °C
15 °C
80 40
0 -40 -80
kHz
80 40
10
20
30
40
50
Temperature (oC)
60
1
0.75
0.5
0.25
0
0 -40 -80
kHz
5
15
25
35
45
Temperature (oC)
55
52
Exchange experiments
‹
2D experiments to correlate the resonance frequency of one
lipid at the beginning and at the end of a delay
ν1
tm
B0
Orientation change
ν2
↔
Frequency change
53
Exchange experiments
• Spectres typiques: t =0s
m
tm=3ms
tm=200μs
tm=12ms
54
Autocorrelation function
‹
“Memory function" of the system
C 2 (t m ) =
‹
5
δ2
∫∫ ν ν S (ν ,ν ; t )d ν d ν
∫∫ S (ν ,ν ; t )d ν d ν
1 2
1
1
2
2
1
m
m
1
2
2
Evolution in the presence of diffusion
⎛ tm ⎞
C 2(tm ) = exp⎜⎜ − ⎟⎟
⎝ td ⎠
R2
td =
6D
Schmidt-Rohr and Spiess (1994) Multidimensional Solid State NMR and Polymers, Academic Press, London.
Picard et al. (1998) Biophys. J., 74, 857.
55
Lateral diffusion in the presence of
obstacles
→ When tm increases, the number of obstacles increases and the
apparent diffusion coefficient decreases
→ Numerical simulations to quantify this effect
56
Random walk on a sphere in the
presence of obstacles
Change
in
percolation
Increasing
number of obstacles
=
Decreasing
size of obstacles
Gel
Fluid
57
Arnold et al. (2004) Biophys. J., 87, 2456.
Spectral simulations
Concentration of
point obstacles
Number (size) of discoidal
immobile domains
Number (size) of fluid
discoidal domains
-50
ppm
-25
0
25
C=0.0
R=500 nm
R=330 nm
C=0.3
R=60 nm
R=180 nm
C=0.6
R=20 nm
R=80 nm
50
-50
ppm
-25
0
25
50
-50
ppm
-25
0
25
50
50
25
0 -25 -50
ppm
50
25
0 -25 -50
ppm
Constant obstacle area fraction of 0.5
50
25
0 -25 58
-50
ppm
Simulations
‹
32 discoidal domains
Different total obstacle areas
0.0
-0.5
-1.0
ln(C2) (a.u.)
‹
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
-1.5
-2.0
-2.5
-3.0
0
2
4
6
8
texch (ms)
10
12
59
Simulations
‹
Different numbers of discoidal immobile domains
Constant obstacle fraction (0.5)
0.0
-0.5
-1.0
ln(C2) (a.u.)
‹
-1.5
no domain
1 domain
64 domains
512 domains
4096 domains
point domains
-2.0
-2.5
-3.0
0
2
4
6
8
texch (ms)
10
12
60
Simulations
‹
Different numbers of fluid discoidal domains
Constant obstacle fraction (0.5)
0.0
-0.5
-1.0
ln(C2) (a.u.)
‹
-1.5
no domain
1 domain
2 domains
4 domains
6 domains
32 domains
-2.0
-2.5
-3.0
0
2
4
6
8
texch (ms)
10
12
61
Simulations
Evolution of Drel as a function of:
‹
Total immobile fraction
Different numbers of domains
Domain radius
Different immobile fractions
1.0
0.9
0.9
Percolation properties
0.8
0.8
0.7
0.7
Drel
Drel
0.6
0.5
0.6
0.4
0.5
0.3
4 domains
32 domains
4096 domains
0.2
0.1
0.0
0.4
0.5
0.6
0.4
0.3
0
0.2
0.4
0.6
0.8
Immobile fraction
1
0
100
200 300 400 500
Domain radius (nm)
600
62
Fit of the experimental spectra (30°C)
Experimental
spectra
32 discoidal obstacles
Diameter: 140 nm
2 confined fluid areas
Diameter: 760 nm
-50
ppm
-25
0
500 μs
25
50
-50
ppm
-25
0
3 ms
25
50
-50
ppm
-25
0
12 ms
25
50
50
25
0 -25 -50
ppm
50
25
0 -25 -50
ppm
50
25
63
0 -25 -50
ppm
Comparison with other techniques
Technique
Domain phase
Diameter
EPR1
fluid
20 nm
NMR (2H)2
gel
>200 nm
Neutron3
gel
10 nm
Monte Carlo4
Gel
>200 nm
FRAP5
Fluid
80 nm
NMR (31P)
Gel
140 nm
1. Sankaram et al. (1992) Biophys. J., 63, 340.
2. Dolainsky et al. (1997) Phys. Rev. E, 55, 4512.
3. Gliss et al. (1998) Biophys. J., 74, 2443.
4. Jorgensen et al. (1995) Biophys. J., 95, 942.
5. Almeida et al. (1992) Biochemistry, 31, 7198.
64
Fit of the autocorrelation
functions
0
Disk obstacles
-0.1
ln (C2) (a.u.)
Disconnected fluid domains
-0.2
Each curve fitted with a fast
and a slow diffusing components
-0.3
-0.4
-0.5
0.5
24°C
30°C
37°C
2.5
4.5
6.5
8.5
texch (ms)
Arnold et al. (2004) Biophys. J., 87, 2456.
10.5
12.5
65
Oriented membranes
‹
Lipids mechanically oriented between glass plates
‹
Bicelle structures oriented in the magnetic field
n
B0
n
Eu3+
66
Bilayers oriented between glass
plates
67
Bilayers oriented between glass
plates
Parallel
orientation
Perpendicular
orientation
B0
0o
90o
B0
40
20
0
-20 ppm
δppm = ƒ (Lipid orientation)
‹
‹
‹
Chemical shift (δppm)
Full width half maximum (FWHMppm)
Powder spectra
1° Head group conformation
2° Bilayer alignment
68
Effect on membrane alignment
Pure lipid
With 14-mer
DLPC
50 40
DMPC
30 20 10 0 -10 -20 -30
δ31P (ppm)
50 40
δ (ppm)
DLPC
DMPC
DPPC
Pure
31.2
31.1
32.0
14-mer
29.7
30.1
30.1
DPPC
30 20 10 0 -10 -20 -30
δ31P (ppm)
FWHM (ppm)
Pure
14-mer
1.9
2.3
1.7
1.9
1.5
3.6
50 40
30 20 10 0 -10 -20 -30
δ31P (ppm)
% Loss of alignment
-15 %
-17 %
-36 %
⇒ Orientation distribution → Effect more pronounced for DPPC
Ouellet et al. (2007) Biochemistry, 46, 6597.
69
2H
NMR spectra
Pure lipid
With 14-mer
0° orientation
90° orientation
DMPC
40
20
0
kHz
DMPC
DPPC
-20
↑ ∆νQ: Order
-40
40
20
0
kHz
-20
-40
40
20
0
-20
kHz
DPPC
-40 40
20
0
-20
kHz
↓ ∆νQ: Disorder
Ouellet et al. (2007) Biochemistry, 46, 6597.
Mani et al. (2004) Biochemistry 43, 13839.
70
-40
Bicelles
‹
‹
Mixture of long chain (DMPC)
DMPC and short chain
(DHPC)
DHPC lipids:
­
Discoidal vesicles
­
Perforated lamellae
­
Ribbons
Spontaneous orientation between 30 and 45°C
­
‹
‹
B0
Study of proteins at physiological temperatures
n
Planar section mimics the surface of the cells
­
Vs curved surface of micelles
­
Preservation of enzymatic activity
Variable size
­
Change in the molar ratio q
Sanders and Landis (1995) Biochemistry, 34, 4030.
Czerski and Sanders (2000) Anal. Biochem., 284, 327.
Raffard et al. (2000) Langmuir, 16, 7655.
q = [long chain lipids]
[short chain lipids]
71
Characteristic spectra, oriented
bicelles
‹
31P
NMR
­
Phospholipid head group
90o
30 20 10 0 -10 -20-30
Chemical shift (ppm)
‹
2H
­
Terminal CD3
NMR
Deuterated acyl chains
CD2 near the polar
head group
↓ ΔνQ = disorder
↑ ΔνQ = order
30
0
Frequency (kHz)
-30
72
Bicelle spectra as a function of
temperature
73
Picard et al. (1999) Biophys. J. 77, 888.
Order parameters
74
Picard et al. (1999) Biophys. J. 77, 888.
Bicelle phase diagrams
‹
Temperature-composition diagrams of DMPC/DHPC
80% (w/w) D2O 100 mM KCl
80% (w/w) D20
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Raffard et al. (2000) Langmuir, 16, 7655.
Mixed bicelles
ε = mole fraction of DHPC
in the bilayer section
76
Triba et al. (2005) Biophys. J. 88, 1887.
Bicelle morphologies
Isotropic bicelles
B0
+La3+
Θ=0°
+Sample
spinning
Θ=54.74°
Θ=90°
77
Zandomeneghi et al. (2003) J. Biomol. NMR 25, 113.
Bicelles flipped with lanthanides
78
Angelis and Opella (2007) Nature Protocols, 2, 2332.
Effect of gramidicin on bicelle
alignment (lipid:peptide 25:1)
79
Marcotte et al. (2006) Chem. Phys. Lipids 139, 137.
Effect of gramidicin on bicelle
alignment (lipid:peptide 10:1)
80
Marcotte et al. (2006) Chem. Phys. Lipids 139, 137.
Fast tumbling bicelles
‹
Fast tumbling bicelles
­
Small (∼ 100 Å)
­
Discoidal morphology
­
Fast rotation motion in solution
­
Isotropic peaks in NMR
Luchette et al. (1997) Biochim. Biophys. Acta 1513, 83.
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Conclusions
‹
‹
Solid-state NMR is well suited to investigate the effects of
membrane peptides and proteins on lipid bilayers
Several techniques available
– MAS
– Static spectra
– Oriented samples
‹
Understanding the effects of membrane peptides and proteins
on lipid bilayers can provide useful and complementary
information of the biological process investigated
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