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Download fileElucidating Potential Energy Surfaces for Singlet O2 Reactions with Protonated, Deprotonated, and Di-Deprotonated Cystine Using a Combination of Approximately Spin-Projected Density Functional Theory and Guided-Ion-Beam Mass Spectrometry
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posted on 2017-07-20, 00:00 authored by Wenchao Lu, I-Hsien “Midas” Tsai, Yan Sun, Wenjing Zhou, Jianbo LiuThe reactivity of
cystine toward electronically excited singlet
O2 (a1Δg) has been long debated,
despite the fact that most organic disulfides are susceptible to oxidation
by singlet O2. We report a combined experimental and computational
study on reactions of singlet O2 with gas-phase cystine
at different ionization and hydration states, aimed to determine reaction
outcomes, mechanisms, and potential energy surfaces (PESs). Ion–molecule
collisions of protonated and di-deprotonated cystine ions with singlet
O2, in both the absence and the presence of a water ligand,
were measured over a center-of-mass collision energy (Ecol) range from 0.1 to 1.0 eV, using a guided-ion-beam
scattering tandem mass spectrometer. No oxidation was observed for
these reactant ions except collision-induced dissociation at high
energies. Guided by density functional theory (DFT)-calculated PESs,
reaction coordinates were established to unravel the origin of the
nonreactivity of cystine ions toward singlet O2. To account
for mixed open- and closed-shell characters, singlet O2 and critical structures along reaction coordinates were evaluated
using broken-symmetry, open-shell DFT with spin contamination errors
removed by an approximate spin-projection method. It was found that
collision of protonated cystine with singlet O2 follows
a repulsive potential surface and possesses no chemically significant
interaction and that collision-induced dissociation of protonated
cystine is dominated by loss of water and CO. Collision of di-deprotonated
cystine with singlet O2, on the other hand, forms a short-lived
electrostatically bonded precursor complex at low Ecol. The latter may evolve to a covalently bonded persulfoxide,
but the conversion is blocked by an activation barrier lying 0.39
eV above reactants. At high Ecol, C–S
bond cleavage dominates the collision-induced dissociation of di-deprotonated
cystine, leading to charge-separated fragmentation. Cross section
for the ensuing fragment ion H2NCH(CO2–)CH2SS• was measured as a function of Ecol, and the mechanism of charge-separated fragmentation
was discussed. It was also found that the reaction of deprotonated
cystine with singlet O2 follows a similar mechanism as
that of di-deprotonated cystine, but with an even higher activation
barrier (0.72 eV).
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Keywords
Singlet O 2 Reactionsdi-deprotonated cystinecenter-of-mass collision energy1 Δ gGuided-Ion-Beam Mass Spectrometrydi-deprotonated cystine ionscharge-separated fragmentationtandem mass spectrometerPEScollision-induced dissociationCHCOsinglet O 2DFTSSE colprotonated cystineSpin-Projected Density Functional Theoryreaction coordinatesNCH