Experimental studies of collective dynamics in lipid bilayers have been challenging due to the energy resolution required to observe these low-energy phonon-like modes. However, inelastic x-ray scattering (IXS) measurements—a technique for probing vibrations in soft and biological materials—are now possible with sub-meV resolution, permitting direct observation of low-energy, phonon-like modes in lipid membranes. Here, IXS measurements with sub-meV energy resolution reveal a low-energy optic-like phonon mode at roughly 3 meV in the liquid-ordered (Lo) and liquid-disordered phases of a ternary lipid mixture. This mode is only observed experimentally at momentum transfers greater than 5 nm−1 in the Lo system. A similar gapped mode is also observed in all-atom molecular dynamics (MD) simulations of the same mixture, indicating that the simulations accurately represent the fast, collective dynamics in the Lo phase. Its optical nature and the Q range of the gap together suggest that the observed mode is due to the coupled motion of cholesterol-lipid pairs, separated by several hydrocarbon chains within the membrane plane. Analysis of the simulations provides molecular insight into the origin of the mode in transient, nanoscale substructures of hexagonally packed hydrocarbon chains. This nanoscale hexagonal packing was previously reported based on MD simulations and, later, by NMR measurements. Here, however, the integration of IXS and MD simulations identifies a new signature of the Lo substructure in the collective lipid dynamics, thanks to the recent confluence of IXS sensitivity and MD simulation capabilities.

中文翻译：

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声子揭示了三元膜中耦合的胆固醇-脂质动力学


由于观察这些低能声子样模式所需的能量分辨率，脂质双层中集体动力学的实验研究一直具有挑战性。然而，非弹性 X 射线散射 （IXS） 测量（一种探测软材料和生物材料振动的技术）现在可以以亚 meV 分辨率实现，从而可以直接观察脂质膜中的低能量声子样模式。在这里，具有亚 meV 能量分辨率的 IXS 测量揭示了在三元脂质混合物的液有序 （Lo） 和液无序相中，在大约 3 meV 处存在低能量光学类声子模式。在 Lo 系统中，这种模式仅在大于 5 nm−1 的动量转移处通过实验观察到。在相同混合物的全原子分子动力学 （MD） 模拟中也观察到类似的间隙模式，表明模拟准确地代表了 Lo 相中的快速集体动力学。它的光学性质和间隙的 Q 范围一起表明，观察到的模式是由于胆固醇-脂质对的耦合运动，由膜平面内的几条烃链隔开。对模拟的分析为六边形堆积的碳氢化合物链的瞬态纳米级子结构中的模式起源提供了分子见解。这种纳米级六边形堆积以前是根据 MD 模拟报道的，后来又通过 NMR 测量报道的。然而，由于 IXS 灵敏度和 MD 模拟能力最近融合在一起，IXS 和 MD 模拟的整合在集体脂质动力学中确定了 Lo 亚结构的新特征。