convert cyanocobalamin to any one of the active vitamin compounds. Cyanocobalamin is one of the most widely manufactured vitamers in the vitamin family (the family of chemicals that function as when put into the body), because cyanocobalamin is the most air-stable of the forms. It is the easiest to crystallize and therefore easiest to purify after it is produced by bacterial fermentation. It can be obtained as dark red crystals or as an amorphous red powder. Cyanocobalamin is hygroscopic in the anhydrous form, and sparingly soluble in water (1:80). It is stable to autoclaving for short periods at . The vitamin coenzymes are unstable in light. After consumption the cyanide ligand is replaced by other groups (adenosyl, methyl) to produce the biologically active forms. The cyanide is converted to thiocyanate and excreted by the kidney. Chemical reactions In the cobalamins, cobalt normally exists in the trivalent state, Co(III). However, under reducing conditions, the cobalt center is reduced to Co(II) or even Co(I), which are usually denoted as and , for reduced and super reduced, respectively. and can be prepared from cyanocobalamin by controlled potential reduction, or chemical reduction using sodium borohydride in alkaline solution, zinc in acetic acid, or by the action of thiols. Both and are stable indefinitely under oxygen-free conditions. appears orange-brown in solution, while appears bluish-green under natural daylight, and purple under artificial light. is one of the most nucleophilic species known in aqueous solution. This property allows the convenient preparation of cobalamin analogs with different substituents, via nucleophilic attack on alkyl halides and vinyl halides. For example, cyanocobalamin can be converted to its analog cobalamins via reduction to , followed by the addition of the corresponding alkyl halides, acyl halides, alkene or alkyne. Steric hindrance is the major limiting factor in the synthesis of the coenzyme analogs. For example, no reaction occurs between neopentyl chloride and , whereas the secondary alkyl halide analogs are too unstable to be isolated. This effect may be due to the strong coordination between benzimidazole and the central cobalt atom, pulling it down into the plane of the corrin ring. The trans effect determines the polarizability of the Co–C bond so formed. However, once the benzimidazole is detached from cobalt by quaternization with methyl iodide, it is replaced by or hydroxyl ions. Various secondary alkyl halides are then readily attacked by the modified to give the corresponding stable cobalamin analogs. The