Vitamin differing reactivities

Under physiological conditions vitamin Bi2-derivatives have been observed in three different oxidation states, Co(ni), Co(II), and Co(I), each possessing different coordination properties and quahtatively differing reactivities [22, 75]. Oxidation-reduction processes are therefore of key importance in the chemistry and biology of B12. Electrochemical methods have been apphed in the synthesis of organometallic B 12-derivatives [86,87], as well as for the purpose of generating reduced forms of protein boxmd B -derivatives [88] and electrode-boimd B -derivatives for analytical apphcations [89]. 

Individual functional groups attached to a partially or fully reduced pyran ring behave much as expected of their aliphatic equivalents but there is often a quantitative difference in their reactivity which enables selective reactions to be carried out on polysubstituted compounds. Many examples of this are known in the tocopherol series which are the most important members of the chroman family. Since they are known by trivial names, these are shown with their structures (674). The most important tocopherol is natural vitamin E or a-tocopherol the four natural tocopherols have 7 -configuration at each of their asymmetric centres at C-2, C-4 and C-8. ... 

Although the basic structure of these synthetic species is very similar to that of the natural corrinoids their reactivity shows several differences thus [Co(A2ODC)] + does not form an adduct with dioxygen or give organometallic derivatives, which are characteristic features of the chemistry of Vitamin B12 and related compounds [13]. This failure has been attributed to a different electronic structure, as confirmed by the very different optical spectra of the two systems (Fig. 32). 

The cycloaddition of 4-phenyl-3//-l,2,4-triazole-3,5(4//)-dione to vitamin D3 (cholecalciferol) occurs regioselectively with the more reactive, least substituted diene. Different ratios of dia-stereomeric f)- and a-adducts 12 were obtained by different authors by working in ethyl acetate7 and in dichloromethane8, although the -adduct always predominated. The diastereoselectivity ratio drops to 1 in the cycloaddition of 4-phenyl-3//-1,2,4-triazole-3,5(47/)-dione to (5 )-chole-calciferol, where the reactive s-cw-diene is more remote from the bicyclic moiety7. Cycloreversion of both adducts by base treatment affords only (5 )-cholecalciferol. 

It appears from Figs. 9.5 and 9.6 that there is a huge variation in colour stability between meat from different sources. A range of intrinsic factors influence the oxidative balance in raw meat and thereby the colour stability of the meat (Bertelsen et al., 2000). Thus the oxidative stability of muscles is dependent on the composition, concentrations, and reactivity of (i) oxidation substrates (lipids, protein and pigments), (ii) oxidation catalysts (prooxidants such as transition metals and various enzymes) and (iii) antioxidants, e.g., vitamin E and various enzymes. For review see Bertelsen et al. (2000). 

Despite its slow reactivity, the ascorbyl radical usually gives up a second electron to produce dehydroascorbate. This molecule is unstable and needs to be caught quickly if it is not to break down spontaneously and irrevocably, and be lost from the body. The continual seeping loss of vitamin C in this way accounts for our need to replenish body pools by daily intake. Even so, we can minimize losses by recycling dehydroascorbate. Several different enzymes bind dehydroascorbate to regenerate vitamin C. These enzymes usually take two electrons from a small peptide called... 

Several chemical and epidemiological studies confirmed that olive oil s beneficial effects are related to high concentration of oleic acid and the presence of vitamin and non-vitamin antioxidants, such as DPE. This compound shows different biological actions one of them is the inhibitory effect on peroxynitrite dependent DNA base modification and tyrosine nitration. Furthermore, DPE counteracts cytotoxicity, caused by reactive oxygen species, ROS, in Caco-2 cells and in erythrocytes [70]. In particular, the Caco-2 cells imitate, in vitro, the food-intestinal tract interaction. 

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