Life at Low D

Lecture III: Monolayers and Membranes

Phospholipids

Amphi (“both”)

philia(“love”)

Amphiphilic Molecules

“There seems to be no natural repulsion between water and air such as to keep them from coming into contact with each other.” (Franklin, 1773)

“...the oil, though not more than a teaspoonful, produced an instant calmover a space several yards square which spread amazingly and extended itself…, making all that quarter of the pond, perhaps half an acre as smooth as a looking glass.”

If a drop of oil is put on a highly polished marble table, or on a looking-glass that lies horizontally, the drop remains in its place, spreading very little.

Interfacial Energy and Surface Tension

TSUF GOAL: Minimize Free Energy

UEnergy

aa

a

TkB200~

Entropy

S

Fluids VDW

H-Bonding

96.0

Solids Covalent

TkB3~

TkB10~

2/0 07.02

TkB

OilH

OH 2

OH 2

Oil

2

0 10

a

0

]2ln[

a

TkF B

Hydrophobic Effect

15

2

40

2/0 07.0

2

TkBOilH

Hydrophobic Area/Molecule 2

300

aS

TkTk

SF BB

OilHA 2530007.0 22/02

Surface Tension

OH 2

2/0 2.02

TkB

AirH

Surface Tension

OH 2

2/0 07.02

TkB

OilH

Monolayer reduces Surface Tension

2/0 2.02

TkB

AirH

)(McCMC

OilH /02

AirH /02

Surface Tension

2/0 07.02

TkB

OilH

Monolayer reduces Surface Tension

2/0 2.02

TkB

AirH

)(McCMC

OilH /02

AirH /02

Self-Assembly

Bilayer Properties

2/0 07.02

TkB

OilH

52

50

Flip-Flop (exposing hydrophilic head to hydrocarbon chain) TkE B12

Water Permeable (cost of water passing through)

96.0

2

12

AS

2/0 07.02

TkB

OilH

TkE B

30

2

50

AS

Bilayer Properties

Edge Energy/Length

30

52

50

TkTkLE BB 23007.0/

2

Driving “Force” for the Formation of closed Structures

Bending Energy

R

BBB

Bend RR

dAR

E 84

4

2

2

22

2

2

TkBB 10 TkE BBend 240