The relationship between ion mobility and collision cross section of peptides and proteins: an experimental and theoretical study

离子迁移率光谱分析技术是由Cohen和Karasek于1970年开发的，作为早期气相离子化学研究的传感器。此后，它被用于检测各种分析物，包括非法药物、化学战剂、爆炸物和环境污染物。离子迁移率是衡量气相离子在电场影响下通过缓冲气体的速度，这取决于两个因素：离子的旋转平均碰撞横截面和离子上存在的电荷。通过测量离子通过已知距离的漂移时间，就有可能以一定程度的精度确定其碰撞截面。在漂移场为直流电位的仪器中，很容易找到离子的碰撞截面与测量的平均漂移时间之间的关系。实验碰撞截面可以与从其他结构研究中获得的坐标预测的截面进行比较，或者从计算测量中获得原子详细的构象信息。离子迁移谱仪中离子的漂移时间与它们的气相碰撞截面之间的关系是很容易理解的，近年来离子迁移谱法与质谱联用（IM-MS）作为一种结构分析工具，特别是用于揭示生物分子的构象，已经变得特别重要。随着软电离方法的发展，生物相关物种的IM-MS研究开始于20世纪90年代中后期，在家用仪器上结合了这两种著名的分析技术。这一时期一些最有影响力的工作是由鲍尔斯、贾罗德、克莱默和希尔完成的，他们的研究为其他人铺平了道路，并促进了商业上可用的移动设备的发展，因为这种技术在生物分析方面的力量变得明显。Waters MS Technologies （Manchester， UK）最近推出了第一款商用集成IM-MS仪器Synapt HDMS。射频应用于离子迁移率分离器内堆叠环形离子波导中的连续电极，提供了一个电位阱，使离子径向限制在器件内。为了推动离子通过该装置，在射频电压的顶部叠加了由一系列瞬态直流电压组成的行波，因此该装置有时被称为行波离子波导- TWIG。这个电压依次施加到一对环形电极上，提供一个可以推动离子通过器件的电位。这些商用设备已经得到了很好的应用。Robinson等人使用基于TWIG的系统评估了多聚体蛋白质的构象，并通过观察单体单元的部分展开同时仍保留了复合物的一些完整性来拆卸复合物。Synapt与家用仪器相比的优点是无可争议的，占空比更短，通过该仪器的传输效率优于大多数家用设备。然而，要从碰撞截面的角度合理地调整Synapt仪器上获得的实验漂移时间，需要仔细校准线性离子迁移率仪器上获得的数据，例如鲍尔斯和克莱默开发的仪器，也存在于Barran实验室。其中一个问题是，可用的碰撞截面数据的蛋白质是有限的，也往往没有很好地验证。这意味着，尽管人们对Synapt的应用非常感兴趣，对复杂的生物结构进行了研究，并且进行了漂亮的初步工作，但结果仍然有些“未经证实”。参见下面的研究策略。

The analytical technique of ion mobility spectrometry was developed by Cohen and Karasek in 1970 as a sensor building on earlier gas-phase ion chemistry investigations. It has since been used to detect a wide range of analytes including illegal drugs, chemical warfare agents, explosives and environmental pollutants. Ion mobility is a measure of how quickly a gas phase ion moves through a buffer gas under the influence of an electric field, and this depends on two factors: the rotationally averaged collision cross section of the ion and the charge present on it. By measuring the drift time of an ion through a known distance it is possible to determine its collision cross section with some degree of accuracy. In instruments where the drift field is a dc potential, the relationship between the collision cross section of an ion and the measured average drift time can be easily found. The experimental collision cross section can be compared to cross sections predicted from co-ordinates obtained from other structural investigations, or from computational measurements to obtain atomistically detailed conformational information. The relationship between the drift times of ions in a ion mobility spectrometer and their gas-phase collision cross section, is well understood, and in recent years ion mobility spectrometry coupled with mass spectrometry (IM-MS) has gained particular importance as a tool for structural analysis and particularly for its use to reveal conformation of biological molecules. After developments in soft ionization methods, IM-MS studies of biological relevant species started in the mid and late 1990's on home built instruments which coupled these two well known analytical techniques. Some of the most influential work in this period was performed by Bowers, Jarrold, Clemmer, and Hill and their investigations have paved the way for others and prompted development of commercially available mobility devices, as the power of this technique for biological analysis became apparent. Waters MS Technologies (Manchester, UK) recently introduced the first commercially available integrated IM-MS instrument the Synapt HDMS. The RF applied to consecutive electrodes in the stacked ring ion guide within the ion mobility separator, provides a potential well which keeps the ions radially confined within the device. In order to propel the ions through the device, a travelling wave comprising a series of transient DC voltages is superimposed on top of the RF voltage, and hence this device is sometimes referred to as a Travelling Wave Ion Guide - TWIG. This voltage is applied sequentially to pairs of ring electrodes providing a potential which can push ions through the device. These commercial available devices have already been used to good effect. Using a TWIG based system, Robinson et al. have assessed conformations of multimeric proteins, and also the disassembly of complexes viewing the partial unfolding of monomer units whilst still retaining some the integrity of the complex. The benefits of the Synapt compared to home built instruments are undisputed, the duty cycles are shorter and the transmission efficiency through this instrument is better than with most home made devices. However, to properly rationalize experimental drift times obtained on Synapt instrumentations in terms of collision cross sections, requires careful calibration with data obtained on a linear ion mobility instrument, such as that developed by Bowers and Clemmer and also present in the lab of Barran. One of the issues with this is that the available collision cross section data for proteins is limited, and also often not well verified. This means that despite the extreme interest in the application of the Synapt to interrogate complex biological structures, and beautiful preliminary work, results are still somewhat 'unverified' This studentship will seek to address this in several ways. See the Research Strategy below.