Über die a-b-Isomerie der Cyano-aquo-corrinoide

Abstract

Two isomers, a and b, of the tetracarbonic acid and some tri-, di- and monocarbonic acids, all derived from cobyric acid, have been crystallized. Two cobinamide monocarbonic acids have also been crystallized. The two crystalline tetracarbonic acids are both stable at 20 °C. They differ from each other in crystal shape. Some anions, carbon monoxide and light catalyze their thermal isomerisation in aqueous medium. The equilibrium a ⇄ b depends upon the pH of the medium only. At pH < 2.5 more than 53% and at pH >4.5 about 20% of the tetracarbonic acid is in the a-form (b = 100—a). The equilibrium shifts therefore as the carboxyls of the tetracarbonic acid (pK=3.15 and 3.85) dissociate. The shift probably results from the action of the newly formed carboxylate anions, which accelerate the reaction (a ⇄ b) in the direction a → b. The isomerisation reaction is slower in more dilute solutions. The 14CN group of the tetracarbonic acid undergoes thermally dependent exchange with the CN group of the cobyric acid. This intermolecular exchange and the isomerisation are perhaps analogous reactions, resembling the formation of cyanocobalamin from aquocobalamin and cyano-aquocorrinoids. The two forms, a and b, of tetracarbonic acid, cobyric acid, cobinamide phosphoric acid and cobinamide differ slightly in their absorption and CD spectra. On the basis of these results it is probable that the CN group of the tetracarbonic acid is bound “above” (in the b-form) or “below” (in the a-form) the plane of the corrin ring and that the CN ligand migrates during a collision of two molecules of tetracarbonic acid. The anion- and light-catalysed isomerisation reactions are probably results of heterolysis and homolysis of the Co — CN bond, respectively.