Structural Studies of Bi2-Derivatives by Nuclear Magnetic Resonance Spectroscopy

Structural Studies of Bi2-Derivatives by Nuclear Magnetic Resonance Spectroscopy 

Nuclear magnetic resonance (NMR) spectroscopy has had a strong influence in the development of B12 chemistry. The early NMR spectroscopic studies established the nature of many noncrystalline Bi2-derivatives, mostly in their CO/3-cyano forms, using one-dimensional analyses. These studies were based on the H- and C-chemical shift values from spectra of several already well-characterized B12-derivatives and used to identify and describe the structure of synthetic and natural analogues of vitamin B12 [74]. The natural corrinoids from a range of bacteria were first characterized by NMR [75,76]. 

Earlier assignment problems regarding B 12-derivatives in aqueous or non-aqueous solutions have now been eliminated by the use of heteronuclear NMR spectroscopy [74,77]. Following on the pioneering studies of coenzyme Bi2 (2) [78,79], Co(I)-heptamethyl-cobyrinate (34) [80] and the noncrystalline Bi2-derivative Co -5 -deoxy-5 -adenosylcobinamide (35) [81], the newer NMR studies have begun to compliment (and in certain aspects rival) x-ray analytical studies of Bi2-derivatives in the sohd state. 

A major factor in the importance of heteronuclear NMR spectroscopy is that the structures (in solution) of noncrystalUne B12 derivatives can be characterized. One of the main examples of this is the natural complete but base-off protonated form of coenzyme B12 (2-H ) [79]. More recently, the solution structures of the organometaUic derivatives pseudocoenzyme B12 (37), adenosyl-factor A (38) [76] and neocoenzyme B12 (39) [83] could be analyzed in great detail. The structures and the base-on/base-off equilibria of a range of protonated base-off cobamides have also been investigated in aqueous solution [74,77]. 

From NOE measurements the first reliable assignment (upper/Co or lower/COa) of the cobalt-bound methyl group in noncrystalline methyl-cob(III)yrinates was determined [84]. Also from NOE data and three-bond coupling constants, detailed and important information on the conformational properties of the nucleotide moiety, the organometaUic group, and 

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