Tocopherol - Forms

Forms

Vitamin E exists in eight different forms, four tocopherols and four tocotrienols. All feature a chromanol ring, with a hydroxyl group that can donate a hydrogen atom to reduce free radicals and a hydrophobic side chain which allows for penetration into biological membranes.

Both the tocopherols and tocotrienols occur in alpha, beta, gamma and delta forms, determined by the number and position of methyl groups on the chromanol ring.

The tocotrienols have the same methyl structure at the ring and the same Greek letter-methyl-notation, but differ from the analogous tocopherols by the presence of three double bonds in the hydrophobic side chain. The unsaturation of the tails gives tocotrienols only a single stereoisomeric carbon (and thus two possible isomers per structural formula, one of which occurs naturally), whereas tocopherols have 3 centers (and eight possible stereoisomers per structural formula, again, only one of which occurs naturally).

Each form has slightly different biological activity. In general, the unnatural l-isomers of tocotrienols lack almost all vitamin activity, and half of the possible 8 isomers of the tocopherols (those with 2S chirality at the ring-tail junction) also lack vitamin activity. Of the stereoisomers which retain activity, increasing methylation, especially full methylation to the alpha-form, increases vitamin activity. In tocopherols, this is due to the preference of the tocophrol binding protein for the alpha-tocopherol form of the vitamin.

As a food additive, tocopherol is labeled with these E numbers:E306 (tocopherol), E307 (α-tocopherol/alpha-tocopherol), E308 (γ-tocopherol/gamma-tocopherol), and E309 (δ-tocopherol/delta-tocopherol). These are all approved in the USA, EU and Australia and New Zealand for use as antioxidants.