Homologation Tocopherol

P-carotene using the Bickoff test and the influence of homologous tocopherols added and exposed to light at 20°C, there was a significant decrease in decomposition dynamics — it was from 4.2 to 5 times slower in the case of a-, y-, and 8-T than in the control sample (Nogala-Kalucka and Zabielski, 1999). 

Kupczyk, B. and Gogolewski, M. (1999). Effects of gamma irradiation and menadione (vit. K3) on dissolution and dimerization of homologous tocopherols. Effect of storage time, Food Sci. TechnoL, 3, 39. 

Differentiation of homologous tocopherols and tocotrienols, with particular emphasis on the resolution of positional isomers... 

Table 9-7 Tocopherol (T) and Tocotrienol (T3) Content of Vegetable Oils and Their Primary Homologs...

Tocopherols Tocopherols are widely distributed in nearly all vegetable oils but not in animal fats. Their basic structure consists of the hypothetic tocol molecule, a chromanol structure with a farnesyl side chain, substituted by one, two, or three methyl groups in the positions 5, 7, or 8. (Figure 5). A homologous... 

Quinones play an important role in both covalent (synthetic) and nonco-valent (synkinetic) assemblies, which are supposed to allow electron transport through membranes and light-induced charge separation processes. With respect to their ability to carry electrons through lipid membranes, quinones with a side chain of more than two isoprene units are far more efficient than homologs with shorter chains. In the reduction of vesicle-entrapped ferricyanide ions by external dithionite, for example, the long side chain may act as the rod of a pendulum (Fig. 7.2.2) leading to fast electron transfer. Tocopherol and other branched-chain hydrocarbons (e.g., phytol,) also disturb egg lecithin... 

FIGURE 28.1 Structures of various homologs of tocopherol and tocotrienol. 

Tocopherols and tocotrienols comprise the group of eight chromanol homologs that possess vitamin E activity in the diet. They are natural monophenolic compounds with varying antioxidant activities. The a-, P-, y-, and 5-tocopherols are characterized by a saturated side chain consisting of three isoprenoid units, whereas their corresponding tocotrienols have double bonds at the 3, 7, and 11 position of the isoprenoid side chain (Figure 5.3). 

Application of HPLC for the quantitation of tocopherols in the pecan provided information on the individual tocopherol homologs (a-, P-, y-, 8-tocopherol). Summation of the four tocopherols yields a measure of the total vitamin E level, as tocotrienols are not present. 

Except 5-methyltocol (Bacharach and Green, 1961) and tocol [2-methyl-2-(4, 8, 12 -trimethyltridccyl)-6-chromanol], all the lower homologs of a-tocopherol bearing only one or two methyl groups on the aromatic nu-... 

Chromatographic separations of phytol isomers, ibuprofen enantiomers, and tocopherol homologs by SMB-SFC have been performed experimentally and were further investigated by simulations [29, 55, 58, 65-67]. The apparatus for experimental separations by SMB-SFC as well as the methods and models for determination of the adsorption equilibrium as the basis for chromatography simulation have been described elsewhere [27, 68]. 

The relative antioxidant activity of different tocopherol homologs has received much attention in the literature. On the basis of induction period... 

K. 1993. Primary structure of alpha-tocopherol transfer protein from rat liver. Homology with cellular retinaldehyde-binding protein. J Biol Chem, 268, 17705-10. 

Lang et al. (1992) stated that although TLC has value as an inexpensive qualitative and semiquantitative assay for vitamin E, HPLC or GC is more suitable for quantitative analysis. However, given proper sample preparation, TLC is suitable for all sample types and for separating tocopherol and tocotrienol homologs. Although one-dimensional systems are usually satisfactory, additional resolution can be achieved by developing in a second direction. Quantification can be done by in situ densitometry. TLC is also useful for sample cleanup prior to other quantitative procedures such as GC or HPLC. 

Vitamin E is a collective term for tocopherols and tocotrienols, a series of potent antioxidants structurally derived from 6-chromanol (Fig. 6). Tocopherols and tocotrienols differ in the degree of saturation of their respective isoprenoid side chains. Among the tocopherols, tocol can be regarded as the unsubstituted parent molecule, whereas 8-, P + 7-, and a-tocopherol form a homologous series of mono-, di-, and trimethyltocols, respectively, P- and y-tocopherols being positional isomers (46). 

Tocopherols and tocotrienols are abundant in seeds, grains, and vegetable oils (46), but only the tocopherols are absorbed by humans to an appreciable extent (47). a-Tocopherol is the predominant form in plasma and is biologically the most active homolog. Although its exact biochemical function in humans is controversial, vitamin E is known to protect membrane lipids from peroxidation by scavenging free radicals and other oxidative species (48). A number of oxidized metabolites, e.g. a-tocopherylquinone have been suggested (49). 

Tocopherol and tocotrienol homologs can be separated on reversed-phase columns but the positional isomers coelute. The tocotrienols as a group elute ahead of the tocopherols, the latter being retained in the order 8-T < P + y-T... 

The combination of SEC with MS has been briefiy addressed in V.C.3. Mobile phases in SEC consist of CO2 and a polar modifier such as ethanol. Oils and fats are either extracted by SEE or dissolved in a nonpolar solvent prior to SEC. A thorough study dedicated to SEC of tocopherols is that of Yarita et al. (144). These investigators examined the retention behavior of a-, P-, y-, and 8-tocopherol on a silica ODS column eluted with mixtures of CO2 and methanol. At low modifier concentrations (0.5% of methanol), P- and y-tocopherol were baseline separated. Increasing the modifier content of the eluent improved the resolution of the other tocopherols but at the expense of that of the P and y pair. The four homologs elute according to the number of methyl groups, but the elution order of P- and y-tocopherol depends on the concentration of the modifier. 

E. Only Spencer and Purdy applied MEKC specifically to the various E vitamers (173). These investigators added p-cyclodextrins to a buffer containing SDS micelles. The four natural vitamin E homologs, including p- and y-tocopherol, were resolved in 30 min. In contrast, positional isomers could not be separated by reversed-phase HPLC using cyclodextrins as additives to the mobile phase. 

Benzofurancarboxylic acids are homologated via the corresponding dianion. An improved route to a key benzopyran-acetic acid intermediate in the Roche synthesis of a-tocopherol has been described. ... 

Sato, Y, Aral, H, Miyata, A, Tokita, S, Y amamoto, K, Tanabe, T and Inoue, K (1993) Primary structure of alpha-tocopherol transfer protein from rat liver. Homology with cellular retinaldehyde-binding protein. J. Biol. Chem., 268, 17705-17710. 

Vitamin E tocopherol) is related to the quinones with the isoprenoid side chain and can easily be oxidized to a quinone (tocoquinone). The formula and synthesis have been discussed in Chapt. XIV-9. Various homologs of tocopherol occur naturally they differ only in the number of methyl groups. Not enough is known about the biochemical mechanism of action of this Adtamin. Several authors believe that it is involved in the respiratory chain (cf. Chapt. X—4). 

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