In addition to being found naturally in foods, alpha-tocopherol can also be synthesized. It is important to know whether alpha-tocopherol is natural or synthetic because the stereochemistry differs between these forms. Alpha-tocopherol contains 3 chiral centers (non-superimposable mirror images) designated as R or S. If this is a new topic to you or you would like a refresher, the following link is a fun, animated chirality game.
The 3 chiral centers in alpha-tocopherol are located at the 2, 4’, and 8’ positions. You can see the full numbering of tocopherols in the link below. In short, the rings are normal numbers and the tail is prime numbers.
The figure below shows the 3 chiral centers without the other numbers.
Figure 9.211 The 2, 4’, and 8’ positions of alpha-tocopherol are chiral centers
In natural alpha-tocopherol, all 3 chiral centers are in the R configuration. Thus, it is designated RRR-alpha-tocopherol. The R’s represent the 2, 4’, and 8’ positions of alpha-tocopherol, respectively, as shown below1.
Figure 9.212 Natural alpha-tocopherol 2, 4’, and 8’ positions are in the R conformation
Synthetic alpha-tocopherol is a racemic (equal) mixture of all the different stereochemical possibilities at the three chiral centers. These are:
The two forms of alpha-tocopherol are designated (these are placed before alpha-tocopherol to indicate whether it is natural or synthetic) as listed below:
- New designation: RRR-alpha-tocopherol (because all 3 positions are RRR)
- Old designation: d-alpha-tocopherol
- New designation: all-rac-alpha-tocopherol (because it is a racemic mixture)
- Old designation: dl-alpha-tocopherol
The old d and dl designations were describing the chemical structure that is sometimes still used. Keep in mind the natural and synthetic are describing the stereochemistry of alpha-tocopherol and not whether it is naturally derived. For example, there are natural alpha-tocopherol derivatives where the derivatives are added through synthetic procedures.
References & Links
- Chirality Game - nobelprize.org/educational_ga...game/game.html
- Tocopherol Numbering - http://www.chem.qmul.ac.uk/iupac/mis...Greek/toc.html