= RRR-alpha-tocopherol
{c}
{title2=$(2R,4'R,8'R)$}
\b[<RRR-alpha-tocopherol>] is the <alpha-tocopherol> configuration $(2R,4'R,8'R)$. The nomenclature identifies it with the naturally occurring stereoisomer described in the recommendations.<Nomenclature of Tocopherols and Related Compounds by IUPAC-IUB Joint Commission on Biochemical Nomenclature>[Recommendations 11 and 12a; Table 1, entry 1]{ref} The prefix records three configurations; “alpha” separately identifies ring methyl substitution.
R and S are descriptors assigned by stereochemical sequence rules. They do not by themselves specify the sign or magnitude of measured optical rotation. For this molecular structure, inverting all three <stereocenters> gives SSS, its <enantiomer>. Inverting only the ring center gives SRR, a <diastereomer>. Thus a change at one center is not the same operation as taking a mirror image of the whole molecule.
An original dilution example begins with 4.00 mg of pure <RRR-alpha-tocopherol>. If material is transferred quantitatively to a final volume of 25.00 mL, its mass concentration is $0.160\,\mathrm{mg\,mL^{-1}}$. Using the <alpha-tocopherol> <molar mass> $430.707\,\mathrm{g\,mol^{-1}}$, its amount concentration is approximately $371.5\,\mathrm{\mu mol\,L^{-1}}$.<Alpha-tocopherol in ChEBI>[CHEBI:22470, Formula and Average Mass]{ref} Identity, purity and final volume are independent inputs to that calculation.
By contrast, 4.00 mg of a stipulated equal eight-form <all-rac-alpha-tocopherol> mixture contains only 0.500 mg of the RRR configuration. Both samples have the same total alpha-tocopherol mass, but different stereoisomer composition. This is a chemical accounting comparison; it does not assign nutritional equivalence or a treatment effect.
A measurement intended to certify the RRR amount must distinguish stereochemistry, rather than reporting only the shared <molecular formula>. Appropriate standards and a selective analytical method determine what the stated purity actually means.
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