Yoshida T, Taga K, Okabayashi H, Kamaya H, Ueda I
Department of Applied Chemistry, Nagoya Institute of Technology, Japan.
Biochim Biophys Acta. 1990 Sep 21;1028(1):95-102. doi: 10.1016/0005-2736(90)90270-x.
Impedance dispersion in liposomes measures the lateral charge transfer of lipid membrane surfaces. Depending on the choice of frequency between 1 kHz and 100 GHz, relaxation of the counterions at the interface, orientation of the head group, and relaxation of the bound and free water are revealed. This study measured the impedance dispersion in dipalmitoylphosphatidylcholine (DPPC) liposomes at 10 kHz. The surface conductance and capacitance showed breaks at pre- and main transition temperatures. Below the pre-transition temperature, the activation energy of the ion movement was 18.1 kJ.mol-1, which corresponded to that of the spin-lattice relaxation time of water (18.0 kJ.mol-1). At temperatures between pre- and main transition it increased to 51.3 kJ.mol-1, and agreed with 46.2-58.0 kJ.mol-1 of the activation energy of the dielectric relaxation of ice. Because the present system was salt-free, the ions were H3O+ and OH-, hence, their behavior represents that of water. The above results show that below the pre-transition temperature, the conductance is regulated by the mobility of free ions, or the number of free water molecules near the interface. On the other hand when the temperature exceeded pre-transition, melting of the surface-bound water crystals became the rate-limiting step for the proton flow. Halothane did not show any effect on the ion movement when the temperature was below pre-transition. When the temperature exceeded pre-transition, 0.35 mM halothane (equilibrium concentration) decreased the activation energy of the ion movement to 29.3 kJ.mol-1. This decrease indicates that halothane enhanced the release of the surface-bound water molecules at pre-transition. The surface-disordering effect of halothane was also shown by depression of the pre-transition temperature and decrease of the association energy among head groups from 9.7 kJ.mol-1 of the control to 5.2 kJ.mol-1 at 0.35 mM.
脂质体中的阻抗色散测量脂质膜表面的横向电荷转移。根据1kHz至100GHz之间频率的选择，可揭示界面处抗衡离子的弛豫、头基的取向以及结合水和自由水的弛豫。本研究测量了二棕榈酰磷脂酰胆碱（DPPC）脂质体在10kHz时的阻抗色散。表面电导和电容在预转变温度和主转变温度处出现断点。在预转变温度以下，离子运动的活化能为18.1kJ·mol-1，这与水的自旋晶格弛豫时间的活化能（18.0kJ·mol-1）相对应。在预转变温度和主转变温度之间，其增加到51.3kJ·mol-1，与冰的介电弛豫活化能46.2 - 58.0kJ·mol-1相符。由于本系统无盐，离子为H3O+和OH-，因此，它们的行为代表了水的行为。上述结果表明，在预转变温度以下，电导由自由离子的迁移率或界面附近自由水分子的数量调节。另一方面，当温度超过预转变温度时，表面结合水晶体的熔化成为质子流动的限速步骤。当温度低于预转变温度时，氟烷对离子运动没有任何影响。当温度超过预转变温度时，0.35mM氟烷（平衡浓度）将离子运动的活化能降低到29.3kJ·mol-1。这种降低表明氟烷在预转变时增强了表面结合水分子的释放。氟烷的表面无序化效应还表现为预转变温度降低以及头基之间的缔合能从对照的9.7kJ·mol-1降低到0.35mM时的5.2kJ·mol-1。
