Studies on the reactivity and selectivity of charged phenyl radicals toward amino acids in a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer

George O Pates, Purdue University

Abstract

The reactivity and selectivity of charged phenyl radicals toward amino acids were examined in the gas phase. The charged phenyl radicals studied were generated in the gas phase in a dual-cell Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer. The radicals were isolated and allowed to react with L-glycine, DL-glycine-(2,2-d2), L-lysine, DL-Lysine-ϵ-15N, L-isoleucine, L-leucine, DL-leucine-(1-d 1), L-proline, and DL-proline-(1-d1). The reaction efficiencies and product branching ratios were measured. Hydrogen atom abstraction, NH2 group abstraction, OH group abstraction, C2H4N abstraction, C5H7NO abstraction, adduct formation and proton transfer were observed. The rate of hydrogen atom abstraction from aliphatic amino acids is influenced by the size of the side-chain of the amino acid. The α-carbon is thus not the only site of hydrogen atom abstraction. Selectivity studies using deuterium-labeled amino acids were carried out to provide further insight on the preferential hydrogen atom abstraction sites in L-glycine, L-leucine and L-proline. Deuterium abstraction was minimal from leucine and proline that were deuterium labeled at the α-carbon. This result supports the conclusion that the hydrogen atom is predominantly abstracted from the side-chain. The presence of an additional NH2 group in an amino acid, as in lysine, influences its' reactivity toward the radicals. The additional NH 2 group can be involved in NH2 group abstraction as well as adduct formation. The reaction efficiencies and product distributions correlate with the vertical electron affinities of the radicals. A higher electron affinity results in a lower transition state energy due to an increased polarity of the transition state. The ionization energy (IE) of the amino acids also plays a role in controlling the rate of the radical reactions by influencing the polarity of the transition state.

Degree

M.S.

Advisors

Kenttamaa, Purdue University.

Subject Area

Analytical chemistry

COinS