MOLECULAR-DYNAMICS CT WATER IN ORIENTED DPPC MULTILAYERS STUDIED BY QUASI-ELASTIC NEUTRON-SCATTERING AND DEUTERIUM-NUCLEAR MAGNETIC-RESONANCE RELAXATION
MOLECULAR-DYNAMICS CT WATER IN ORIENTED DPPC MULTILAYERS STUDIED BY QUASI-ELASTIC NEUTRON-SCATTERING AND DEUTERIUM-NUCLEAR MAGNETIC-RESONANCE RELAXATION
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DOI：
10.1063/1.466422
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发表时间：
1994-02-15
影响因子：
4.4
通讯作者：
BAYERL, TM
中科院分区：
文献类型：
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作者：
KONIG, S;SACKMANN, E;BAYERL, TM
在两个时域内研究了不同水化程度下1，2-二棕榈酰-sn-甘油-3-磷酸胆碱（DPPC）高度取向多层膜间水的动力学行为。非相干准弹性中子散射（QENS）和氘核磁共振（NMR）纵向（T-1）弛豫被用来研究水的高频运动（10（-9）-10（-11）s时间尺度）及其各向异性，而2 H-NMR横向（T-2）弛豫被用来获得低频动力学过程（微秒时间尺度）的信息。我们的研究结果表明，在低水合（3至4个水分子/脂质）的高频动力学（皮秒时间尺度）可以被理解为仅仅是一个单轴旋转的水分子紧密结合到DPPC头基团的相关时间τ（rot）在55摄氏度和1 +/- 0.1埃的旋转半径约为62 ps，但没有可检测的平移自由度。~ 2 H-NMR T-1数据（纳秒时间尺度）可以根据上述相关时间的快速旋转和旋转轴的较慢重定向（相关时间τ 1近似为6 ns）来满意地解释。QENS和2 H-NMR T-1测量都提供了该过程的表观活化能E（a）= 32 +/- 1.0 kJ/mol。增加多层的水合水平使旋转运动基本上不变，但能够实现额外的平移运动，其可以被认为是非结合水的跳跃扩散过程（在44 ℃下扩散系数D = 16 +/-1 × 10（-10）m（2）/s，平均停留时间τ（0）= 2.0 +/- 0.5 ps）。值得注意的是，这种扩散在QENS实验的特征长度尺度上是完全各向同性的（等于或小于10埃）。温度的变化表明，相态的脂质水分子的高频动力学没有显着的影响。在取向DPPC多层膜的相变温度T-m附近的温度下，水（D2 O）的2 H-NMR四极分裂的测量清楚地显示了结晶L（β '）相和流体L（α）相在T-m两侧高达4 ℃的范围内共存。当我们在低于饱和度的水化水平下工作时，中间的P-β '（“波纹”）相被抑制。在共存范围内，由于水的扩散，水的交换发生在结晶和流体脂质域之间。这种交换导致T-m处的2 H-NMR横向弛豫时间T-2的显著最小值，因为这种低频过程近似满足两个位点化学交换过程的临界阻尼条件。
The dynamics of water between highly oriented multilayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) has been studied in two time domains at different hydration levels. Incoherent quasielastic neutron scattering (QENS) and deuterium-nuclear magnetic resonance (NMR) longitudinal (T-1) relaxation were employed to investigate both the high-frequency motions of water (10(-9)-10(-11) s time scale) and their anisotropy, while 2H-NMR transverse (T-2) relaxation was used for obtaining information on low frequency dynamical processes (microsecond time scale). Our results show that high frequency dynamics (picosecond-time scale) at low hydration (three to four water molecules per lipid) can be understood solely as a uniaxial rotation of the water molecules tightly bound to DPPC head groups with a correlation time tau(rot) approximate to 62 ps at 55 degrees C and a rotational radius of 1 +/- 0.1 Angstrom, but with no detectable translational degrees of freedom. The 2H-NMR T-1 data (nanosecond-time scale) can be explained satisfactorily on the basis of fast rotations with the correlation time above and a slower reorientation of the rotational axis (correlation time tau 1 approximate to 6 ns). Both QENS and 2H-NMR T-1 measurements provide an apparent activation energy of E(a) = 32 +/- 1.0 kJ/mol for this process. Increasing the hydration level of the multilayers leaves the rotational motion essentially unchanged, but enables additional translational motion which can be considered as a jump diffusion process (diffusion coefficient D = 16 +/- 1X10(-10) m(2)/s at 44 degrees C and a mean residence time of tau(o) = 2.0 +/- 0.5 ps) of nonbound water. It is interesting to note that this diffusion is completely isotropic on the characteristic length scale of this QENS experiment (equal to or less than 10 Angstrom). Temperature variation shows that the phase state of the lipids has no significant effect on the high frequency dynamics of the water molecules. Measurements of the 2H-NMR quadrupolar splitting of water (D2O) at temperatures around the phase transition temperature T-m of the oriented DPPC multilayers clearly show a coexistence of the crystalline L(beta') phase and of the fluid L(alpha) phase over a range of up to 4 degrees C at both sides of T-m. The intermediate P-beta' (''ripple'') phase is suppressed as we worked at hydration levels below saturation. In the coexistence range, exchange of water takes place between crystalline and fluid lipid domains due to water diffusion. This exchange causes a pronounced minimum of the 2H-NMR transverse relaxation time T-2 at T-m since this low frequency process satisfies approximately a critical damping condition for a two-site chemical exchange process.