Tocopherols
The most important naturally occurring compound with vitamin E activity is α-tocopherol. It has 3 chirality centres (2, 4', 8') at which the methyl groups are in R configuration.
The naturally occurring α-tocopherol is usually accompanied by small amounts of β-, γ- and δ-tocopherol, which differ in the number and position of the methyl groups on the ring.
The most common synthetic Vit. E consists of 8 stereoisomers, with R,R,R-α-tocopherol accounting for 12.5 %. The mixture is known as all-rac-α-tocopherol.
Quantitatively, the effect of the various tocopherols differs greatly. Compared to R,R,R-α-tocopherol (= 100 %), all other naturally occurring tocopherols have less than 50 % of the activity, all-rac-α-tocopherol 74 % and the frequently used all-rac-α-tocopherol acetate, which is esterified with acetic acid to increase stability, 67 %.
In order to take these different Vit. E activities into account, they are often stated in I.U. or U.S.P. (United States Pharmacopeia).