Dynamic motion in crown ether dendrimer complexes: a "spacewalk" on the molecular scale.
Dynamic motion in crown ether dendrimer complexes: a "spacewalk" on the molecular scale.
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冠醚树枝状聚合物复合物的动态运动：分子尺度上的“太空行走”。
DOI：
10.1002/anie.200902437
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发表时间：
2009
影响因子：
--
通讯作者：
C. Schalley
中科院分区：
文献类型：
--
作者：
H. D. Winkler;D. P. Weimann;A. Springer;C. Schalley
布朗运动、分子的旋转和分子内的振动是热运动的典型形式。快速的化学平衡，如氨氮原子的反转，烷烃中构象异构体的相互转化，或在弱结合的非共价复合物中的高度动态的缔合/解离过程也是热诱导的。在非共价复合物的背景下，它是迷人的，以检查是否复杂的客体分子之间的多主题主机的不同结合位点的迁移是可能的，以及如何像这样的运动可以监测。本文中，前五代（G1-G5）聚氨基丙烯胺（POPAM）树枝状聚合物充当原型多主题宿主。我们解决的问题，冠醚是否可以直接从结合位点移动到树枝状聚合物周边的结合位点，而没有中间的解离/重新缔合（图1）。此外，如果这种分子“太空行走”确实是可能的，它提出了一个问题，即它是通过什么机制进行的。在解决方案中，这种复合物内结合位点跳跃的检测是具有挑战性的，如果不是不可能的，因为它总是叠加的解离/重新缔合平衡。因此，有必要将复合物彼此分离并与相应的游离结构单元分离，以抑制任何复合物间的客体交换反应。质谱仪内部的高真空非常适合于实现复合物的分离，因为那里的复合物是带相同电荷的，因此通过电荷排斥有效地彼此分离。此外，可以排除与中性冠醚分子的反应。冠醚/树枝状大分子复合物的碎片将是中性冠醚在气相中出现的唯一来源。因此，它们的分压太低，以至于在它们被泵走之前在仪器内停留的短时间内无法有效地重新附着。然而，这种方法带来的困难是，任何分子内过程都不会改变络合离子的分子质量，因此仍然无法通过简单测定质荷比（m/z）来检测。因此，需要气相反应来探测客体的运动。这样的反应必须a）在低于络合物解离能的能量下进行，B）引起质量移动，和c）与客体运动直接相关。为了实现这一想法，我们选择POPAM树枝状聚合物作为多主题支架。这些树枝状聚合物具有高度支化的洋葱层型结构（图1）。从每一代（Gn）到下一代，外围氨基的数量从G1树枝状聚合物中的4个增加到G5中的64个。详细研究了它们的气相化学。在不存在溶剂化剂的情况下，质子化可能发生在内部叔胺而不是外围伯NH 2基团处。研究树枝状分子在气相中的主客体化学通常是一个具有挑战性的和基本上未探索的研究领域。到目前为止，只有几个例子。在我们的研究中，[18]冠-6作为客体，它在溶液和气相中与伯铵离子结合。树枝状冠醚/铵复合物如图1所示。[18]冠-6和第四代（G4）POPAM树枝状聚合物的化学结构。开始与1，4-二氨基丁烷核心，分支的第n壳发散地生长在（n-1）代树枝状聚合物通过两个迈克尔加成的丙烯腈到每个分支和随后的氢解还原的腈基团。红色箭头表示本研究的主要问题：冠醚能否沿着POPAM树枝状聚合物的外围自由移动而不发生复合物的中间解离？由于这一过程是在质谱仪内部的高真空中进行的，我们称之为分子“太空行走”。
Brownian motion, the rotation of molecules, and vibrations within molecules are typical forms of thermal motion. Fast chemical equilibria, such as the inversion at the ammonia nitrogen atom, the interconversion of conformers in alkanes, or highly dynamic association/dissociation processes in weakly bound noncovalent complexes are also thermally induced. In the context of noncovalent complexes, it is fascinating to examine whether an intracomplex migration of a guest molecule between different binding sites of a multitopic host is possible and how a motion like this could be monitored. Herein, the first five generations (G1–G5) of polyamino propylene amine (POPAM) dendrimers serve as prototypical multitopic hosts. We address the question, whether crown ethers can directly move from binding site to binding site on the dendrimers periphery without intermediate dissociation/reassociation (Figure 1). Furthermore, if this molecular “spacewalk” is indeed possible, it raises the question as to by what mechanism it proceeds. In solution, the detection of such an intracomplex binding-site hopping is challenging if not impossible, because it is always superimposed by dissociation/reassociation equilibria. Therefore, it is necessary to isolate the complexes from each other and from the corresponding free building blocks to suppress any intercomplex guest-exchange reactions. The high vacuum inside a mass spectrometer is ideally suited to achieve the isolation of the complexes as the complexes there are like-charged and thus efficiently separated from each other by charge repulsion. Also, reactions with neutral crown ether molecules can be excluded. Fragmentation of the crown ether/dendrimer complexes would be the only source for the appearance of neutral crown ethers in the gas phase. Therefore, their partial pressure is much too low to result in an efficient reattachment during the short time they spend inside the instrument before being pumped away. However, this approach comes with the difficulty that any intramolecular process does not change the complex ion s molecular mass and thus remains undetectable by a simple determination of the mass-to-charge ratio (m/z). Therefore, a gas-phase reaction is required that probes the guest s motion. Such a reaction must a) proceed energetically below the complex dissociation energy, b) cause a mass shift, and c) be directly linked to the guest movement. To realize this idea, we chose POPAM dendrimers as the multitopic scaffold. These dendrimers have highly branched onion-layer-type structures (Figure 1). From each generation (Gn) to the next, the number of peripheral amino groups doubles from four in theG1 dendrimer to 64 inG5. Their gasphase chemistry has been studied in detail. In the absence of a solvating agent, protonation is likely to occur at interior tertiary amines rather than the peripheral primary NH2 groups. To examine the host–guest chemistry of dendritic molecules in the gas phase is generally a challenging and byand-large unexplored field of research. Only a few examples exist to date. In our study, [18]crown-6 serves as the guest, it binds to primary ammonium ions in solution, and in the gas phase. Dendritic crown ether/ammonium complexes are Figure 1. Chemical structure of [18]crown-6 and a fourth generation (G4) POPAM dendrimer. Starting with a 1,4-diaminobutane core, the nth shell of branches is divergently grown on the (n 1)th generation dendrimer by two Michael additions of acrylnitrile to each branch and subsequent hydrogenolytic reduction of the nitrile groups. The red arrows symbolize the main question of the present study: Can crown ethers move freely along the periphery of POPAM dendrimers without intermediate dissociation of the complex? As this process proceeds in the high vacuum inside a mass spectrometer, we refer to it as a molecular “spacewalk”.