Studies On Coenzyme B₁₂ Models And Their Supramolecules With Cyclodextrins

Coenzyme B₁₂(5'-deoxyadenosylcobalamin, AdoCbl) serves as a cofactor for various enzymatic reactions. A common feature of these reactions is the 1,2-interchange of a hydrogen atom and another substituent on adjacent carbon atoms of the substrate. One of the key steps in those reactions is that the distortion of corrin ring of the cofactor is induced by enzymes, and then the cobalt-carbon bond is broken to produce a 5'-deoxyadenosyl radical. The rate of homolytic cleavage of the cobalt-carbon bond in the enzyme-bound cofactor is about 10¹⁰ times larger than in free cofactor. However, how the protein accelerates the Co-C bond cleavage and the mechanisms for the coenzyme B₁₂-dependent enzymatic reactions are still unclear now. Therefore, efforts have been made for investigation on a large number of coenzyme B₁₂ models encompassing the structure and function relationship for the coenzyme B₁₂. Recently, the special interests are paid for the artificial coenzyme B₁₂/enzyme models. In this thesis, the following themes that relate to the mechanism for coenzyme B₁₂-dependent rearrangements are discussed:(1).Investigation on a-cyclodextrin/alkyl(aqua) cobaloxime inclusion complexes as a new kind of coenzyme B₁₂/enzyme modelsCyclodextrins(CDs), a series of oligosaccharides consisting of six or more D-glucopyranose residues α -1,4-linked in a cyclic array, are one of the most fascinating naturally occurring molecular receptors. They have been taken as artificial enzyme models. In this work, a series of novel 1:1 inclusion complexes α-cyclodextrin/ alkyl(aqua)cobaloxime (α-CD/H₂OCo(DH)₂R, R = n-C₃H₇, i-C₄H₉, n-C₄H₉, n-C₅H₁₁), in which alkylcobaloxime is a kind of widely studied coenzyme B₁₂ modes, has been synthesized and characterized by elemental analysis. One-dimensional NMR investigation revealed that upfield shifts are induced for H-3, H-5 at α-CD and downfield shifts are induced for most hydrogens at alkylcobaloximes. Connectivity signal in two-dimensional NOE spectroscopy between alkyls and H-5 of α-CD have been observed. It suggested that alkyl groups were included into α-CD. According to that the chemical shifts for the inclusion complexes vary on changing the concentration of the 1:1 adduct, we have determined the formation constant of inclusion complexes. It was revealed that the longer the alkyl chain is, the deeper inclusion of the R group...