The coenzyme B-12 derivative N1-methyl-5 '-deoxyadenosylcobalamin:Synthesis, characterization, stability towards dimroth rearrangement, andCo-C thermolysis product and kinetic studies

摘要

The mechanism of Co-C heterolysis of adenosylcobalamin (AdoCbl) is studied, specifically the effect of a positive charge produced by methylation (CH:, a "sticky proton" analog of H+ addition) to the most basic adenine N1 nitrogen site in AdoCbl. This is accomplished by the synthesis, characterization and Co-C thermolysis in the presence of the radical trap TEMPO of the N1 methylated AdoCbl derivative, [N1-methyl-5'-deoxyadenosycobalamin](+), 5. A variety of needed additional syntheses and other control experiments are also reported, including effective purification methods using ultrafiltration membranes, procedures that may prove to be the more generally useful part of the synthetic work reported. The thermolysis results with 5 are unequivocal in showing that a positive charge at the N1 nitrogen in the adenosyl group of AdoCbl has no effect within +/-8% on the products or rates of Co-C thermal cleavage. The data are most consistent with a recently suggested, new mechanism for AdoCbl Co-C heterolysis at less acidic pH values, one not involving the protonation of the beta-ribose oxygen that is known to occur under strongly acidic conditions. In addition, the present study illustrates some of the positive features, and also some of the pitfalls, in the approach where CH3+ is used as a "sticky proton" analog in mechanistic studies.