How non-covalent interactions contribute to gas-phase bond energies.
What is the design behind Vitamin B12?
Homolytic dissociation of the weak Co-C bond in Vitamin B12 (adenosylcobalamine), an essential cofactor in a large number of enzymes, initiates radical reactions. Modulation of the bond dissociation energy is accordingly a key element in control of enzymatic activity, although one would ordinarily consider homolytic bond strengths to be largely insensitive to environmental effects. Nevertheless, the rate of homolytic cleavage of the Co-C bond can be modulated by at least 12 orders-of-magnitude. The lecture covers basic physical and physical organic methods for the determination of bond dissociation energies, focusing on work in the gas phase, and comparisons to quantum chemical methods that model structure, spectroscopy and thermochemistry of medium-to-large-sized molecules, applied to the specific problem of cobinamides. The results, including benchmarking of the methods to assess reliability, suggest an elegant method by which non-covalent interactions can achieve the range of variation in bond energies, and therefore also rates, seen in the natural system. We believe that the non-covalent interactions provide the basis for control of enzymatic radical reactions