Stereochemistry of the methyl group in (R)-3-methylitaconate derived by rearrangement of 2-methylideneglutarate catalysed by a coenzyme B-12-dependent mutase

2-Methylidenegtutarate mutase is an adenosylcobalamin (coenzyme B-12)-dependent enzyme that catalyses the equilibration of 2-methylideneglutarate with (R)-3-methylitaconate. This reaction is believed to occur via protein-bound free radicals derived from substrate and product. The stereochemistry of the formation Of the methyl group of 3-mechylitaconate has been probed using a 'chiral methyl group'. The methyl group in 3-([H-2(1),H-3]methyl)itaconate derived from either (R)- or (S)-2-methylidene[3-H-2(1),3-H-3(1)]glutarate was a 50:50 mixture of (R)- and (S)-forms. It is concluded that the barrier to rotation about the C-C bond between the methylene radical centre and adjacent C-atom in the product-related radical [(CH)-C-.,CH(-O2CC=CH2)CO2-] is relatively low, and that the interaction of the radical with cob(II)alamin is minimal. Hence, cob(II)alamin is a spectator of the molecular rearrangement of the substrate radical to product radical.