SPIN LABELING OF VITAMIN B

Several new paramagnetic (CoIII) corrinoids have been synthesized by the reaction of piperidine nitroxide derivatives with cobamides and cobinamides. The synthesis of the Co-Cσ bond was effected by the reaction of 4-bromoacetamido-2,2,6,6-tetramethylpiperidine-N-oxyl with strongly nucleophilic Cob(I)-alamin(B12-s). The corresponding cobinamide was prepared by basic hydrolysis of this nitroxalkylcobamide. A variety of cobinamides were shown to react with nitroxide derivatives by the displacement of water from the 6th coordination position of the cobalt atom, allowing the nitroxide group itself to coordinate to the cobalt. Modificalions of the electron spin resonance spectra of nitroxide radicals which result from association with B12 are discussed. Electron spin resonance has been used to study the kinetics of the homolytic cleavage of Co-C bond for both cobamide and cobinamide derivatives. These spin-labeled cobamides and cobinamides are of interest as probes in order to study the effect of the environment on binding of vitamin B12 in enzymes. The compounds which contain nitroxide as a coordinate ligand represent the first example of a stable, well-characterized, nitroxide-transition metal complex. © 1969, American Chemical Society. All rights reserved.