Electron-transfer dissociation

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An ion trap mass spectrometer with electron transfer dissociation capability Bruker Amazon Speed ETD.jpg
An ion trap mass spectrometer with electron transfer dissociation capability
Peptide fragmentation notation Peptide fragmentation.gif
Peptide fragmentation notation

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). [1] Similar to electron-capture dissociation, ETD induces fragmentation of large, multiply-charged cations by transferring electrons to them. [2] ETD is used extensively with polymers and biological molecules such as proteins and peptides for sequence analysis. [3] Transferring an electron causes peptide backbone cleavage into c- and z-ions while leaving labile post translational modifications (PTM) intact. [4] The technique only works well for higher charge state peptide or polymer ions (z>2). [2] However, relative to collision-induced dissociation (CID), ETD is advantageous for the fragmentation of longer peptides or even entire proteins. [5] This makes the technique important for top-down proteomics. The method was developed by Hunt and coworkers at the University of Virginia. [6]

Contents

History

Electron-capture dissociation (ECD) was developed in 1998 to fragment large proteins for mass spectrometric analysis. [7] Because ECD requires a large amount of near-thermal electrons (<0.2eV), originally it was used exclusively with Fourier transform ion cyclotron resonance mass spectrometry (FTICR), the most expensive form of MS instrumentation. [8] Less costly options such as quadrupole time-of-flight (Q-TOF), quadrupole ion trap (QIT) and linear quadrupole ion trap (QLT) instruments used the more energy-intensive collision-induced dissociation method (CID), resulting in random fragmentation of peptides and proteins. [9] In 2004 Syka et al. announced the creation of ETD, a dissociation method similar to ECD, but using a low-cost, widely available commercial spectrometer. The first ETD experiments were run on a QLT mass spectrometer with an electrospray ionization (ESI) source. [10]

Principle of operation

Several steps are involved in electron transfer dissociation. Usually a protein mixture is first separated using high performance liquid chromatography (HPLC). Next multiply-protonated precursor molecules are generated by electrospray ionization and injected into the mass spectrometer. (Only molecules with a charge of 2+ or greater can be used in ETD.) In order for an electron to be transferred to the positive precursor molecules radical anions are generated and put into the ion trap with them. During the ion/ion reaction an electron is transferred to the positively-charged protein or peptide, causing fragmentation along the peptide backbone. Finally the resultant fragments are mass analyzed. [11]

Radical anion preparation

In the original ETD experiments anthracene (C14H10) was used to generate reactive radical anions through negative chemical ionization. [10] Several polycyclic aromatic hydrocarbon molecules have been used in subsequent experiments, with fluoranthene currently the preferred reagent. [12] Fluoranthene has only about 40% efficiency in electron transfer, however, so other molecules with low electron affinity are being sought. [11]

Injection and fragmentation

Multiply-charged precursor ion reacts with radical anion ETD cartoon.tiff
Multiply-charged precursor ion reacts with radical anion

When the precursor cations (proteins or peptides) and radical anions are combined in the ion trap an electron is transferred to the multiply-charged cation. This forms an unstable positive radical cation with one less positive charge and an odd electron. [13] Fragmentation takes place along the peptide backbone at a N− Cα bond, resulting in c- and z-type fragment ions. [14]

Protein or peptide radical cation fragments into c-ion and z-ion ETD Fragmentation.tiff
Protein or peptide radical cation fragments into c-ion and z-ion

Mass analysis

Fragmentation caused by ETD allows more complete protein sequence information to be obtained from ETD spectra than from CID tandem mass spectrometry. Because many peptide backbone c- and z- type ions are detected, almost complete sequence coverage of many peptides can be discerned from ETD fragmentation spectra. [15] Sequences of 15-40 amino acids at both the N-terminus and the C-terminus of the protein can be read using mass-to-charge values for the singly and doubly charged ions. These sequences, together with the measured mass of the intact protein, can be compared to database entries for known proteins and to reveal post-translational modifications. [16]

Instrumentation

Schematic diagram of LTQ with ETD LTQ schematic.tiff
Schematic diagram of LTQ with ETD
Bruker high capacity ion trap with ETD (schematic diagram) Bruker HCT-schematicJune2008.PNG
Bruker high capacity ion trap with ETD (schematic diagram) 

Electron transfer dissociation takes place in an ion trap mass spectrometer with an electrospray ionization source. The first ETD experiments at the University of Virginia utilized a radio frequency quadrupole linear ion trap (LQT) modified with a chemical ionization (CI) source at the back side of the instrument (see diagram at right). [10] Because a spectrum can be obtained in about 300 milliseconds, liquid chromatography is often coupled with the ETD MS/MS. [11] The disadvantage of using LQT is that the mass resolving power is less than that of other mass spectrometers. [14]

Subsequent studies have tried other instrumentation to improve mass resolution. Having a negative CI source at the back of the instrument interfered with the high-resolution analyzer in LQT-Orbitrap and quadrupole time-of-flight (QTOF), so alternate ionization methods for the radical anions have been introduced. [11]

In 2006 a group at Purdue University led by Scott McLuckey used a quadrupole/time-of-flight (QqTOF) tandem mass spectrometer with pulsed nano-ESI/atmospheric pressure chemical ionization (APCI) dual ionization source using radical anions of 1,3-dinitrobenzene as the electron donor. [17] Later a lab at the University of Wisconsin adapted a hybrid quadrupole linear ion trap-orbitrap mass spectrometer to use ETD. This method also used a front-end ionization method for the radical anions of 9-anthracenecarboxylic acid via pulsed dual ESI sources. [18]

As ETD is increasingly popular for protein and peptide structure analysis, implementation on easily available ion-trap mass spectrometers coupled with high resolution mass analyzers continues to evolve. [19]

Applications

Proteomics

ETD is widely used in the analysis of protein and large peptides. Important post translational modifications including phosphorylation, glycosylation and disulfide linkages are all analyzed using ETD. [20]

Polymer chemistry

Although MS-based analyses of polymers have largely been performed using single-stage MS, tandem MS has also been used to characterize polymer components. CID is the most common method of dissociation used, but ETD has been used as a complementary method. Unique bond cleavages resulting from ETD supply valuable diagnostic information. [2]

See also

Related Research Articles

Mass spectrometry (MS) is an analytical technique that measures the mass-to-charge ratio of ions. The results are typically 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.

Electron ionization Ionization technique

Electron ionization is an ionization method in which energetic electrons interact with solid or gas phase atoms or molecules to produce ions. EI was one of the first ionization techniques developed for mass spectrometry. However, this method is still a popular ionization technique. This technique is considered a hard ionization method, since it uses highly energetic electrons to produce ions. This leads to extensive fragmentation, which can be helpful for structure determination of unknown compounds. EI is the most useful for organic compounds which have a molecular weight below 600. Also, several other thermally stable and volatile compounds in solid, liquid and gas states can be detected with the use of this technique when coupled with various separation methods.

Tandem mass spectrometry

Tandem mass spectrometry, also known as MS/MS or MS2, is a technique in instrumental analysis where two or more mass analyzers are coupled together using an additional reaction step to increase their abilities to analyse chemical samples. A common use of tandem-MS is the analysis of biomolecules, such as proteins and peptides.

Matrix-assisted laser desorption/ionization ionization technique that uses a laser energy absorbing matrix to create ions from large molecules with minimal fragmentation

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 large 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.

Electron-capture dissociation

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.

Protein 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.

Top-down proteomics

Top-down proteomics is a method of protein identification that either uses an ion trapping mass spectrometer to store an isolated protein ion for mass measurement and tandem mass spectrometry (MS/MS) analysis or other protein purification methods such as two-dimensional gel electrophoresis in conjunction with MS/MS. Top-down proteomics is capable of identifying and quantitating unique proteoforms through the analysis of intact proteins. The name is derived from the similar approach to DNA sequencing. During mass spectrometry intact proteins are typically ionized by electrospray ionization and trapped in a Fourier transform ion cyclotron resonance, quadrupole ion trap or Orbitrap mass spectrometer. Fragmentation for tandem mass spectrometry is accomplished by electron-capture dissociation or electron-transfer dissociation. Effective fractionation is critical for sample handling before mass-spectrometry-based proteomics. Proteome analysis routinely involves digesting intact proteins followed by inferred protein identification using mass spectrometry (MS). Top-down MS (non-gel) proteomics interrogates protein structure through measurement of an intact mass followed by direct ion dissociation in the gas phase.

Robert Graham Cooks is the Henry Bohn Hass Distinguished Professor of Chemistry in the Aston Laboratory of Mass Spectrometry at Purdue University. He is an ISI Highly Cited Chemist, with over 1,000 publications and an H-index of 94.

Laser spray ionization

Laser spray ionization refers to one of several methods for creating ions using a laser interacting with a spray of neutral particles or ablating material to create a plume of charged particles. The ions thus formed can be separated by m/z with mass spectrometry. Laser spray is one of several ion sources that can be coupled with liquid chromatography-mass spectrometry for the detection of larger molecules.

Triple quadrupole mass spectrometer

A triple quadrupole mass spectrometer (TQMS), is a tandem mass spectrometer consisting of two quadrupole mass analyzers in series, with a (non-mass-resolving) radio frequency (RF)–only quadrupole between them to act as a cell for collision-induced dissociation. This configuration is often abbreviated QqQ, here Q1q2Q3.

Electron capture ionization is the ionization of a gas phase atom or molecule by attachment of an electron to create an ion of the form . The reaction is

Selected reaction monitoring

Selected reaction monitoring (SRM) is a method used in tandem mass spectrometry in which an ion of a particular mass is selected in the first stage of a tandem mass spectrometer and an ion product of a fragmentation reaction of the precursor ion is selected in the second mass spectrometer stage for detection.

Linear ion trap

The linear ion trap (LIT) is a type of ion trap mass spectrometer. In a linear ion trap, ions are confined radially by a two-dimensional radio frequency (RF) field, and axially by stopping potentials applied to end electrodes. Linear ion traps have high injection efficiencies and high ion storage capacities.

Fragmentation (mass spectrometry) of chemical structures

In mass spectrometry, fragmentation is the dissociation of energetically unstable molecular ions formed from passing the molecules in the ionization chamber of a mass spectrometer. The fragments of a molecule cause a unique pattern in the mass spectrum. These reactions are well documented over the decades and fragmentation pattern is useful to determine the molar weight and structural information of the unknown molecule.

Joshua Coon is a professor of chemistry and biomolecular chemistry and the inaugural holder of the Thomas and Margaret Pyle Chair at the University of Wisconsin–Madison, and an affiliate of the Morgridge Institute for Research.

Collision-induced dissociation A 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.

In bio-informatics, a peptide-mass fingerprint or peptide-mass map is a mass spectrum of a mixture of peptides that comes from a digested protein being analyzed. The mass spectrum serves as a fingerprint in the sense that it is a pattern that can serve to identify the protein. The method for forming a peptide-mass fingerprint, developed in 1993, consists of isolating a protein, breaking it down into individual peptides, and determining the masses of the peptides through some form of mass spectrometry. Once formed, a peptide-mass fingerprint can be used to search in databases for related protein or even genomic sequences, making it a powerful tool for annotation of protein-coding genes.

In mass spectrometry, de novo peptide sequencing is the method in which a peptide amino acid sequence is determined from tandem mass spectrometry.

Matrix-assisted ionization

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. Initial ionization occurs as the pressure drops within the inlet tube.

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