Solid–solid phase transitions (SSPTs) have been widely observed in crystals of organic or inorganic small-molecules. Although SSPTs in macromolecular crystals have been reported, the majority involve local atomic changes, such as those induced by changes in hydration. SSPTs driven by large conformational changes, however, can be more difficult to characterize since they often significantly disrupt lattice packing interactions. Such drastic changes make the cooperativity of molecular motion at the atomic level less easily achieved and more dependent on intrinsic properties of the crystal that define lattice order. Here, we investigate the effect of crystal size on the uniformity of SSPT in thin plate-like crystals of the adenine riboswitch aptamer RNA (riboA) by monitoring changes in crystal birefringence upon the diffusion of adenine ligand. The birefringence intensity is directly related to molecular order and the concurrent changes to polarizability of molecules that results from structural changes throughout the phase transition. The riboA crystals were loosely grouped into three categories (small, medium, and large) based on the surface area of the crystal plates. The time width of transition increased as a function of crystal size, ranging from ∼13 s for small crystals to ∼40 s for the largest crystal. Whereas the transitions in small crystals (<10 μm²) were mostly uniform throughout, the medium and large crystals exhibited large variations in the time and width of the transition peak depending on the region of the crystal being analyzed. Our study provides insight into the spatiotemporal behavior of phase transitions in crystals of biological molecules and is of general interest to time-resolved crystallographic studies, where the kinetics of conformational changes may be governed by the kinetics of an associated SSPT.

中文翻译：

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相变均匀性对使用双折射表征的晶体尺寸的依赖性

固-固相变（SSPT）已在有机或无机小分子晶体中广泛观察到。尽管大分子晶体中的 SSPT 已有报道，但大多数涉及局部原子变化，例如由水合作用变化引起的原子变化。然而，由大构象变化驱动的 SSPT 可能更难以表征，因为它们通常会显着破坏晶格堆积相互作用。这种剧烈的变化使得原子水平上分子运动的协同性更不容易实现，并且更依赖于定义晶格顺序的晶体的固有特性。在这里，我们通过监测腺嘌呤配体扩散时晶体双折射的变化，研究晶体尺寸对腺嘌呤核糖开关适体 RNA (riboA) 薄板状晶体中 SSPT 均匀性的影响。双折射强度与分子顺序以及整个相变过程中结构变化导致的分子极化率的同时变化直接相关。根据晶体板的表面积，riboA 晶体被松散地分为三类（小、中和大）。转变时间宽度随着晶体尺寸的变化而增加，范围从小晶体的~13秒到最大晶体的~40秒。尽管小晶体 (<10 μ m ² )中的转变 基本上是均匀的，但中型和大晶体在转变峰的时间和宽度方面表现出很大的变化，具体取决于所分析的晶体区域。我们的研究提供了对生物分子晶体相变时空行为的深入了解，并且对时间分辨晶体学研究具有普遍意义，其中构象变化的动力学可能受相关 SSPT 动力学的控制。