Unimolecular dissociation of gas-phase small sulfinyl radical ions upon low energy collision-induced dissociation

Chasity B Love, Purdue University

Abstract

Protein radicals play important roles in biological systems including their involvement in enzyme catalytic sites, aging, neurodegenerative diseases, and oxidative damage to proteins. Understanding the structure and reactivity of protein radicals, give insight into their reaction dynamics and structural modifications. Several solution studies have been used to study protein radicals, but their transient nature present difficulties. To over the limitations of solution phase studies, gas-phase atnmospheric pressure radical reactions in an electrospray ionization plume were implemented to study the unimolecular dissociation of small cysteinyl radical ions. Coupling radical reactions with tandem mass spectrometry allows more structural information to be obtained for these long-lived radical species. Cysteinyl sulfinyl radical ions are new members of the gas-phase amino acid radical ion family. To explore the effect of small structural changes on the fragmentation behavior, three sulfur containing amino acids were explored (cysteine, homocysteine, and penicillamine). Intact, N-acetylated, O-methylated cysteinyl sulfinyl radical ions were investigated by low-energy collision-induced dissociation. The dominant fragmentation channel for the protonated cysteine sulfinyl radical ions was a radical-directed C&agr;-Cβ homolytic cleavage, resulting in the formation of glycyl radical ions and loss of CH2SO. Whereas, the major channel for homocysteine and penicillamine sulfinyl radical ions were charge-directed fragmentation. Homocysteine sulfinyl radical ions resulted in a major loss of H2O followed by a consecutive SH loss. Penicillamine sulfinyl radical ions resulted in two complementary fragmentation channels, formation of a glycyl radical and sulfenic acid. Interestingly, N-acetylation significantly changed the major fragmentation pathway. This tells us that location of charge (proton) significantly modulate the competition between radical- and charge-directed fragmentations. Stable isotopic labeling was used to provide insight to the reaction mechanisms and computational calculations were used to provide energetics to the possible mechanisms.

Degree

Ph.D.

Advisors

Xia, Purdue University.

Subject Area

Physical chemistry

COinS