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Pulse Radiolysis and Ultra‐High‐Performance Liquid Chromatography/High‐Resolution Mass Spectrometry Studies on the Reactions of the Carbonate Radical with Vitamin B 12 Derivatives
Author(s) -
Dassanayake Rohan S.,
Shelley Jacob T.,
Cabelli Diane E.,
Brasch Nicola E.
Publication year - 2015
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201406269
Subject(s) - chemistry , corrinoid , corrin , radiolysis , reaction rate constant , tetrapyrrole , photochemistry , mass spectrometry , hydrogen atom abstraction , hydrogen atom , medicinal chemistry , radical , organic chemistry , ring (chemistry) , kinetics , alkyl , chromatography , biochemistry , quantum mechanics , methylation , methyltransferase , gene , enzyme , physics
The reactions of the carbonate radical anion (CO 3 . − ) with vitamin B 12 derivatives were studied by pulse radiolysis. The carbonate radical anion directly oxidizes the metal center of cob(II)alamin quantitively to give hydroxycobalamin, with a bimolecular rate constant of 2.0×10 9 M −1 s −1 . The reaction of CO 3 . − with hydroxycobalamin proceeds in two steps. The second‐order rate constant for the first reaction is 4.3×10 8 M −1 s −1 . The rate of the second reaction is independent of the hydroxycobalamin concentration and is approximately 3.0×10 3 s −1 . Evidence for formation of corrinoid complexes differing from cobalamin by the abstraction of two or four hydrogen atoms from the corrin macrocycle and lactone ring formation has been obtained by ultra‐high‐performance liquid chromatography/high‐resolution mass spectrometry (UHPLC/HRMS). A mechanism is proposed in which abstraction of a hydrogen atom by CO 3 . − from a carbon atom not involved in the π conjugation system of the corrin occurs in the first step, resulting in formation of a Co III C‐centered radical that undergoes rapid intramolecular electron transfer to form the corresponding Co II carbocation complex for about 50 % of these complexes. Subsequent competing pathways lead to formation of corrinoid complexes with two fewer hydrogen atoms and lactone derivatives of B 12 . Our results demonstrate the potential of UHPLC combined with HRMS in the separation and identification of tetrapyrrole macrocycles with minor modifications from their parent molecule.