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

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.