a ¼
T xx þT yy þT zz
3
. Original value of a in non-polar type is 13.8 G. On the other hand, a
vale becomes 15.6 G in aqueous phase because dipolar moment of water molecule
changes N-O to polar type. Consequently, separated ESR signal of TEMPO is
observed. Partition between aqueous phase and depend on phase transition as well
as temperature change. Therefore TEMPO is used to study phase behavior such as
phase transition and phase separation [5]. TEMPO parameter depends on polarity
around nitroxide radical which generates ESR signal. And measurement of the
separated peaks H and P are used to obtain TEMPO parameter. Height of each
peak H and P corresponds to content of TEMPO spin probe in hydrophobic
environment and polar environment, respectively. Phase behavior of lipid membranes is experimentally realized on change of TEMPO parameter. Partition coefficient of TEMPO spin probe between lipid phase and aqueous phase. The partition
coefficient, k is shown in following equation.
k ¼
C
½ Š in lipid bilayef
C
½ Š in aqeous phase
Fig. 3.11 TEMPO
parameter
Partition of TEMPO spin
probes depend on phase
structure of lipid bilayer
membranes. The probes
distribute preferentially in
aqueous phase during gel
phase, and they distribute
preferentially in lipid phase
during fluid phase
3.6 Magnetic Resonance
39