Stokely Kevin, Manz Alyssa S, Kaufman Laura J
Department of Chemistry, Columbia University, New York, New York 10027, USA.
J Chem Phys. 2015 Mar 21;142(11):114504. doi: 10.1063/1.4915267.
Supercooled liquids are proposed to be dynamically heterogeneous, with regions exhibiting relaxation time scales that vary in space and time. Measurement of the distribution of such time scales could be an important test of various proposed theories of vitrification. Single molecule fluorescence experiments attempt to uncover this distribution, typically by embedding single molecule probes into these systems and monitoring their individual rotational relaxations from a computed autocorrelation function (ACF). These ACFs may exhibit stretched exponential decays, with the value of the stretching exponent assumed to report the set of dynamical environments explored by the probe. Here, we use simulated trajectories of rotation to investigate how the time scale of dynamic exchange relative to underlying relaxation time scales in the system affects probe ability to report the distribution relaxation of time scales present. We find that dynamically heterogeneous regions must persist for approximately 50 times the median relaxation time scale for a single molecule to accurately report the full distribution of time scales it has experienced. In systems with faster dynamic exchange, single molecule ACFs average over successive environments, limiting the reported heterogeneity of the system. This leads to degeneracies in stretching exponent for systems with different underlying relaxation time distributions. We show that monitoring single molecule median stretching exponent as a function of trajectory length or simultaneously measuring median stretching exponent and measured relaxation time distribution at a given trajectory length can resolve these degeneracies, revealing the underlying set of relaxation times as well as median exchange time.
过冷液体被认为是动态非均匀的，其区域表现出随空间和时间变化的弛豫时间尺度。测量这种时间尺度的分布可能是对各种提出的玻璃化理论的重要检验。单分子荧光实验试图揭示这种分布，通常是通过将单分子探针嵌入这些系统中，并从计算的自相关函数（ACF）监测它们各自的旋转弛豫。这些ACF可能表现出拉伸指数衰减，拉伸指数的值被认为可以报告探针所探索的动态环境集。在这里，我们使用旋转模拟轨迹来研究系统中动态交换的时间尺度相对于潜在弛豫时间尺度如何影响探针报告存在的时间尺度分布弛豫的能力。我们发现，动态非均匀区域必须持续存在约50倍于中值弛豫时间尺度，单个分子才能准确报告其经历的时间尺度的完整分布。在具有更快动态交换的系统中，单分子ACF在连续环境上平均，限制了所报告的系统非均匀性。这导致具有不同潜在弛豫时间分布的系统在拉伸指数上出现简并。我们表明，监测单分子中值拉伸指数作为轨迹长度的函数，或在给定轨迹长度下同时测量中值拉伸指数和测量的弛豫时间分布，可以解决这些简并问题，揭示潜在的弛豫时间集以及中值交换时间。
