Amphiphiles Copyright Stuart Lindsay 2008 Hydrophobic tail Polar head Phospholipid.

Amphiphiles

Copyright Stuart Lindsay 2008

Hydrophobic tailHydrophobic tail

Polar headPolar headPhospholipidPhospholipid

Self-assembled amphiphilic structuresSelf-assembled amphiphilic structures

Copyright Stuart Lindsay 2008(From Molecular Cell Biology, 4th ed. By H. Lodish, A. Berk, S.L. Zipursky, P. Matsudara, D. Baltimore, J. Darnell. © 2000, W.H. Freeman and Company. Used with permission)

MD Simulation of vesicle formationMD Simulation of vesicle formation


(Reprinted with permission from Molecular dynamics simulation of the spontaneous formation of a small DPPC vesicle in water in atomistic detail, A.H de Vries et al., A.E. Mark, and S.J. Marrink, J. Am. Chem. Soc. 2004 126: 4488. Published 2006 by American Chemical Society)

Association KineticsAssociation Kinetics

NA Xkv 11

Single-step aggregation

N/Xkv ND 2

To dissociate into N monomers divide aggregate concentration by N to get monomer equivalent

NN

NX

X

k

kK

12

1

In equilibrium the two rates are equal so the equilibrium constant is

N

XnX n1

Association rate Dissociation rate

110

1 XlnRT)X()X(

N

XlnRT

N

X

N

X NNN 0

)X(NN

XKlnRTG N

1000

)X(NN

XN

N 00

)X()X(Tk

Nexp

Tk/)X(Nexp

Tk/)X(NexpK N

BBN

B 01

00

10

Critical Micelle ConcentrationCritical Micelle Concentration

NXXC 1

In units of mole fraction the total mole fraction of solute is:

N

NK

XCX

1

11

)(

N

NKX

1

1

1

The maximum value of C-X1 is unity

Above this critical micelle concentration all added monomer Above this critical micelle concentration all added monomer is turned into aggregatesis turned into aggregates


N

BNN Tk/)X()X(expXNX 01

01

Chemical potential must be lower in aggregate!Chemical potential must be lower in aggregate!

For:

NN NXX 1

)X()X(Tk

Nexp

Tk/)X(Nexp

Tk/)X(NexpK N

BBN

B 01

00

10

NN

NX

X

k

kK

12

1

with X1<1

)X()X( N0

10

Monomers predominte!

Size-dependent chemical potentialSize-dependent chemical potential

pB

N N

Tk 00

The chemical potential depends on the size of the aggregate:

0 Chemical potential for an infinite N aggregate

Tk

)X()X(

B

N0

10

Bond energy (in KT units)

pp : dimensionality and shape of the aggregate

NNpN XNNXNX exp/11(exp 1

)1

3

100

N

TkBN

For a spherical aggregate:

γ = interfacial energy

Tk

r

B

24

And so:

Critical Micelle Concentration RevisitedCritical Micelle Concentration Revisited

NNpN XNNXNX exp/11(exp 1

)1

Since XN can never exceed 1, X1 cannot exceed exp( -) and:

Tk

rX

Bcrit

2

1

4exp for a spherical aggregate

For a water/methane interface: γ ≈50mJ·m-2, r≈0.2 nm, T=300K

J.r 202 10524 α ≈ 6 002501 .X crit

Packing effects depend on geometryPacking effects depend on geometry

c

v

a0

lc is the length of the hydrocarbon chain

is the volume occupied by the hydrocarbon chainA0 is the area of the head group

Shape of aggregatesShape of aggregates

3

1

0

ca

v

2

1

3

1

0

ca

v

12

1

0

ca

v

10

ca

v

Spherical micellesSpherical micelles

Non-spherical micellesNon-spherical micelles

Vesicles or bilayersVesicles or bilayers

‘‘Inverted cones’Inverted cones’

ca

v

0

A dimensionless shape factor

Short hydrocarbon chains

Long or double hydrocarbon chains

Lipid bilayer structure – the mitochondrianLipid bilayer structure – the mitochondrian

Pyruvate oxidation – 30 ATPs vs 2


(EM image is reproduced with permission from Chapter 4 of The genetic basis of human disease by G. Wallis published by the Biochemical Society 1999. Copyrighted by the Biochemical Society. http://www.biochemj.org.)

Self-assembled monolayers


Chemisorption of long-chain amphiphilic molecules (both hydrophobic and hydrophilic functionalities) at surfaces.

→ creation of long-range order

• active head group for chemisorption

• activated surfaces

Self-assembled monolayersSelf-assembled monolayers

• long-chain alkanethiolates (SH end group)

STM image of a dodecanthiol SAM on Au(111) (40nm·40nm)

• I step: attachment of the sulfur atom to the gold surface driving force: Au-S interaction (≈40 kcal·mol-1)

X(CH2)nSH + Au0 → X(CH2)nS- + Au+ + ½H2

• Sulfur atoms for long.chain alkanethiolates (n>11) formed a hexagonally packed arrangement on the Au(111) surface.

Kinetic studies on SAM formation show that the adsorption process is consistent with a first-order Langmuir isotherm: the growth rate is proportional to the number of unoccupied gold sites.

The methylene groups tilt at an angle of 30° degrees from the surface normal to maximize the favorable Van der Waals interactions between adjacent chains.

Bulky or polar groups terminating the alkyl chain may reduce the packing density and overall order of the SAM.

Long-lasting self-healing dynamics

• II step: lateral organization of the alkyl chains to form a densely packed monolayer. driving force: Van der Waals lateral interactions

A series of STM images of a single octanedithiol molecule inserted into an octanethiol monolayer.

Sulfur with the gold atom attached to it moves over the surface in almost liquid-like manner.

The consequence of the mobility of the sulfur-gold bond is a substantial restructuring of the gold surface resulting in the formatiion of pits on the surface that are one gold atom in depth.

Nanoparticles kinetically trappedNanoparticles kinetically trapped

CdSe quantum dots from two phase synthesis with Ostwald ripening (diameter: 8 nm)

Si nanowires from Au/Si eutectic seeded on Au NP


Helix repeat: 3.4 nm

DNA NanotechnologyDNA Nanotechnology


About 8 bases must be paired for a double helix to be stable at room Temperature.


A DNA-based four-way crossover structures producing a rigid planar tile. The distance between adjacent tile is 20nm.The structure, imaged by AFM, is produced by spontaneous self-assembly of the individual crosses.

DNA OrigamiDNA Origami


A long template strand is annealed with a numebr of short strands that either form cross-links at fixed points (loops) or fill regions to form double helices.

Amphiphiles Copyright Stuart Lindsay 2008 Hydrophobic tail Polar head Phospholipid.

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