4.2 Phase Transition of Lipid Bilayer Membranes
Lipid bilayer membranes are assembly of a lot of number (sufficient for statistical
phenomena) of lipid molecules and they show phase transition like first order phase
transition and it is an interest research subject from physical viewpoint. From the
definition of first order phase transition, Gibbs free energy G for temperature change
is continuous under constant condition of pressure and its differential,
∂G
∂T
À Á
P
is
discontinuity. So ΔG ¼ 0 and ΔG ¼ ΔH ¼ À T t ΔS becomes latent heat ΔH ¼ T t ΔS.
When 2 phases are independent from each other, Gibbs free energy of these phases
are shown as function of temperature, T and pressure, P as follows G 1 ¼ f 1 (T, P),
G 2 ¼ f 2 (T, P). In case of fluid phase and gel phase, they become G L ¼ f L (T,
P) ¼ H L À TS L and G S ¼ f S (T, P) ¼ H S À TS S , and difference of the Gibbs free
energy between these phases is shown as G S À G L ¼ (H S À H L ) À T(S S À S L ).
Enthalpy in gel phase is smaller than that in fluid phase and entropy in solid phase is
smaller than fluid phase because of higher orderliness in gel phase. Therefore,
transition from fluid phase to gel phase occurs when a certain temperature, T exists
for negative (G S À G L ). This transition reduces the free energy and the transition to
gel phase proceeds spontaneously. Temperature, T t of first order phase transition is
obtained as intersection of Gibbs free energy curves of these phases. DSC measurement of phase transition in biomembranes is focused on lipid membranes. And one
component of lipid dispersed in water such as dipalmitoylphosphatidylcholine
(DPPC) shows clear phase transitions (Fig. 4.3). DPPC molecules dispersed in
water by voltexing form multi-lamellar membranes and they show four phase
transitions; sub transition (crystal phase, L c to gel phase, L β
0 ), pre-transition (gel
phase to ripple phase) and main transition (ripple phase to fluid phase, L α ). These
phase transitions occur in keeping regular structure of multi-lamellar structure and
show characteristic change on X-ray diffraction pattern (Fig. 4.4). Fatty acyl chains
of DPPC molecules are arranged all-trans structure in gel phase and some gauche
structures appear in fluid phase. This phenomenon is melting of acyl chains of
phosphatidylcholine molecules. Structural change of phase transition is observed
by freeze-fractured electron microscope as jumbled structured of L α phase and
parallel lines of L β
0 phase (Fig. 4.5). Phase transition from fluid phase to hexagonal
II phase in phosphatidylethanolamine liposomes by lowering temperature. Relaxation process of this phase transition is slow and long time is required to complete the
transition. Phase transition of liposomes is also observed as change of density
observed by dilatometer (Fig. 4.6). And phase transition from gel phase to fluid
phase shows decrease of density of lipid bilayers because of extension of distance
between lipid molecules and distance between lamellas. Each lipid molecular species
has individual temperature of phase transition. And the temperature depends on
length and degree of unsaturation of fatty acyl chains. Phase transition temperature
of phosphatidylcholine changes from À7
C of DLPC to 54
C of DSPC. However,
4.2 Phase Transition of Lipid Bilayer Membranes
61
Lipid bilayer membranes are assembly of a lot of number (sufficient for statistical
phenomena) of lipid molecules and they show phase transition like first order phase
transition and it is an interest research subject from physical viewpoint. From the
definition of first order phase transition, Gibbs free energy G for temperature change
is continuous under constant condition of pressure and its differential,
∂G
∂T
À Á
P
is
discontinuity. So ΔG ¼ 0 and ΔG ¼ ΔH ¼ À T t ΔS becomes latent heat ΔH ¼ T t ΔS.
When 2 phases are independent from each other, Gibbs free energy of these phases
are shown as function of temperature, T and pressure, P as follows G 1 ¼ f 1 (T, P),
G 2 ¼ f 2 (T, P). In case of fluid phase and gel phase, they become G L ¼ f L (T,
P) ¼ H L À TS L and G S ¼ f S (T, P) ¼ H S À TS S , and difference of the Gibbs free
energy between these phases is shown as G S À G L ¼ (H S À H L ) À T(S S À S L ).
Enthalpy in gel phase is smaller than that in fluid phase and entropy in solid phase is
smaller than fluid phase because of higher orderliness in gel phase. Therefore,
transition from fluid phase to gel phase occurs when a certain temperature, T exists
for negative (G S À G L ). This transition reduces the free energy and the transition to
gel phase proceeds spontaneously. Temperature, T t of first order phase transition is
obtained as intersection of Gibbs free energy curves of these phases. DSC measurement of phase transition in biomembranes is focused on lipid membranes. And one
component of lipid dispersed in water such as dipalmitoylphosphatidylcholine
(DPPC) shows clear phase transitions (Fig. 4.3). DPPC molecules dispersed in
water by voltexing form multi-lamellar membranes and they show four phase
transitions; sub transition (crystal phase, L c to gel phase, L β
0 ), pre-transition (gel
phase to ripple phase) and main transition (ripple phase to fluid phase, L α ). These
phase transitions occur in keeping regular structure of multi-lamellar structure and
show characteristic change on X-ray diffraction pattern (Fig. 4.4). Fatty acyl chains
of DPPC molecules are arranged all-trans structure in gel phase and some gauche
structures appear in fluid phase. This phenomenon is melting of acyl chains of
phosphatidylcholine molecules. Structural change of phase transition is observed
by freeze-fractured electron microscope as jumbled structured of L α phase and
parallel lines of L β
0 phase (Fig. 4.5). Phase transition from fluid phase to hexagonal
II phase in phosphatidylethanolamine liposomes by lowering temperature. Relaxation process of this phase transition is slow and long time is required to complete the
transition. Phase transition of liposomes is also observed as change of density
observed by dilatometer (Fig. 4.6). And phase transition from gel phase to fluid
phase shows decrease of density of lipid bilayers because of extension of distance
between lipid molecules and distance between lamellas. Each lipid molecular species
has individual temperature of phase transition. And the temperature depends on
length and degree of unsaturation of fatty acyl chains. Phase transition temperature
of phosphatidylcholine changes from À7
C of DLPC to 54
C of DSPC. However,
4.2 Phase Transition of Lipid Bilayer Membranes
61