Cyanocobalamin, CNCbl

The Bj2 vitamers consist of a group of organometallic compounds that have a common cor-rinoid structure and vary in the substituent bound to the central cobalt atom (Fig. 7) (167,168). The principal naturally occurring Bl2 vitamers are hydroxocobalamin (HOCbl), methylcobal-amin (MeCbl), and adenosylcobalamin (AdoCbl). Cyanocobalamin (CNCbl) is the form commonly used for clinical, pharmaceutical, and food fortification purposes, due to its greater relative stability. 

There is substantial literature on cobalt-containing compounds in which a carbon-cobalt bond is in an axial position trans to usually an N-donor ligand, with the four equatorial positions occupied by N-donors too. This coordination arrangement is present in adenosylcobalamin (AdoCbl, coenzyme B12) and in cyanocobalamin (CNCbl, vitamin B12). The trans N-donor is dimethylbenzimidazole (DMBz) attached to a nucleotide (Scheme 2). 

The Bi2 cofactorsknown so far are alkylcobalamins (RCbl) consisting of a cobaltcorrinoid with a pendant nucleotide (with different purine base), which occupies five or six coordination sites of an octahedral Co(lli). The sixth position is occupied by the R group or by a CN ligand in the cyanocobalamin (CNCbl), the vitamin B12... 

Cyanocobalamin appears to be the most stable of the B12 vitamers (167,168). It can be autoclaved at 120°C in aqueous solution at pH 4-7. It is susceptible to degradation and loss of vitamin activity under alkaline conditions. Short exposure to UV or visible light causes conversion to HOCbl prolonged exposure results in irreversible decomposition. CNCbl is soluble in water, short-chain alcohols, and phenol, but it is insoluble in acetone, chloroform, and ether. 

CNCbl = cyanocobalamin HOCbl = hydroxocobalamin MeCbl = methylcobalamin AdoCbl = adenosylcobalamin. Column specifications expressed as (length X ID, particle size manufacturer) when reported in original publication. n = number of determinations. 

The abbreviations used are AdoCbl, adenosylcobalamin CNCbl, cyanocobalamin Cbl(II) cob(II)alamin AdoCH3, 5 -deoxyadenosine AdoCH2, 5 -deoxyadenosyl radical. 

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