ADOCOBINAMIDE, THE BASE-OFF ANALOG OF COENZYME B-12 (ADOCOBALAMIN) .2. PROBING THE BASE-ON EFFECT IN COENZYME B-12 VIA COBALT-CARBON BOND THERMOLYSIS PRODUCT AND KINETIC-STUDIES AS A FUNCTION OF EXOGENOUS PYRIDINE BASES

The thermolysis of the Co-C bond in adocobinamide (AdoCbi(+)BF(4)(-)) in anaerobic ethylene glycol has been studied as a function of a serie s of para-substituted pyridine axial bases using the TEMPO radical-tra pping method. In contrast to the slower rates of Co-C cleavage previou sly found for benzylcobinamide, neopentylcobinamide, and the (alpha-ph enylethyl)cobaloxime coenzyme B-12 models, for AdoCbi(+) the rate of t otal Co-C cleavage becomes faster as the para-substituted pyridines be come more electron-donating, Specifically, the 110 degrees C k(obsd) f or AdoCbi(+)BF(4)(-) total Co-C cleavage increased 23-fold on going fr om 1 M pyridine (py) to 1 M p-(dimethylamino)-pyridine (Me(2)N-py). Ho wever, HPLC product studies reveal that the percentage of abiological Co-C heterolysis increases (to a limiting value); that is, Co-C hetero lysis is a major reason for the observed rate increase seen for Me(2)N -py. Deconvolution of the k(obsd) rate constant into its heterolysis a nd homolysis components yields values of the 110 degrees C k(heterolys is) and k(homolysis) for AdoCb(+). base for Me(2)N-py and pyridine. Th ese data in turn reveal that the AdoCbi(+)BF(4)(-) base-on homolysis r ate constant does not increase within experimental error as one goes f rom py to the more basic Me(2)N-py (k(on,h)=8(3)x10(-4) and 7(1)x10(-4 ) s(-1), respectively), but that the base-on heterolysis rate constant changes by 17-fold (k(on,het)=0.4(0.1)x10(-4) and 7(1)x10(-4) s(-1) f or py and Me(2)N-py, respectively). The plausible biological significa nce of these results is then discussed, notably the heretofore unsubst antiated idea, first suggested by Mealli, Sabat, and Marzilli, that on e major evolutionary pressure for selecting and appending the 5,6-dime thylbenzimidazole axial base in coenzyme B-12 is because this base lim its Co-C bond heterolysis, thereby promoting the biologically relevant Co-C cleavage reaction, Co-C homolysis.