Toft Lindkvist Thomas, Djavani-Tabrizi Iden, Chen Li, Brøndsted Nielsen Steen
Department of Physics and Astronomy, Aarhus University, Aarhus, Denmark.
State Key Laboratory and Institute of Elemento-Organic Chemistry College of Chemistry, Nankai University, Tianjin, 300071, P. R. China.
Chempluschem. 2024 Dec;89(12):e202400448. doi: 10.1002/cplu.202400448. Epub 2024 Oct 23.
Various techniques are available to illuminate geometric structures of molecular ions in gas phase, such as Förster Resonance Energy Transfer (FRET) informing on distances between two dyes covalently attached to a molecule. Typically, cationic rhodamines, which absorb and emit visible light, are used for labeling. Extensive work has revealed that the transition energy of a rhodamine is intricately linked to its nearby microenvironment, with nearby charges causing Stark-shifted emission. This occurs because the inter-dye Coulomb interaction is weaker in the excited state (S) than in the ground state (S) due to the increase in polarizability upon excitation. Therefore, absorption and emission spectra, along with FRET efficiencies, provide insights into structural motifs. At room temperature, multiple conformers often co-exist, leading to overlapping absorption bands among different conformers and broad spectra. To study specific conformers, it is necessary to isolate them, for example, using ion-mobility spectrometry. Another approach is to reduce temperature, which results in spectral narrowing and distinct absorption bands, allowing for the selection of specific conformers through selective excitation. Here, we describe the instrumentation used for cryogenically cold FRET experiments and discuss recent results for small model systems, as well as future directions for a technique still in its infancy.
有多种技术可用于照亮气相中分子离子的几何结构，例如福斯特共振能量转移（FRET），它能提供有关共价连接到分子上的两种染料之间距离的信息。通常，吸收和发射可见光的阳离子罗丹明用于标记。大量研究表明，罗丹明的跃迁能量与其附近的微环境密切相关，附近的电荷会导致斯塔克位移发射。之所以会这样，是因为激发态（S）中的染料间库仑相互作用比基态（S）中的弱，这是由于激发时极化率增加所致。因此，吸收光谱、发射光谱以及FRET效率能提供有关结构基序的信息。在室温下，多种构象异构体常常共存，导致不同构象异构体之间的吸收带重叠且光谱较宽。为了研究特定的构象异构体，有必要将它们分离出来，例如使用离子迁移谱法。另一种方法是降低温度，这会导致光谱变窄并出现明显的吸收带，从而可以通过选择性激发来选择特定的构象异构体。在此，我们描述用于低温冷FRET实验的仪器，并讨论小型模型系统的最新结果以及这项仍处于起步阶段的技术的未来发展方向。
