The ionization behaviour of DL-alpha-tocopherol (vitamin E) in model membranes: micelles and vesicles

The ionization behaviour of DL-alpha-tocopherol (vitamin E) has been investigated in dodecyltrimethylammonium bromide (DTAB), hexadecyltrimethylammonium chloride (CTAC) and dodecyldimethyl propiobetaine (DPB) micelles and didodecyldimethylammonium bromide (DDAB) and dimyristoylphosphatidylcholine (DMPC) vesicles. The pKa values for DL-alpha-tocopherol in the aqueous self-assembled solutions of DTAB, CTAC, DPB, DDAB and DMPC were 12.0 +/- 0.1, 11.7 +/- 0.1, 13.1 +/- 0.1, 11.0 +/- 0.2 and greater than 14.5, respectively. It is shown how these pKa results confirm that DL-alpha-tocopherol exists predominantly in the un-ionized form when localized in any type of micelle or vesicle at physiological pH values.