Forschungsneutronenquelle Heinz Maier-Leibnitz (FRM II), Technische Universität München, Garching bei München, Germany.
J Am Chem Soc. 2010 Mar 17;132(10):3232-3. doi: 10.1021/ja907581s.
The motion of phospholipids has previously been studied on many time scales due to the significance for living cells and technological applications. The motions on a pico- to nanosecond time scale were determined by quasielastic neutron scattering (QENS) to be much faster than the ones on the microsecond scale covered by fluorescence recovery after photobleaching (FRAP). This was explained by assuming that the molecules rattle fast in a cage of neighbors (observed with QENS) from which they escape once in a while; this escape was then the primary step of the slower diffusion measured by FRAP. However, nanosecond MD simulation studies could not observe any escape events; recent findings even suggested that the long-range motion in phospholipid membranes on short time scales is not diffusive but has flow-like characteristics. To check this novel view, we have repeated the QENS experiments with today's significantly improved instrumentation. By using the advantage of QENS that allows tuning of the observation time in the pico- to nanosecond range, it was possible to study the evolution of motions in this time frame. Localized motions, e.g., of the head and tail groups, appear separated from the long-range motion and do not obfuscate the analysis as they do in a mean squared displacement plot. The results for the long-range motion are indeed compatible with flow patterns, whereas the localized motions can account for the fast motions interpreted as motions in a cage before. Hereby, we give experimental evidence for a completely different mechanism of long-range motion on short time scales in phospholipid membranes.
由于磷脂对活细胞和技术应用具有重要意义，因此先前已经在许多时间尺度上研究了磷脂的运动。通过准弹性中子散射（QENS）确定的皮秒到纳秒时间尺度上的运动比荧光恢复后光漂白（FRAP）覆盖的微秒时间尺度上的运动快得多。这是通过假设分子在邻居的笼子中快速摇晃（通过 QENS 观察到），并且它们偶尔会从中逃脱来解释的；这种逃脱是由 FRAP 测量的较慢扩散的主要步骤。然而，纳秒 MD 模拟研究未能观察到任何逃脱事件；最近的发现甚至表明，在短时间尺度上，磷脂膜中的长程运动不是扩散的，而是具有流动特征。为了验证这种新观点，我们使用今天显著改进的仪器重复了 QENS 实验。通过利用 QENS 可以在皮秒到纳秒范围内调整观察时间的优势，可以研究该时间范围内运动的演变。局部运动，例如头和尾基团的运动，与长程运动分开，并且不会像均方根位移图中那样使分析变得混乱。长程运动的结果确实与流动模式兼容，而局部运动可以解释为以前解释为在笼子中的快速运动。通过这种方式，我们为磷脂膜中短时间尺度上长程运动的完全不同机制提供了实验证据。
