Cyanogen bromide is the inorganic compound with the formula (CN)Br or BrCN. It is a colorless solid that is widely used to modify biopolymers, fragment proteins and peptides (cuts the C-terminus of methionine), and synthesize other compounds. The compound is classified as a pseudohalogen. Synthesis, basic properties, and structure The carbon atom in cyanogen bromide is bonded to bromine by a single bond and to nitrogen by a triple bond (i.e. ). The compound is linear and polar, but it does not spontaneously ionize in water. It dissolves in both water and polar organic solvents. Cyanogen bromide can be prepared by oxidation of sodium cyanide with bromine, which proceeds in two steps via the intermediate cyanogen (): 2 NaCN + Br2 -> (CN)2 + 2 NaBr (CN)2 + Br2 -> 2 (CN)Br When refrigerated the material has an extended shelflife. Like some other cyanogen compounds, cyanogen bromide undergoes an exothermic trimerisation to cyanuric bromide (). This reaction is catalyzed by traces of bromine, metal salts, acids and bases. For this reason, experimentalists avoid brownish samples. Cyanogen bromide is hydrolyzed to release hydrogen cyanide and hypobromous acid (CN)Br + H2O -> HCN + HOBr Biochemical applications The main uses of cyanogen bromide are to immobilize proteins, fragment proteins by cleaving peptide bonds, and synthesize cyanamides and other molecules. Protein immobilization Cyanogen bromide is often used to immobilize proteins by coupling them to reagents such as agarose for affinity chromatography. Because of its simplicity and mild pH conditions, cyanogen bromide activation is the most common method for preparing affinity gels. Cyanogen bromide is also often used because it reacts with the hydroxyl groups on agarose to form cyanate esters and imidocarbonates. These groups are reacted with primary amines in order to couple the protein onto the agarose matrix, as shown in the figure. Because cyanate esters are more reactive than are cyclic imidocarbonates, the amine will react mostly with the ester, yielding isourea derivatives, and partially with the less reactive imidocarbonate, yielding substituted imidocarbonates. The disadvantages of this approach include the toxicity of cyanogen bromide and its sensitivity to oxidation. Also, cyanogen bromide activation involves the attachment of a ligand to agarose by an isourea bond, which is positively charged at neutral pH and thus unstable. Consequently, isourea derivatives may act as weak anion exchangers. Protein cleavage Cyanogen bromide hydrolyzes peptide bonds at the C-terminus of methionine residues. This reaction is used