B12 chemistry

A number of model Vitamin B12 systems have been prepared and 

Studied. An aetioporphyrin I cobalt(III) methyl complex has been prepared [128] and methyl cobalt(III) phthalocyanine is also known [129]. The dimethylglyoxime complex, 7.23, where R = CN and L = PhsP, pyri- 

The nature of the reduced cobaloxim species in aqueous solution is unknown but it is suggested that there may be both a hydride and an anionic complex in equilibrium [130], e.g. 

A number of cobaloxim-metal complexes, [Co(D2H2)BR], where R = PhsSi, PhsGe, PhsSn and PhsPb, have been described [132]. 

A related, tetradentate ligand bis(acetylacetone)ethylenediamine, (MeCOCH2CMe=NCH2)2 (abbreviated to BAE), also stabilizes cobalt-alkyl complexes, including the pentaco-ordinate (BAE)CoR [132a]. 

---

The corrin ring is the cobalt-binding molecule in vitamin B12 (Figure 3.12). With regards to vitamin B12 chemistry, much can be learned through the study of simpler analogues. 

Because of its immense scope, a detailed description of corrins (and vitamin B12) will not be presented here. The reader is instead referred to reviews of B12 chemistry and its biosynthesis that have appeared recently. Further, because they are more directly related to the corrins than are the corroles, the chemistry of the dehydrocorrins will not be discussed here. Also not included in this review are the so-called artificial porphyrins of Floriani, et al. (e.g., 2.5), since it is deemed by these authors in their review that these macrocycles are more dehydrocorrin-like than corrole-like in their nature. Other systems omitted here include the spiro porphyrins of Battersby and coworkers, the tetraphosphole macrocycles of Mathey and coworkers and the tetrapyrrolic systems of Bartczak and Smith and co-workers. Thus, the emphasis will be on those contracted porphyrins that are most removed, in structural and chemical terms, from the macrocyclic unit found in coenzyme B12 and its analogs. 

The synthesis of (7-alkylcobalts from the reaction of Co hydrides and alkenes is relevant to the use of Co hydrides in alkene hydroformylation and is of interest because of the biological significance of carbon-cobalt bonds in vitamin B12 chemistry. The reaction of HCo(CO)4 with simple alkenes involves hydrometalation as a key step in hydrogenation, isomerization or hydroformylation of alkenes . ff-Alkylcobalt compounds are detectable intermediates in reactions of HCo(CO)4 and alkenes by Ir, but... 

From studies of the non-enzymic chemistry of B,2 coenzyincs and of model systems noted below, a body of knowledge about basic B12 chemistry... 

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]. 

Figure 7.1. Dependence of catabolism of some amino acids, fatty acids and thymine upon methylmalonyl-CoA mutase. Reprinted with pennission from A. Stroinski, Medical aspects of vitamin B12, pp. 335-370 in Z. Schneider and A. Stroinski (eds.) Comprehensive B12 Chemistry. Biochemistry. Nutrition. Ecology. Medicine. 1987 Walter de Gruyter.

Schneider Z (1987a) Biosynthesis of vitamin B. In Schneider Z and Stroinski A (eds) Comprehensive B12. Chemistry. Nutrition. Ecology. Medicine, pp 93-110. Walter de Gruyter, Berlin, New York... 

Vitamin B12 chemistry. The structure of the 5,6-dimethylbenzimidazole-cobamide-coenzyme, as found in the wet crystals, has been determined by X-ray and neutron diffraction (see Figure 59) [123, 124]. A schematic... 

Chemistry of vitamin B12 and related inorganic model systems. D. G. Brown, Prog. Inorg. Chem., 1973,18, 177-286 (268). 

B12 and related organocobalt chemistry formation and cleavage of cobalt-carbon bonds. P. J. Toscano and L. G. Marzillo, Prog. Inorg. Chem., 1984, 31,105-204 (677). 

Much research focuses on the structures, properties, and uses of the complexes formed between d-metal ions acting as Lewis acids and a variety of Lewis bases, partly because they participate in many biological reactions. Hemoglobin and vitamin B12, for example, are both complexes—the former of iron and the latter of cobalt (Box 16.1). Complexes of the d-metals are often brightly colored and magnetic and are used in chemistry for analysis, to dissolve ions (Section 11.13), in the... 

Pratt, J. M. Coordination Chemistry of the B12 Dependent Isomerase Reactions Wiley New York, 1982. 

Pratt, J. M. Inorganic Chemistry of Vitamin B12 Academic Press New York, 1972. 

Tests to exclude possible causes of dementia include a depression screen, vitamin B12 deficiency, thyroid function tests [thyroid-stimulating hormone (TSH) and free triiodothyronine and thyroxine], complete blood cell count, and chemistry panel.21... 

Co within all compounds of the so-called cobalamin (or B12) family. The biological functions of cobalamin cofactors are defined by their axial substituents either a methyl or an adenosyl group. Both cofactors participate in biosynthesis the former in methyl transfer reactions while the latter is a free radical initiator, abstracting H atoms from substrates. Decades after their initial characterization, the fascination with the biological chemistry of cobalamins remains.1109... 

The mechanistic and structural chemistry of B12 may be separated into (i) investigations of cobalamin cofactors both apart from and in complex with their enzymes, and (ii) biomimetic model complexes, both structural and functional. 

Most of the NMR work reported on B 12-derivatives has been concerned with interpreting spectra and assigning resonance positions. In certain cases some valuable information concerning the chemistry of B12 has been obtained. We will discuss the nuclear magnetic resonance work which has been reported for B12 plus some of our own unpublished results with particular emphasis on those results which give some insight into vitamin B 12-chemistry. 

---