Synthesis and characterization of isolable thiolatocobalamin complexes relevant to coenzyme B12-dependent ribonucleoside triphosphate reductase

The syntheses, isolation and characterization of cyclohexylthiolatocobalamin (C6H11SCbl), glutathionylcobalamin (GluSCbl), and cysteinylcobalamin (CysSCbl) are reported in 75, 55, and 65% yield, respectively. Characterization was achieved using elemental analyses, L-SIMS (liquid secondary ion mass spectrometry), UV-visible spectroscopy and, for the more stable C6H11SCbl and GluSCbl, our recently established 1H NMR method (which emphasizes the readily interpreted aromatic region of the cobalamin's 1H NMR spectrum). Preliminary evidence is presented for clean homolysis of the RS-Co bond in C6H11SCbl, GluSCbl, and CysSCbl to give RS. and .Co(II)Cbl radical pairs analogous to those that are intermediates in ribonucleoside triphosphate reductase (RTPR). A summary is provided which emphasizes the seven variables identified to date, underlying the successful syntheses and isolation of thiolatocobalamins, variables which make the one-step syntheses of RSCbls considerably more complex than they initially appear. Also briefly discussed are the analogous protein-S-Cbl complexes that are seen as side-products in RTPR, and the probability that such side-products are formed when HOCbl.HX is used as a possible 'active-site inhibitor' complex with B12-dependent enzymes.