Formaldehyde mediated proton-transport catalysis in the ketene–water radical cation CH2C(O)OH2+

Abstract

Previous studies have shown that the solitary ketene-water ion CH 2 C( O)OH 2•+ (1) does not isomerize into CH 2 C(OH) 2 •+ (2), its more stable hydrogen shift isomer. Tandem mass spectrometry based collision experiments reveal that this isomerization does take place in the CH 2 O loss from low-energy 1,3-dihydroxyacetone ions (HOCH 2 ) 2 C O•+ . A mechanistic analysis using the CBS-QB3 model chemistry shows that such molecular ions rearrange into hydrogen-bridged radical cations [CH 2 C( O)O(H)-H· · ·OCH 2 ]•+ in which the CH 2 O molecule catalyzes the transformation 1 → 2 prior to dissociation. The barrier for the unassisted reaction, 29 kcal mol −1 , is reduced to a mere 0.6 kcal mol −1 for the catalysed transformation. Formaldehyde is an efficient catalyst because its proton affinity meets the criterion for facile proton-transport catalysis.

Carboxyl-Catalyzed Prototropic Rearrangements in Histidine Peptide Radicals upon Electron Transfer: Effects of Peptide Sequence and Conformation

“…The facile isomerization of h2a is due to the catalytic effect of the carboxyl group which functions as a proton donor in the COOH form and a proton acceptor in the COO form. Analogous catalyzed isomerizations have been reported for ion−molecule complexes in dissociating gas-phase ions . The prototropic isomerization in radical h2a can be viewed as an autocatalytic process analogous to enzymatic acid−base catalyzed reactions involving His and acidic residues .…”

Proton affinity and enthalpy of formation of formaldehyde

“…Knowledge of accurate PA values should be useful in many applications, for example in MS and MS/MS experimental studies including those related to proteomics. Certain transformation reactions important in MS, like those enhanced by proton-transport catalysis, also require an accurate knowledge of PA values of species used for the catalysis, for example that of formaldehyde [151].…”

Peptide Radicals and Cation Radicals in the Gas Phase

“…For example, the c / z ion–molecule complex from (Gly-Phe-NH 2 + 2H) +• was found to have an extremely low barrier for catalyzed interfragment proton transfer that isomerized the enolimine tautomer of the c fragment into the more stable amide form . Analogous catalyzed prototropic isomerizations in ion–molecule complexes have been reported for a large number of fragment ions. − …”