AUTHOR=Zheng Mingyu , Zhang Xiaoping , Cheng Yihao , Sun Lili , Zhang Xinglei 

TITLE=Hydroxyl transfer versus cyclization reaction in the gas phase: Sequential loss of NH3 and CH2CO from protonated phenylalanine derivatives

JOURNAL=Frontiers in Chemistry

VOLUME=10

YEAR=2023

URL=https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2022.1094329

DOI=10.3389/fchem.2022.1094329

ISSN=2296-2646

ABSTRACT=<p>Collisional activation of protonated phenylalanine derivatives deamination products leads to hydroxyl skeletal rearrangement <italic>versus</italic> cyclization reaction, and to form hydroxylbenzyl cation <italic>via</italic> elimination of CH<sub>2</sub>CO. To better clarify this unusual fragmentation reaction, accurate mass measurements experiments, native isotope experiments, multiple-stage mass spectrometry experiments, different substituents experiments, and density functional theory (DFT) calculations were carried out to investigate the dissociation mechanistic pathways of protonated phenylalanine derivatives deamination products. In route 1, a three-membered ring-opening reaction and a 1,3-hydroxyl transfer (from the carbonyl carbon atom to the interposition carbon atom of carbonyl) occurs to form 3-hydroxy-1-oxo-3-phenylpropan-1-ylium, followed by dissociation to lose CH<sub>2</sub>CO to give hydroxy (phenyl)methylium. In route 2, a successive cyclization rearrangement reaction and proton transfer occur to form a 2-hydroxylphenylpropionyl cation or protonated 2-hydroxy-4H-benzopyran, followed by dissociation to lose CH<sub>2</sub>CO or CH≡COH to give 2-hydroxylbenzyl cation. In route 3, a successive hydroxyl transfer (from the carbonyl carbon atom to the ortho carbon atom on benzene) and two stepwise proton transfer (1,2-proton transfer to the ipso-carbon atom of the phenyl ring followed by 1,3-proton transfer to the ortho carbon atom of carbonyl) occurs to form a 2-hydroxylphenylpropionyl cation, which subsequently dissociates to form 2-hydroxylbenzyl cation by elimination of CH<sub>2</sub>CO. DFT calculations suggested that route 1 was more favorable than route 2 and route 3 from a thermodynamic point of view.</p>