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DRI DIETARY REFERENCE INTAKES FOR Vitamin C, Vitamin E, Selenium, and Carotenoids
FIGURE 6-2all rac-α-Tocopherol structures. Shown are the eight different stereoisomers in synthetic vitamin E (all rac-α-tocopherol): RRR-, RSR-, RRS-, RSS-, SRR-, SSR-, SRS-, and SSS-. All eight stereoisomers are formed in equal amounts. One stereoisomer, RRR-α-tocopherol, is also naturally present in food. The structure differences occur in the side chain and most importantly at the ring/tail junction.
or γ-tocopherols, dimethyl [2 methyl groups on the chromanol ring at different positions]; and δ-tocopherol, monomethyl [1 methyl group on the chromanol ring]) and 4 tocotrienols (α-tocotrienol, trimethyl; β- or γ-tocotrienols, dimethyl; and δ-tocotrienol, monomethyl) (IUPAC-IUB Joint Commission on Biochemical Nomenclature, 1974). The tocopherols are characterized by a substituted, hydroxylated ring system (chromanol ring) with a long, saturated (phytyl) side chain (Figure 6-1). Tocotrienols differ from tocopherols only in that they have an unsaturated side chain. All tocopherols that occur naturally in foods have the RRR stereochemistry in the side chain. However, the various forms of vitamin E are not inter-