Electron-capture dissociation

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Schematic diagram of the combined ECD FTICRMS and IRMPD experimental setup Schematic diagram of the combined ECD FTICRMS and IRMPD experimental setup.png
Schematic diagram of the combined ECD FTICRMS and IRMPD experimental setup

Electron-capture dissociation (ECD) is a method of fragmenting gas-phase ions for structure elucidation of peptides and proteins in tandem mass spectrometry. It is one of the most widely used techniques for activation and dissociation of mass selected precursor ion in MS/MS. It involves the direct introduction of low-energy electrons to trapped gas-phase ions. [1] [2]

Contents

History

Electron-capture dissociation was developed by Roman Zubarev and Neil Kelleher while in Fred McLafferty's lab at Cornell University. Irradiation of melittin 4+ ions and ubiquitin 10+ ions (trapped in FT-MS cell) by laser pulses not only resulted in peculiar c', z fragmentation but also charge reduction. It was suggested that if FT cell is modified to trap cations and electrons simultaneously, secondary electrons emitted by UV photons increases the charge reduction effect and c′, z• fragmentation. Replacing UV laser with EI source led to the development of this new technique. [3]

Principles

Electron-capture dissociation typically involves a multiply protonated molecule M interacting with a free electron to form an odd-electron ion. Liberation of the electric potential energy results in fragmentation of the product ion.

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Rate of electron capture dissociation not only depends on the frequency of ion–electron fragmentation reactions but also on the number of ions in an ion–electron interaction volume. Electron current density and cross-section of ECD is directly proportional to fragmentation frequency. [4] [5] An indirectly heated dispenser cathode used as an electron source results in larger electron current and larger emitting surface area. [6] [7]

ECD devices can be of two forms. It can trap analyte ions during the ECD stage or can undergo flow through mode where dissociation takes place as analyte ions flows continuously through the ECD region. Flow through mode has advantage over other mode because nearly all the analyte ion beam is used. However, that decreases the efficiency of ECD for flow through mode. [8]

ECD produces significantly different types of fragment ions (although primarily c- and z-type, b-ions have been identified in ECD [9] ) than other MS/MS fragmentation methods such as electron-detachment dissociation (EDD) (primarily a and x types), [10] [11] [12] collision-induced dissociation (CID) (primarily b [13] and y type) and infrared multiphoton dissociation. CID and IRMPD introduce internal vibrational energy in some way or another, causing loss of post-translational modifications during fragmentation. In ECD, unique fragments (and complementary to CID) are observed, [14] and the ability to fragment whole macromolecules effectively has been promising.

Although ECD is primarily used in Fourier transform ion cyclotron resonance mass spectrometry, [15] investigators have indicated that it has been successfully used in an ion-trap mass spectrometer. [16] [17] [18] ECD can also do rapid integration of multiple scans in FTICR-MS if put in a combination with external accumulation. [6]

ECD is a recently introduced MS/MS fragmentation technique and is still being investigated. [19] [20] The mechanism of ECD is still under debate but appears not to necessarily break the weakest bond and is therefore thought to be a fast process (nonergodic) where energy is not free to relax intramolecularly. Suggestions have been made that radical reactions initiated by the electron may be responsible for the action of ECD. [21] In a similar MS/MS fragmentation technique called electron-transfer dissociation, the electrons are transferred by collision between the analyte cations and reagent anions. [22] [23] [24]

Applications

Disulfide bond cleavage

ECD itself and combined with other MS is very useful for proteins and peptides containing multiple disulfide bonds. FTICR combined with ECD helps to recognize peptides containing disulfide bonds. ECD could also access important sequence information by activation of higher charged proteins. Moreover, disulfide bond cleavage takes place by ECD of multiply charge proteins or peptides produced by ESI. [25] Electron capture by these proteins releases H atom, captured by the disulfide bond to cause its dissociation. [26]

ECD with UV-based activation increases the top-down MS sequence coverage of disulfide bond containing proteins and cleaves a disulfide bond homolytically to produce two separated thiol radicals. This technique was observed with insulin and ribonuclease, which led them to cleave up to three disulfide bonds and increase the sequence coverage. [27]

Post-translational modifications

ECD-MS fragments can retain posttranslational modifications such as carboxylation, phosphorylation [28] [29] and O-glycosylation. [6] [30] [31] ECD has the potential to do the top-down characterization of the major types of posttranslational modifications in proteins. It successfully cleaved 87 of 208 backbone bonds and provided the first direct characterization of a phosphoprotein, bovine β casein, simultaneously restricting the location of five phosphorylation sites. It has advantages over CAD to measure the degree of phosphorylation with a minimum number of losses of phosphates and for phosphopeptide/phosphoprotein mapping, which makes ECD a superior technique. [32]

Schematic diagram of Atmospheric pressure electron capture dissociation (AP-ECD) source Schematic of AP-ECD source.png
Schematic diagram of Atmospheric pressure electron capture dissociation (AP-ECD) source

Coupling of ECD with separation techniques

ECD has been coupled with capillary electrophoresis (CE) to gain insight into structural analysis of mixture of peptides and protein digest. [33] Micro-HPLC combined with ECD FTICR was used to analyze pepsin digest of cytochrome c. [34] Sequence tags were provided by analysis of a mixture of peptides and tryptic digest of bovine serum albumin when LC ECD FTICR MS was used. [35] Additionally, LC-ECD-MS/MS is provides longer sequence tags than LC-CID-MS/MS for identification of proteins. [14] ECD devices using radio frequency quadrupole ion trap are relevant for high-throughput proteomics. [36] [8] Recently, Atmospheric pressure electron capture dissociation (AP-ECD) is emerging as a better technique because it can be implemented as a stand-alone ion-source device and doesn't require any modification of the main instrument. [37] [38]

Proteomics

Analysis of proteins can be done by either using top-down or bottom-up approach. However, better sequence coverage is provided by top-down analysis. [39] Combination of ECD with FTICR MS has resulted in popularity of this approach. It has also helped in determining the multiple modification sites in intact proteins. [40] [41] Native electron capture dissociation (NECD) was used to study cytochrome c dimer [42] and has been recently used to elucidate iron-binding channels in horse spleen ferritin. [43]

Synthetic polymers

ECD studies of polyalkene glycols, polyamides, polyacrylates and polyesters are useful for understanding composition of polymer samples. It has become a powerful technique to analyze structural information about precursor ions during MS/MS for synthetic polymers. ECD's single bond cleavage tendency makes the interpretation of product ion scans simple and easy for polymer chemistry. [44]

See also

Related Research Articles

<span class="mw-page-title-main">Mass spectrometry</span> Analytical technique based on determining mass to charge ratio of ions

Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a mass spectrum, a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is used in many different fields and is applied to pure samples as well as complex mixtures.

<span class="mw-page-title-main">Tandem mass spectrometry</span> Type of mass spectrometry

Tandem mass spectrometry, also known as MS/MS or MS2, is a technique in instrumental analysis where two or more stages of analysis using one or more mass analyzer are performed with an additional reaction step in between these analyses to increase their abilities to analyse chemical samples. A common use of tandem MS is the analysis of biomolecules, such as proteins and peptides.

Fourier-transform ion cyclotron resonance mass spectrometry is a type of mass analyzer (or mass spectrometer) for determining the mass-to-charge ratio (m/z) of ions based on the cyclotron frequency of the ions in a fixed magnetic field. The ions are trapped in a Penning trap (a magnetic field with electric trapping plates), where they are excited (at their resonant cyclotron frequencies) to a larger cyclotron radius by an oscillating electric field orthogonal to the magnetic field. After the excitation field is removed, the ions are rotating at their cyclotron frequency in phase (as a "packet" of ions). These ions induce a charge (detected as an image current) on a pair of electrodes as the packets of ions pass close to them. The resulting signal is called a free induction decay (FID), transient or interferogram that consists of a superposition of sine waves. The useful signal is extracted from this data by performing a Fourier transform to give a mass spectrum.

<span class="mw-page-title-main">Matrix-assisted laser desorption/ionization</span> Ionization technique

In mass spectrometry, matrix-assisted laser desorption/ionization (MALDI) is an ionization technique that uses a laser energy-absorbing matrix to create ions from large molecules with minimal fragmentation. It has been applied to the analysis of biomolecules and various organic molecules, which tend to be fragile and fragment when ionized by more conventional ionization methods. It is similar in character to electrospray ionization (ESI) in that both techniques are relatively soft ways of obtaining ions of large molecules in the gas phase, though MALDI typically produces far fewer multi-charged ions.

Infrared multiple photon dissociation (IRMPD) is a technique used in mass spectrometry to fragment molecules in the gas phase usually for structural analysis of the original (parent) molecule.

<span class="mw-page-title-main">History of mass spectrometry</span>

The history of mass spectrometry has its roots in physical and chemical studies regarding the nature of matter. The study of gas discharges in the mid 19th century led to the discovery of anode and cathode rays, which turned out to be positive ions and electrons. Improved capabilities in the separation of these positive ions enabled the discovery of stable isotopes of the elements. The first such discovery was with the element neon, which was shown by mass spectrometry to have at least two stable isotopes: 20Ne and 22Ne. Mass spectrometers were used in the Manhattan Project for the separation of isotopes of uranium necessary to create the atomic bomb.

Hydrogen–deuterium exchange is a chemical reaction in which a covalently bonded hydrogen atom is replaced by a deuterium atom, or vice versa. It can be applied most easily to exchangeable protons and deuterons, where such a transformation occurs in the presence of a suitable deuterium source, without any catalyst. The use of acid, base or metal catalysts, coupled with conditions of increased temperature and pressure, can facilitate the exchange of non-exchangeable hydrogen atoms, so long as the substrate is robust to the conditions and reagents employed. This often results in perdeuteration: hydrogen-deuterium exchange of all non-exchangeable hydrogen atoms in a molecule.

<span class="mw-page-title-main">Electron-transfer dissociation</span>

Electron-transfer dissociation (ETD) is a method of fragmenting multiply-charged gaseous macromolecules in a mass spectrometer between the stages of tandem mass spectrometry (MS/MS). Similar to electron-capture dissociation, ETD induces fragmentation of large, multiply-charged cations by transferring electrons to them. ETD is used extensively with polymers and biological molecules such as proteins and peptides for sequence analysis. Transferring an electron causes peptide backbone cleavage into c- and z-ions while leaving labile post translational modifications (PTM) intact. The technique only works well for higher charge state peptide or polymer ions (z>2). However, relative to collision-induced dissociation (CID), ETD is advantageous for the fragmentation of longer peptides or even entire proteins. This makes the technique important for top-down proteomics. The method was developed by Hunt and coworkers at the University of Virginia.

<span class="mw-page-title-main">Protein mass spectrometry</span> Application of mass spectrometry

Protein mass spectrometry refers to the application of mass spectrometry to the study of proteins. Mass spectrometry is an important method for the accurate mass determination and characterization of proteins, and a variety of methods and instrumentations have been developed for its many uses. Its applications include the identification of proteins and their post-translational modifications, the elucidation of protein complexes, their subunits and functional interactions, as well as the global measurement of proteins in proteomics. It can also be used to localize proteins to the various organelles, and determine the interactions between different proteins as well as with membrane lipids.

<span class="mw-page-title-main">Top-down proteomics</span>

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<span class="mw-page-title-main">Ambient ionization</span>

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<span class="mw-page-title-main">Fragmentation (mass spectrometry)</span>

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<span class="mw-page-title-main">Collision-induced dissociation</span> Mass spectrometry technique to induce fragmentation of selected ions in the gas phase

Collision-induced dissociation (CID), also known as collisionally activated dissociation (CAD), is a mass spectrometry technique to induce fragmentation of selected ions in the gas phase. The selected ions are usually accelerated by applying an electrical potential to increase the ion kinetic energy and then allowed to collide with neutral molecules. In the collision, some of the kinetic energy is converted into internal energy which results in bond breakage and the fragmentation of the molecular ion into smaller fragments. These fragment ions can then be analyzed by tandem mass spectrometry.

<span class="mw-page-title-main">Atmospheric pressure photoionization</span> Soft ionization method

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<span class="mw-page-title-main">Matrix-assisted ionization</span>

In mass spectrometry, matrix-assisted ionization is a low fragmentation (soft) ionization technique which involves the transfer of particles of the analyte and matrix sample from atmospheric pressure (AP) to the heated inlet tube connecting the AP region to the vacuum of the mass analyzer.

Kristina Håkansson is an analytical chemist known for her contribution in Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry for biomolecular identification and structural characterization. Currently, she holds the position of Professor of Chemistry at University of Michigan. Her research focuses on mass spectrometry, primarily identification and characterization of protein posttranslational modifications by complementary fragmentation techniques such as electron-capture dissociation (ECD)/negative ion ECD (niECD) and infrared multiphoton dissociation (IRMPD) at low (femtomole) levels.

Ying Ge is a Chinese-American chemist who is a Professor of Cell and Regenerative Biology at the University of Wisconsin–Madison. Her research considers the molecular mechanisms that underpin cardiac disease. She has previously served on the board of directors of the American Society for Mass Spectrometry. In 2020 Ge was named on the Analytical Scientist Power List.

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