Hydrogen Bonding Constrains Free Radical Reaction Dynamics at Serine and Threonine Residues in Peptides

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• 作者：Daniel A. Thomas ; Chang Ho Sohn ; Jinshan Gao ; J. L. Beauchamp
• 刊名：Journal of Physical Chemistry A
• 出版年：2014
• 出版时间：September 18, 2014
• 年：2014
• 卷：118
• 期：37
• 页码：8380-8392
• 全文大小：693K
• ISSN：1520-5215

文摘

Free radical-initiated peptide sequencing (FRIPS) mass spectrometry derives advantage from the introduction of highly selective low-energy dissociation pathways in target peptides. An acetyl radical, formed at the peptide N-terminus via collisional activation and subsequent dissociation of a covalently attached radical precursor, abstracts a hydrogen atom from diverse sites on the peptide, yielding sequence information through backbone cleavage as well as side-chain loss. Unique free-radical-initiated dissociation pathways observed at serine and threonine residues lead to cleavage of the neighboring N-terminal C_伪鈥揅 or N鈥揅_伪 bond rather than the typical C_伪鈥揅 bond cleavage observed with other amino acids. These reactions were investigated by FRIPS of model peptides of the form AARAAAXAA, where X is the amino acid of interest. In combination with density functional theory (DFT) calculations, the experiments indicate the strong influence of hydrogen bonding at serine or threonine on the observed free radical chemistry. Hydrogen bonding of the side-chain hydroxyl group with a backbone carbonyl oxygen aligns the singly occupied 蟺 orbital on the 尾-carbon and the N鈥揅_伪 bond, leading to low-barrier 尾-cleavage of the N鈥揅_伪 bond. Interaction with the N-terminal carbonyl favors a hydrogen-atom transfer process to yield stable c and z^{鈥?/sup> ions, whereas C-terminal interaction leads to effective cleavage of the C_伪鈥揅 bond through rapid loss of isocyanic acid. Dissociation of the C_伪鈥揅 bond may also occur via water loss followed by 尾-cleavage from a nitrogen-centered radical. These competitive dissociation pathways from a single residue illustrate the sensitivity of gas-phase free radical chemistry to subtle factors such as hydrogen bonding that affect the potential energy surface for these low-barrier processes.}