STUDY OF THE THERMAL-DECOMPOSITION, AXIAL-BASE EQUILIBRIUM, AND CO-C BOND-DISSOCIATION ENERGY OF 2',5'-DIDEOXYADENOSYLCOBALAMIN IN ETHYLENE-GLYCOL

The anaerobic thermal decomposition for 2',5'-dideoxyadenosylcobalamin (2'-dAdoCbl) with an added 2,2,6,6-tetramethylpipyridinyl-1-oxy(TEMPO ) radical trap in ethylene glycol has been studied. The temperature de pendence of the rate for 2'dAdoCbl Co-C homolysis was obtained from 85 -115 degrees C, Delta H-on(not equal) = 36.8 +/- 0.9 kcal . mol(-1), a nd Delta S-on(not equal) = 22.1 +/- 1.5 cal . mol(-1) . K-1. The tempe rature dependence of the axial-base equilibrium of 2'-dAdoCbl in ethyl ene glycol was independently determined from 10-75 degrees C, Delta H = 6.9 +/- 0.2 kcal . mol(-1) . K-1. The Co-C bond dissociation energy was estimated at 32.4 +/- 1.5 kcal . mol(-1) for 2'-dAdoCbl in ethylen e glycol if one assumes a cage efficiency factor Fc = 1. Comparing the Co-C BDE of 2'-dAdoCbl with that for 5'-deoxyadenosylcobalamin (30.0 +/- 2 kcal . mol(-1)), the cage chemistry effect probably can account for this similar to 2 kcal . mol(-1). (C) 1997 Elsevier Science Inc.