Hexagonal phase formation

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Hexagonal phase formation in a lipid/fatty acid
mixture atomistic study
Volker Knecht
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Questions (1) can lamellar-HII phase behavior of
lipid systems be reproduced by atomistic
simulations? (2) what is appropriate method of
testing the phase behavior of a lipid model? (3)
how does inverted hexagonal phase form?
lamellar (La/Lb)
stalk
inverted hexagonal (HII)
Siegel, D.P., 1999, Biophys. J. 76291-313
Volker Knecht
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Phase diagram of DPPC/PA(H) 12 / water
J.M. Seddon et al., Biochim. Biophys. Acta 1327
(1997), 131-147
Volker Knecht
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Molecular dynamics simulations
Volker Knecht
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Molecular dynamics simulations
Volker Knecht
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Gel (Lb) phase formation
T10oC
t 0-40ns
Volker Knecht
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Inverted hexagonal (HII) phase formation
T110oC
t 0-10ns
Volker Knecht
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Result inverted hexagonal (HII) phase
inverted hexagonal phase formed during 10 ns of
simulation at 110oC multiple copies of unit
cell shown
Volker Knecht
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Self-aggregated phases
at extreme temperatures good agreement with
experiment
Volker Knecht
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Transition from Lb to HII phase
T120oC
t 0-20ns
Volker Knecht
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Stalk formation
T120oC
t 0-20ns
only acyl chains shown
Volker Knecht
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Transition from stalk intermediate to Lb or HII
phase
0 ns
stalk
Volker Knecht
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Stalk formation between two independent bilayers
DPPC
PA(H)
Volker Knecht
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Stalk structure
DPPC
PA(H)
Volker Knecht
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Summary of hexagonal phase formation study

• atomistic molecular dynamics simulations
  reproduce
• binary (Lb/HII) phase diagram of DPPC/PA(H) 12
  mixture
• overall lipid phases formed from stalk
  intermediate
• agree well with experiment
• appropriate method to study phase behavior in
  such
• simulations
• transition from L? to HII phase proceeds via
  metastable L?
• and transient stalk intermediate

Volker Knecht