Sharma V K, Mamontov E, Anunciado D B, O'Neill H, Urban V
†Biology and Soft Matter Division, Neutron Sciences Directorate, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States.
‡Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India.
J Phys Chem B. 2015 Mar 26;119(12):4460-70. doi: 10.1021/acs.jpcb.5b00220. Epub 2015 Mar 17.
The dynamics of phospholipids in unilamellar vesicles (ULVs) is of interest in biology, medical, and food sciences, since these molecules are widely used as biocompatible agents and a mimic of cell membrane systems. We have investigated the nanoscopic dynamics of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) phospholipid in ULVs as a function of temperature using elastic and quasielastic neutron scattering (QENS). The dependence of the signal on the scattering momentum transfer, which is a critical advantage of neutron scattering techniques, allows the detailed analysis of the lipid motions that cannot be carried out by other means. In agreement with a differential scanning calorimetry measurement, a sharp rise in the elastic scattering intensity below ca. 296 K indicates a phase transition from the high-temperature fluid phase to the low-temperature solid gel phase. The microscopic lipid dynamics exhibits qualitative differences between the solid gel phase (in a measurement at 280 K) and the fluid phase (in a measurement at a physiological temperature of 310 K). The analysis of the data demonstrates the presence of two types of distinct motions: the entire lipid molecule motion within a monolayer, also known as lateral diffusion, and the relatively faster internal motion of the DMPC molecule. The lateral diffusion of the entire lipid molecule is Fickian in character, whereas the internal lipid motions are of localized character, which is consistent with the structure of the vesicles. The lateral motion slows down by an order of magnitude in the solid gel phase, whereas for the internal motion not only the time scale but also the character of the motion changes upon the phase transition. In the solid gel phase, the lipids are more ordered and undergo uniaxial rotational motion. However, in the fluid phase, the hydrogen atoms of the lipid tails undergo confined translation diffusion rather than uniaxial rotational diffusion. The translational, but spatially localized, diffusion of the hydrogen atoms of the lipid tails is a manifestation of the flexibility of the chains acquired in the fluid phase. Because of this flexibility, both the local diffusivity and the confinement volume for the hydrogen atoms increase in the linear fashion from near the lipid's polar headgroup to the end of its hydrophobic tail. Our results present a quantitative and detailed picture of the effect of the gel-fluid phase transition on the nanoscopic lipid dynamics in ULVs. The data analysis approach developed here has a potential for probing the dynamic response of lipids to the presence of additional cell membrane components.
单层囊泡（ULV）中磷脂的动力学在生物学、医学和食品科学领域备受关注，因为这些分子被广泛用作生物相容性试剂以及细胞膜系统的模拟物。我们使用弹性和准弹性中子散射（QENS）研究了ULV中1,2 - 二肉豆蔻酰 - sn - 甘油 - 3 - 磷酸胆碱（DMPC）磷脂的纳米级动力学随温度的变化。信号对散射动量转移的依赖性是中子散射技术的一个关键优势，它能够对脂质运动进行详细分析，而这是其他方法无法做到的。与差示扫描量热法测量结果一致，在约296 K以下弹性散射强度急剧上升，表明从高温流体相到低温固体凝胶相的相变。微观脂质动力学在固体凝胶相（在280 K测量）和流体相（在生理温度310 K测量）之间表现出质的差异。数据分析表明存在两种不同类型的运动：单层内整个脂质分子的运动，也称为横向扩散，以及DMPC分子相对较快的内部运动。整个脂质分子的横向扩散具有菲克性质，而内部脂质运动具有局部性质，这与囊泡的结构一致。在固体凝胶相中，横向运动减慢了一个数量级，而对于内部运动，不仅时间尺度而且运动特征在相变时都会发生变化。在固体凝胶相中，脂质排列更有序，进行单轴旋转运动。然而，在流体相中，脂质尾部的氢原子进行受限的平移扩散而不是单轴旋转扩散。脂质尾部氢原子的平移但空间局部的扩散是流体相中链获得的灵活性的体现。由于这种灵活性，氢原子的局部扩散系数和受限体积从脂质的极性头部基团附近到其疏水尾部末端以线性方式增加。我们的结果给出了凝胶 - 流体相变对ULV中纳米级脂质动力学影响的定量和详细图景。这里开发的数据分析方法有潜力探测脂质对其他细胞膜成分存在的动态响应。
