1 3 J Biol Inorg Chem DOI 10.1007/s00775-017-1474-z ORIGINAL PAPER Comparative studies of reaction of cobalamin (II) and cobinamide (II) with sulfur dioxide Ilia A. Dereven’kov 1 · Pavel A. Ivlev 1 · Cristina Bischin 2 · Denis S. Salnikov 1 · Radu Silaghi-Dumitrescu 2 · Sergei V. Makarov 1 · Oscar I. Koifman 1 Received: 26 March 2017 / Accepted: 7 June 2017 © SBIC 2017 In vivo functions of Cbls are directly connected with redox reactions of cobalt ion. Cobalt ion is capable to adopt 3+, 2+, and 1+ oxidation states; all of those are biologi- cally relevant [1]. Methyl-transfer requires generation of Cbl(I) reacting with methyl donors, whereas Cbl(II) is formed during scission of Co–C bond during isomerization cycle. Cbl(I) and Cbl(II) exhibit remarkable reactivity to numerous biological oxidants, which action alters functions of Cbl-dependent enzymes. Cbl(I) reduces multiple organic (e.g., halogen-containing [4, 5], epoxides [6, 7], unsatu- rated compounds [8]), and inorganic (e.g., NO 2 - [9, 10], NO 3 - [9, 11], NO [10], ONOO - [12], N 2 O [13], NH 2 OH [14], SO 3 2- , S 2 O 3 2- [15], S 2 O 4 2- [15, 16], and disulfides [17]) compounds. Cbl(II) is reactive mainly towards free radicals (e.g., O 2 •- [18, 19], NO • [20], NO 2 • [21], SO 2 •- [22], S •- [23]), or strong oxidants (e.g., NO + [20], ONOO - [24]). Ligand exchange reactions with Cbl(II) are also reported [20, 25]. Reduction and oxidation of sulfite [sulfite species are abbreviated as “S(IV)”] are important steps in the natural sul- fur cycle. Six-electron sulfite reduction to hydrogen sulfide is assisted by siroheme containing sulfite reductases [26, 27]. Due to complexity of sulfite reduction cycle, its several steps remain elusive and DFT calculations were employed to fill these gaps [28, 29]. Reactions of SO 2 with U(III) [30] or Yb(III) [31] lead to dithionito complexes; molybdenum hydride complexes react with SO 2 to give Mo–SO 2 H or μ-dithionito complex depending on conditions [32]. Recently, we reported that Cbl(I) is extremely reactive towards S(IV) species under physiologically relevant con- ditions. Reaction results in the formation of Cbl(II)–SO 2 •- complex [15]. Co(III) species of corrinoids gives complexes with sulfite [33–35], and several of those are considered to be sulfite sensors [36]. Nevertheless, reactions of Co(II) species with S(IV) species remain unstudied. Abstract The kinetics of reactions of cobalamin (II) and cobinamide (II) with sulfur dioxide was studied by UV– visible (UV–vis) spectroscopy. Reaction results in oxi- dation of Co(II) center and involves two aquated SO 2 moieties. The final product is suggested to be complex Co(III)–S 2 O 4 •– . The absence of corrin ring modifications during the reactions was proved. Keywords Cobalamin · Cobinamide · Sulfur dioxide · Redox reactions · Kinetics Abbreviations Cbl Cobalamin Cbi Cobinamide DMBI 5,6-dimethylbenzimidazole S(IV) Sulfite species Introduction Cobalamins (Cbls; Fig. 1a) are essential cobalt complexes in vivo [1]. In mammalian cells, Cbls are involved in the synthesis of methionine from homocysteine (as a cofactor of methionine synthase [2]) and in isomerization of methyl- malonyl-CoA to succinyl-CoA (as a cofactor of methylmal- onyl-CoA mutase [3]). * Denis S. Salnikov densal@isuct.ru 1 Institute of Macroheterocyclic Compounds, Ivanovo State University of Chemistry and Technology, Sheremetevskiy str. 7, 153000 Ivanovo, Russia 2 Department of Chemistry and Chemical Engineering, “Babes-Bolyai” University, Str. Arany Janos Nr. 11, 400028 Cluj-Napoca, Romania