Frauenfelder Hans, Fenimore Paul W, Young Robert D
Theoretical Biology and Biophysics Group, Los Alamos National Laboratory, Los Alamos, NM 87545; and
Theoretical Biology and Biophysics Group, Los Alamos National Laboratory, Los Alamos, NM 87545; and.
Proc Natl Acad Sci U S A. 2014 Sep 2;111(35):12764-8. doi: 10.1073/pnas.1411781111. Epub 2014 Aug 18.
Quasielastic incoherent neutron scattering (QENS) is an important tool for the exploration of the dynamics of complex systems such as biomolecules, liquids, and glasses. The dynamics is reflected in the energy spectra of the scattered neutrons. Conventionally these spectra are decomposed into a narrow elastic line and a broad quasielastic band. The band is interpreted as being caused by Doppler broadening due to spatial motion of the target molecules. We propose a quantum-mechanical model in which there is no separate elastic line. The quasielastic band is composed of sharp lines with twice the natural line width, shifted from the center by a random walk of the protein in the free-energy landscape of the target molecule. The walk is driven by vibrations and by external fluctuations. We first explore the model with the Mössbauer effect. In the subsequent application to QENS we treat the incoming neutron as a de Broglie wave packet. While the wave packet passes the protons in the protein and the hydration shell it exchanges energy with the protein during the passage time of about 100 ns. The energy exchange broadens the ensemble spectrum. Because the exchange involves the free-energy landscape of the protein, the QENS not only provides insight into the protein dynamics, but it may also illuminate the free-energy landscape of the protein-solvent system.
准弹性非相干中子散射（QENS）是探索生物分子、液体和玻璃等复杂系统动力学的重要工具。动力学反映在散射中子的能谱中。传统上，这些能谱被分解为一条窄的弹性线和一个宽的准弹性带。该带被解释为由目标分子的空间运动引起的多普勒展宽。我们提出了一个量子力学模型，其中不存在单独的弹性线。准弹性带由具有两倍自然线宽的锐线组成，这些锐线在目标分子的自由能景观中因蛋白质的随机游走而偏离中心。这种游走由振动和外部涨落驱动。我们首先用穆斯堡尔效应探索该模型。在随后应用于QENS时，我们将入射中子视为德布罗意波包。当波包穿过蛋白质中的质子和水合壳时，它在大约100纳秒的通过时间内与蛋白质交换能量。能量交换拓宽了系综谱。由于这种交换涉及蛋白质的自由能景观，QENS不仅能深入了解蛋白质动力学，还可能阐明蛋白质 - 溶剂系统的自由能景观。
