Development of Electron Based Dissociation Techniques in Mass Spectrometry for the Structural Characterisation of Small Organic Ions and Modified Proteins

Abstract

The work detailed herein describes the developments made using electrons to initiate bond dissociation via electronic excitation. Electron Capture Dissociation (ECD) uses low energy electrons to analyse multiply charged cations, providing a complementary series of product ions to vibrational excitation techniques Collision-Induced Dissociation (CID) and Infrared Multiphoton Dissociation (IRMPD). ECD has been adapted to analyse small singly charged ions by increasing the electron energy, known as Electron-Induced Dissociation (EID). The effect of electron energy has been studied, indicating optimal results occurring at 18 - 20 eV. EID has been carried out on a range of small organic molecules, resulting in a high degree of fragmentation. EID results suggest that bond dissociation can occur via multiple dissociation mechanisms, forming a combination of odd-electron and even-electron species, resulting in a unique set of product ions. Developments have been made to combine Liquid Chromatography (LC) with EID in order to analyse complex mixtures of small organic molecules with a wide dynamic concentration range. In-depth analysis has been carried out by LC-EID and LC-CID on the pharmaceutical reaction mixture of cediranib, allowing structural inferences to be made for the ten unknown low abundance components observed in the sample, proving each compound to be analogous to cediranib.

Investigations of protein modifications have focussed on two proteins; the Multiple transferrable resistance Regulator (MtrR) protein and the Matrix (M) protein. ECD and CID successfully identified and located an insertion on the flexible N-terminus of the M protein that could not be resolved by X-ray crystallography. Mass spectrometry analysis has been used to identify the chemical alterations of both proteins resulting from reactions with small molecules, and ECD has been used to confirm the location of the reaction site for one modified peptide.