Weiss Thomas M, Chen Poe-Jou, Sinn Harald, Alp Ercan E, Chen Sow-Hsin, Huang Huey W
Department of Physics and Astronomy, Rice University, Houston, Texas 77251-1892, USA.
Biophys J. 2003 Jun;84(6):3767-76. doi: 10.1016/S0006-3495(03)75105-4.
We investigated the application of inelastic x-ray scattering (IXS) to lipid bilayers. This technique directly measures the dynamic structure factor S(q,omega) which is the space-time Fourier transform of the electron density correlation function of the measured system. For a multiatomic system, the analysis of S(q,omega) is usually complicated. But for multiple bilayers of lipid, S(q,omega) is dominated by chain-chain correlations within individual bilayers. Thus IXS provides a unique probe for the collective dynamics of lipid chains in a bilayer that cannot be obtained by any other method. IXS of dimyristoyl phosphatidylcholine and dimyristoyl phosphatidylcholine + cholesterol at two different concentrations were measured. S(q,omega) was analyzed by three-mode hydrodynamic equations, including a thermal diffusive mode and two propagating acoustic modes. We obtained the dispersion curves for the phonons that represent the collective in-plane excitations of lipid chains. The effect of cholesterol on chain dynamics was detected. Our analysis shows the importance of having a high instrument resolution as well as the requirement of sufficient signal-to-noise ratio to obtain meaningful results from such an IXS experiment. The requirement on signal-to-noise also applies to molecular dynamics simulations.
我们研究了非弹性X射线散射（IXS）在脂质双层中的应用。该技术直接测量动态结构因子S(q,ω)，它是被测系统电子密度关联函数的时空傅里叶变换。对于多原子系统，S(q,ω)的分析通常很复杂。但对于脂质的多个双层，S(q,ω)主要由单个双层内的链-链相关性决定。因此，IXS为双层中脂质链的集体动力学提供了一种独特的探测手段，这是其他任何方法都无法实现的。我们测量了两种不同浓度下的二肉豆蔻酰磷脂酰胆碱以及二肉豆蔻酰磷脂酰胆碱+胆固醇的IXS。通过三模流体动力学方程对S(q,ω)进行了分析，其中包括一个热扩散模式和两个传播声模式。我们获得了代表脂质链集体面内激发的声子色散曲线。检测到了胆固醇对链动力学的影响。我们的分析表明，要从这样的IXS实验中获得有意义的结果，高仪器分辨率以及足够的信噪比都很重要。对信噪比的要求同样适用于分子动力学模拟。
