Stability 305 tocopherols

Selected Substituent-Stabilized Tocopherols and Conjugatively Stabilized Ortho-Quinone Methides... 

Ascorbic Acid and Tocopherol. In certain foods and beverages ascorbic acid and AP synergized other antioxidants and tocopherols, and some of this data was reviewed (5,35). Both a- and y-tocopherol, similar to ascorbic acid, react with Cu and Fe and as a result ascorbic acid may sacrificially stabilize tocopherols (36). However, ascorbic acid transforms -tocopheroxide to a-tocopherol and the dimeric keto ether to "bi-a-tocopheryl, thus regenerating the two antioxidant species from oxidized tocopherols (37). Packer et al. (38) mixed trichloromethylper-oxy radical with -tocopherol under pulse radiolysis to form tocopherol radical (absorbs at 425 nm). When ascorbic acid was added, ascorbic radical formed, absorbing at 360 nm the 425-nm absorption was lost. The rapid interaction may recycle tocopherol at the expense of ascorbic acid. 

Parcerisa, J., Rafecas, M., Castellote, A.I., Codony, R., Farran, A., Garcia, J., Gonzalez, C., Ldpez, A., Romero, A., and BoateUa, J., Influence of variety and geographical origin on the lipid fraction of hazelnuts (Corylus avellana L.) from Spain (III). Oil stability, tocopherol content and some mineral contents (Mn, Fe, Cu), Food Chem., 53, 71-74, 1995. 

Following the action of extraordinary stimulants (hypoxic hypoxia, hypoxia + hyperoxia, hypodynamia + hyperthermia), animals demonstrate an accumulation of malonic dialdehyde with a simultaneous fall of antiradical activity of the liver tissue. A preliminary introduction to rats of acetylene amine 3,4,5-tris(morpho-linopropynyl)-l-methylpyrazole 103 and also of tocopherol antioxidant and gutumine antihypoxant averts activation of the lipid peroxidation processes. The inhibition of peroxidation with this agent is mediated by stabilization of ly-zosomal and mitochondrial membranes. Unsaturated amines prevent destruction of the organelle membranes provoked by UV irradiation and incubation at 37°C (pH4.7)(78MIl). 

Another approach to safer stabilization is to use a biological antioxidant such as vitamin E (a-tocopherol is the active form of vitamin E, AO-9, Table la). It is essentially a hindered phenol which acts as an effective chain breaking donor antioxidant, donating a hydrogen to ROO to yield a very stable tocopheroxyl radical, a-Tocopherol is a very effective melt stabilizer in polyolefins that offers high protection to the polymer at very low concentration [41], (Table 2). 

PENG Y s, PENG Y M, MCGEE D L and ALBERTS D s (1994) Carotenoids, tocopherols and retinoids in human buccal mucosal cells intra- and inter-individual variability and storage stability. J Clin Nutr 59(3) 636-43. 

N-Nitrosamine inhibitors Ascorbic acid and its derivatives, andDC-tocopherol have been widely studied as inhibitors of the N-nitrosation reactions in bacon (33,48-51). The effect of sodium ascorbate on NPYR formation is variable, complete inhibition is not achieved, and although results indicate lower levels of NPYR in ascorbate-containing bacon, there are examples of increases (52). Recently, it has been concluded (29) that the essential but probably not the only requirement for a potential anti-N-nitrosamine agent in bacon are its (a) ability to trap NO radicals, (b) lipophilicity, (c) non-steam volatility and (d) heat stability up to 174 C (maximum frying temperature). These appear important requirements since the precursors of NPYR have been associated with bacon adipose tissue (15). Consequently, ascorbyl paImitate has been found to be more effective than sodium ascorbate in reducing N-nitrosamine formation (33), while long chain acetals of ascorbic acid, when used at the 500 and lOOO mg/kg levels have been reported to be capable of reducing the formation of N-nitrosamines in the cooked-out fat by 92 and 97%, respectively (49). 

Detailed Formation Pathway and Stabilization of the Tocopherol-Derived o-QM 3 and Other o-QMs... 

FIGURE 6.17 Oxidation of a-tocopherol (1) conventionally leads to its spiro dimer (9) via ortho-quinone methide 3 (path A). The zwitterionic o-QM precursor 3a is stabilized by NMMO in complex 17, which upon rapid heating produces small amounts of new dioxocine dimer 18 (path B). Acid treatment of 18 causes quantitative conversion into spiro dimer 9, via o-QM 3 (path C). 

It was shown that complexes 19 of the zwitterionic precursors of ortho-quinone methides and a bis(sulfonium ylide) derived from 2,5-di hydroxyl 1,4 benzoquinone46 were even more stable than those with amine N-oxides. The bis(sulfonium ylide) complexes were formed in a strict 2 1 ratio (o-QM/ylide) and were unaltered at —78 °C for 10 h and stable at room temperature under inert conditions for as long as 15—30 min (Fig. 6.18).47 The o-QM precursor was produced from a-tocopherol (1), its truncated model compound (la), or a respective ortho-methylphenol in general by Ag20 oxidation in a solution containing 0.50-0.55 equivalents of bis(sulfonium ylide) at —78 °C. Although the species interacting with the ylide was actually the zwitterionic oxidation intermediate 3a and not the o-QM itself, the term stabilized o-QM was introduced for the complexes, since these reacted similar to the o-QMs themselves but in a well defined way without dimerization reactions. 

The stabilization of the zwitterionic o-QM precursors is due to electrostatic interactions. It was reasonable to assume that also the other methods of stabilizing the zwitterions might be viable, and indeed it was confirmed that both steric and electronic effects are able to stabilize such intermediates. In 5-(4-octyl)-y-tocopherol (5a-butyl-5a-propyl-a-tocopherol, 21), the octyl group acts as a flywheel, which impedes the rotation of the C-5a moiety into the ring plane as compared to the parent zwitterions with the unsubstituted exocyclic methylene group. The situation is... 

Stabilization of the zwitterionic intermediate in o-QM formation can also occur intramolecularly. In this case, the stabilizing moieties must be able to dissipate the positive charge at the benzylic group by a resonance effect and prevent rotation of the exocyclic methylene group by a steric blocking. One example for such a temporary stabilization is the nitration of a-tocopheryl acetate (25) by concentrated HNO3, which produced 6-0-acetyl-5-nitro-a-tocopherol (27) in quite good yields,48 the... 

An interesting feature of 5-tocopherylacetic acid (51) and its derivatives was their appreciable thermal stability up to 200 °C. In contrast to 5a-substituted tocopherols carrying an electronegative substituent at C-5a, the homopolar C—C bond in the C2-unit at the 5-position of the tocopherol skeleton was shown to be very stable. Thermal decomposition of 51 at temperatures above 250 ° C caused a complete breakdown of the chroman structure, the C3-unit consisting of C-2, C-2a, and C-3 being eliminated as propyne, the side chain as 4,8,12-trimethyltridec-l-ene (Fig. 6.38). Fragmentation... 

FIGURE 6.45 Inability of 5a-substituted derivatives to form structures analogous to o-QM 3 causes increased oxidative stability as in compounds 71 and 72. 5-(p-Hydroxyphenyl)-y-tocopherol (73) is oxidized to the conjugatively stabilized o-QM 74, the phenylogous a-tocored (75). 

FIGURE 6.46 Oxidation chemistry of 5-(4-methylphenyl)- y-tocopherol (76), establishing a reaction system phenylogous to a-tocopherol (1), with quinone methide 77 and benzyl bromide 78 being the conjugatively stabilized, phenylogous counterparts of o-QM 3 and 5a-bromo-a-tocopherol (46), respectively. 

The presence of tocopherols,114 as well as caffeic acid,108 accounts for the remarkable stability of green coffee bean oils toward oxidation. In coffee beans from different origins, a-tocopherol concentrations are in the range 89 to 188 pg/kg and (p + y)-tocopherol concentrations are in the range 252 to 530 pg/kg.114 Since p- and y-tocopherols have better antioxidant properties than a-tocopherol,114 it is not surprising to see coffee oil patented as an antioxidant material.115117... 

A study carried out in Ireland which also compared oxidative stability and a-tocopherol levels in organic, free-range and conventional breast meat burgers from France, Ireland and the UK showed significant and greater variability between different samples from the same production system, than between production systems. They concluded that oxidative stability and a-tocopherol... 

The effect of vitamin E supplementation on a-tocopherol and /(-carotene concentrations in tissues from pasture- and grain-fed cattle was also elucidated with HPLC analysis. The investigation was motivated by the fact that a-tocopherol influences beneficially meat colour and stability [53], and the presence of /(-carotene can modify the amount of a-tocopherol in tissues [54], Samples were extracted with hexane and the concentration of /(-carotene was assessed by HPLC. Some data are listed in Table 2.20. It was concluded from the data that... 

Choy et al. [193] reported that vitamin A (retinoic acid), vitamin C (ascorbic acid) and vitamin E (tocopherol) could be intercalated into Zn/Al LDHs by the coprecipitation method. In solutions, these vitamins are normally all sensitive to light, heat and oxygen, and it was proposed that incorporating the molecules into a layered inorganic lattice may lead to their stabilization, resulting in a wider range of potential apphcations. 

The solid matrix of SLN protects the drug from hydrolysis and oxidation. Chemical stability of tocopherol and retinol improves considerably [17,39], with tocopherol improving by 57% compared with an aqueous dispersion. The degree of retinol stabilization depends on the nature of lipid and surfactant [39]. For each drug, the optimal preparation has to be defined individually. 

---