Roger N. Hayes

COLLISION-INDUCED DISSOCIATION

Publisher Summary This chapter discusses collision-induced dissociation. The determination of molecular structure by mass spectrometry requires that the molecule be ionized and undergo structurally informative fragmentations. Fragmentation may be induced by the ionization process itself or by some other means of excitation. Often the optimum methods of ion formation do not impart sufficient energy to cause appreciable fragmentation of the molecular ion. The internal energy of the ion in these circumstances may be increased by collisional activation (CA). Tandem mass spectrometry (MS/MS), in all its various configurations, has emerged as the most important technique for acquiring collisional-activated dissociation (CAD) [also termed collision-induced dissociation (CID)] mass spectra. The overall CAD process can be separated into two consecutive steps occurring on well-separated time scales. CAD mass spectra may be acquired on any of four distinct classes of mass spectrometer: magnet sector, quadrupole, time-of-flight (TOF), and Fourier transform (FT) mass spectrometers. There are also a number of hybrid configurations employing a combination of sector and quadrupole analyzers.

Collision-induced dissociations of deprotonated α-amino acids. The occurrence of specific proton transfers preceding fragmentation

Abstract The collision-induced fragmentations noted in the negative ion mass spectra of deprotonated α-amino acids are interpreted in terms of specific proton transfers to the carboxylate centre followed by simple fragmentations through ion complexes. In certain cases decarboxylation reactions occur, as well as internal nucleophilic displacement through the charged carboxylate centre.

Collision induced dissociations of deprotonated peptides: Dipeptides containing phenylalanine, tyrosine, histidine and tryptophan

Abstract The presence and position of Phe, Tyr, His and Trp residues in dipeptides may be determined from a consideration of the collisional activation mass spectra of the (M  H) − ions, formed by fast atom bombardment. Product ion and deuterium labelling studies have been used in an attempt to elucidate fragmentation mechanisms some of which are complex.

Collision induced dissociations of deprotonated peptides: dipeptides containing serine or threonine

The collisional activation mass spectra of (M — H)− ions from dipeptides containing serine or threonine show respective losses of CH2O or MeCHO from the α chains, thus allowing ready identification of these amino acids. The C terminal amino acid may be identified by the general dipeptide fragmentation NH2CH(R′)CONHCH(R2)CO−2 → NH2C(R1)CONHCH(R2)CO2H → −NHCH(R2)CO2H + NH2C(R1)CO, except in certain cases when the C terminal amino acid is threonine.

Elimination of Molecular Hydrogen and Methane from Collision-Activated Alkoxide Negative Ions in the gas Phase. Anab initioand Isotope Effect Study

Ab initio calculations indicate that the collisional induced losses of molecular hydrogen from the ethoxide negative ion and methane from the t- butoxide negative ion to be stepwise processes in which the key intermediates are [H-… MeCHO ] and [Me-…Me2CO] respectively. Deuterium kinetic isotope effects observed for these and other alkoxide negative ions are in accord with the operation of a stepwise reaction.

Negative ion fragmentations of deprotonated heterocycles. The isothiazole, thiazole, isoxazole, and oxazole ring systems

Abstract The major collision-induced dissociations of deprotonated isothiazole occur from the 5-anion, while deprotonated thiazole fragments almost equally through the 2- and 5-anions. Both 5-anions fragment by a simple retro cleavage yielding HC 2 S − and HCN. The 5-anion of isothiazole and the 2-anion of thiazole also rearrange to the common intermediate − SCHCHCN which decomposes by losses of H 2 , HCN and H 2 S. There is no evidence for direct interconversion of isothiazole and thiazole anions. The spectra of deprotonated methylisothiazoles are complex, but the major fragmentations are of ring deprotonated ions and are generally analogous to the parent systems. The fragmentation behaviour of deprotonated isoxazole and oxazole is analogous to that of the isothiazole and thiazole systems.

A new four-centre reaction of alkanol–alkoxide negative ions. The reaction of [RO–⋯ HOR] with alkoxysilanes. An ion cyclotron resonance study

Alkanol–alkoxide negative ions [R1O ⋯ H ⋯ OR2]– react with alkoxysilanes Me 3SiOR3 to produce both [M+R1O–] and [M+R2O–] ions of trigonal bipyramidal geometry. When R1 < R 2 and either R1 or R2 Pr, the four-centre reaction [R1O ⋯ H ⋯ OR2]–+ Me3SiOR3→[R2O ⋯ H ⋯ OR3]–+ Me3SiOR1 is observed. Addition of [R1O ⋯ H ⋯ OR2]– in the reverse direction is not detected. Analogous reactions do not occur between Me3SiX and [R1O ⋯ H ⋯ OR2]– when X = F, NHR, NR2,SiMe3, alkyl, allyl, propargyl, benzyl, or aryl, but [R1O ⋯ H ⋯ X]– ions of small abundance are formed when X = HO, OCOMe, OCN, and SR. Cyclic ethers react with alkanol–alkoxide negative ions by reaction (i;n= 2–4). [graphic ommitted]

Does the Lossen rearrangement occur in the gas phase

Deprotonated hydroxamic acids and cognate systems undergo a number of rearrangement processes upon collisional activation. It is proposed that ions RCONOH undergo the Lossen rearrangement to form [(OCNR)HO–], and that this reactive intermediate may decompose to form the ionic products HO–, [(R – H)–NCO], NCO– and RNH–. Alternatively, hydrogen transfer yields RCONHO– which may undergo a three-centre reaction to produce RCO2NH–; this species may decompose to yield both RCO2– and [(R – H)CO2]˙–. Fragmentation processes have been investigated by both deuterium labelling and product ion studies.

The four-centre reactions of alkanol-alkoxide negative ions with alkyl ethers and orthoacetates: an ion cyclotron resonance study

Alkoxide–alkanol negative ions [R1O–⋯ HOR2] react with carbon ethers (e.g., Me3COR3) by the four-centre reactions [R1O–⋯ HOR2]+ Me3COR3→[R2O–⋯HOR3]+ Me3COR1, and [R1O–⋯ HOR2]+ Me3COR3→[R1O–⋯ HOR3]+ Me3COR2. The former reaction predominates when R1 < R2. In the case of di- and tri-ethers, the four-centre reaction can, in principle, compete with the alternative six-centre reaction. For example, for Me2C(OR3)2, [R1O–⋯HOR2]+ Me2C(OR3)2→ Me2CO + R1OR3+[R2R–⋯ HOR3]. Evidence is presented which shows that the six-centre reaction does not occur.

Isomerisation reactions of β-ethyl anions in the gas phase

Evidence based primarily on the technique of collisional activation mass spectrometry is presented which suggests that substituted β-ethyl anions (XCH2CH2–) are at best transient species in the gas phase often rearranging to more stable isomers, e.g., MeCOCH2CH2–→ MeCOCHMe, PhCOCH2CH2–→ PhCOCHMe, HOCOCH2CH2–→ MeCH2CO2– and CH2CHCH2CH2→–CH2–CHCH–Me. The β-methoxycarbonylethyl anion MeOCOCH2CH2– is presumably unstable with respect to its radical since no peak corresponding to either this species or a stable isomer is observed.