 All-rac-alpha-tocopherol

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.^[ref] It is a description of stereochemistry, rather than a different alpha ring-methylation pattern.

The three stereocenters at $2$, $4^{\prime }$ and $8^{\prime }$ 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.