ABSTRACT
RATIONALE: McLafferty rearrangements occur in radical cations of molecules containing a carbonyl group and a γ hydrogen atom but are not common in the [M+H](+) ions of carbonyl compounds. We propose to investigate the collision-induced dissociation (CID) of the [M+H](+) ions of nicotinoyl and picolinoyl amides of 1- and 2-phenylethylamines to explore the possibility of McLafferty-type rearrangement. METHODS: The compounds for study were synthesized by the reaction of methyl nicotinate or methyl picolinate with 1- and 2-phenylethylamines. The CID mass spectra of electrospray ionization (ESI)-generated protonated molecules were obtained using a QSTAR XL quadrupole time-of-flight (QTOF) mass spectrometer, and density functional theory (DFT) calculations using the B3LYP method were employed to elucidate the fragmentation mechanisms. The total electronic and thermal energies of intermediate transition states (TSs) and product ions are reported relative to those of the [M+H](+) ions. RESULTS: CID of the [M+H](+) ions of N-[nicotinoyl]-2-phenylethylamine (1) yielded product ions of m/z 105 (1-phenylethyl cation) and 123 ([M+H-styrene](+) cation). The competitive formation of the ions of m/z 123 and 105 is proposed to involve a McLafferty-type rearrangement. Similarly, the [M+H](+) ions of the isomeric compound 2 and the N-[picolinoyl] phenylethyl amines (3 and 4) dissociate to yield ions of m/z 123 and 105. CONCLUSIONS: A molecule of styrene was eliminated from the ESI-generated [M+H](+) ions of N-[nicotinoyl]phenylethylamines and the isomeric N-[picolinoyl]phenylethylamines, through a mechanism involving a McLafferty-type 1,5-H shift. The transition state energy for the 1,5-H shift is less for the amides of 1-phenylethylamine than for the amides of 2-phenylethylamine. The process occurs as a charge remote process and the presence of the pyridine ring is essential for the process.
ABSTRACT
RATIONALE: The collisional-induced dissociations (CID) of the [M+H]+ ions of molecules having benzyl groups attached to N-atoms have been proposed to involve migration of the benzyl group through the intermediacy of ion/neutral complexes (INCs). We report the investigation of the mechanism of dissociation of protonated N-benzyl- and N-(1-phenylethyl)tyrosine amides by electrospray ionization (ESI) tandem mass spectrometry (MS/MS) and density functional theory (DFT) calculations. METHODS: The amides were synthesized from the corresponding amino acids and amines. The ESI-MS/MS spectra were recorded using an Agilent QTOF 6540 mass spectrometer. The DFT calculations were performed by using Gaussian 09 software. The structures of the [M+H]+ ions, intermediates, products and transition states (TS) were optimized at the B3LYP/6-31G(d,p) level of theory. RESULTS: CID of the [M+H]+ ions of N-benzyltyrosine amide yields two product ions due to rearrangements: (i) the [M+H-74]+ ion (m/z 197) due to benzyl migration to the hydroxyphenyl ring and (ii) the [M+H-45]+ ion (m/z 226) due to benzyl migration to the NH2 group. DFT calculations suggest that the rearrangements occur through an INC in which the benzyl cation is the cation partner. The [M+H]+ ion of N-(1-phenylethyl)tyrosine amide rearranges to an INC of the 1-phenylethyl cation. Subsequent elimination of styrene occurs by transfer of a proton from the 1-phenylethyl cation to the neutral partner. CONCLUSIONS: The [M+H]+ ions of both N-benzyl (1) and N-(1-phenylethyl) (2) tyrosine amide rearrange into INCs. The dissociation of [M+H]+ ion of 1 yields the benzyl cation and [M+H-74]+ and [M+H-45]+ due to benzyl migration to the hydroxyphenyl ring and NH2 group, respectively. However, the formation of the [M+H-74]+ ion is not observed when the aromatic ring is deactivated. The [M+H]+ ion of 2 either dissociates to form the 1-phenylethyl cation or [M+H-styrene]+ . Copyright © 2015 John Wiley & Sons, Ltd.
