Large Scale Structures Group, Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States; Shull-Wollan Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States; Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996, United States.
Large Scale Structures Group, Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States; Shull-Wollan Center, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States.
Chem Phys Lipids. 2020 Oct;232:104979. doi: 10.1016/j.chemphyslip.2020.104979. Epub 2020 Sep 25.
Short-wavelength collective molecular motions, also known as phonons, have recently attracted much interest in revealing dynamic properties of biological membranes through the use of neutron and X-ray scattering, infrared and Raman spectroscopies, and molecular dynamics simulations. Experimentally detecting unique vibrational patterns such as, shear phonon excitations, viscoelastic crossovers, transverse acoustic phonon gaps, and continuous and truncated optical phonon modes in cellular membranes, to name a few, has proven non-trivial. Here, we review recent advances in liquid thermodynamics that have resulted in the development of the phonon theory of liquids. The theory has important predictions regarding the shear vibrational spectra of fluids, namely the emergence of viscoelastic crossovers and transverse acoustic phonon gaps. Furthermore, we show that these vibrational patterns are common in soft (non-crystalline) materials, including, but not limited to liquids, colloids, liquid crystals (mesogens), block copolymers, and biological membranes. The existence of viscoelastic crossovers and acoustic phonon gaps define the self-diffusion properties of cellular membranes and provide a molecular picture of the transient nature of lipid rafts (Bolmatov et al., 2020). Importantly, the timescales (picoseconds) for the formation and dissolution of transient lipid rafts match the lifetime of the formation and breakdown of interfacial water hydrogen bonds. Apart from acoustic propagating phonon modes, biological membranes can also support more energetic non-propagating optical phonon excitations, also known as standing waves or breathing modes. Importantly, optical phonons can be truncated due to the existence of finite size nanodomains made up of strongly correlated lipid-cholesterol molecular pairs. These strongly coupled molecular pairs can serve as nucleation centers for the formation of stable rafts at larger length scales, due to correlations of spontaneous fluctuations (Onsager's regression hypothesis). Finally and importantly, molecular level viscoelastic crossovers, acoustic phonon gaps, and continuous and truncated optical phonon modes may offer insights as to how lipid-lipid and lipid-protein interactions enable biological function.
短波长集体分子运动，也称为声子，最近通过使用中子和 X 射线散射、红外和拉曼光谱以及分子动力学模拟，在揭示生物膜的动态特性方面引起了广泛关注。实验上检测到独特的振动模式，如剪切声子激发、粘弹性转变、横向声学声子间隙以及连续和截断的光学声子模式等，在细胞膜中，仅举几例，已被证明并非易事。在这里，我们回顾了液体热力学的最新进展，这些进展导致了声子液体理论的发展。该理论对流体的剪切振动谱有重要的预测，即粘弹性转变和横向声学声子间隙的出现。此外，我们表明，这些振动模式在软（非晶态）材料中很常见，包括但不限于液体、胶体、液晶（介晶）、嵌段共聚物和生物膜。粘弹性转变和声学声子间隙的存在定义了细胞膜的自扩散性质，并提供了脂质筏（Bolmatov 等人，2020）瞬态性质的分子图像。重要的是，形成和溶解瞬态脂质筏的时间尺度（皮秒）与界面水分子氢键形成和破坏的寿命相匹配。除了声传播声子模式外，生物膜还可以支持更具能量的非传播光学声子激发，也称为驻波或呼吸模式。重要的是，由于由强相关的脂质-胆固醇分子对组成的有限大小的纳米域的存在，光学声子可以被截断。这些强耦合分子对可以作为在较大长度尺度上形成稳定筏的成核中心，这是由于自发波动的相关性（Onsager 的回归假设）。最后，重要的是，分子水平的粘弹性转变、声学声子间隙以及连续和截断的光学声子模式可能提供有关脂质-脂质和脂质-蛋白质相互作用如何实现生物功能的见解。
