= All-rac-alpha-tocopherol
\b[<All-rac-alpha-tocopherol>] designates an <alpha-tocopherol> stereoisomer mixture containing all four pairs of <enantiomers>. The nomenclature does not require exactly equal proportions of the pairs.<Nomenclature of Tocopherols and Related Compounds by IUPAC-IUB Joint Commission on Biochemical Nomenclature>[Recommendation 12e and Table 1, entry 4]{ref} It is a description of <stereochemistry>, rather than a different alpha ring-methylation pattern.
The three <stereocenters> at $2$, $4'$ and $8'$ permit the configurations RRR, RRS, RSR, RSS, SRR, SRS, SSR and SSS, with the letters read in that order. The <enantiomer> pairs are RRR/SSS, RRS/SSR, RSR/SRS and RSS/SRR. Inverting every center gives the partner; changing only some centers generally gives a <diastereomer>.
For an original equal-composition model, 80 micromoles of the mixture contains 10 micromoles of each configuration. The <RRR-alpha-tocopherol> fraction is $1/8$, while the total fraction with R at position 2 is $4/8$. These values follow from the additional equal-proportions assumption. A sample with four racemic pairs of unequal abundance can satisfy the all-rac description while having a different RRR fraction.
All eight configurations share a <molecular formula> and <molar mass>. A total-mass measurement cannot recover their distribution. A stereochemically discriminating separation or another validated selective measurement supplies additional information. Conversely, the chemical fraction of one configuration is not automatically its fraction of biological effect: that conversion would require a stated biological response model, including differences in transport, retention or metabolism.
The distinction is useful when comparing analytical specifications. “Alpha” fixes ring methylation; “all-rac” specifies a class of stereoisomer mixtures; a reported concentration specifies amount on the assay's chosen basis. None can replace the others.
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