November 2, 2022

Chemical and physical properties of vitamin E

Vitamin E is the major lipid-soluble component in the cell antioxidant defence system and is exclusively obtained from the diet. The tocopherols are viscous oils at room temperature, insoluble in water but soluble in ethanol and aprotic solvents. Vitamin E is a slightly yellow to amber, practically odorless and tasteless, clear, viscous oil, which darkens on exposure to air or light by oxidation.

Vitamin E is a generic term denoting eight different isomers among which α-tocopherol is the most important and most active.

The melting point of RRR-α-tocopherol is 3 °C. The optical rotations of tocopherols are very small and depend on the nature of the solvent. RRR-α-tocopherol (RRR-α-T), is known to be more bioactive than all-rac-α-tocopherol (all-rac-α-T), a synthetic racemic mixture of 8 stereoisomers.

Tocopherols are easily oxidized and can be destroyed by peroxides ozone and permanganate in a process catalyzed by light and accelerated by polyunsaturated fatty acids and metal salts.

The ultraviolet absorption spectra of tocopherols and tocotrienols in ethanol show an absorption maximum at 292–298 nm, while the infrared spectra show OH (2.8 ± 3.0 µm) and CH (3.4 ± 3.5 µm) stretching and a characteristic band at 8.6 µm.

Vitamin E is a fat-soluble vitamin that consists of a group of tocols and tocotrienols with hydrophobic character, but possessing a hydroxyl substituent that confers an amphipathic character on them.

The interactions between vitamin E and water are hydrophobic in nature; hence, vitamin E is found to be insoluble in water since vitamin E is non-polar and water is polar in nature. Solubility is reported for α-tocopherol in aqueous mixtures containing ethanol at 33 °C. It has very low solubility in pure water owing to hydrophobic repulsion.

α-Tocopherol is fluorescent with an emission maximum about 325 nm in a hydrophobic solution. The bond dissociation energy of α-tocopherol's O–H bond is 77.1 kcal mol−1.31 The pKa values for α-, β-, γ-, and δ-tocopherol in a micellar solution were reported as 13.1, 12.8, 12.7, and 12.6 respectively.

The α-tocopherol are unstable to air and light, particularly when in alkaline media. α-tocopherol acid succinate is insoluble in water, slightly soluble in alkaline solutions, soluble in alcohol, in ether, in acetone and in vegetable oils; very soluble in chloroform.

The partition coefficient of α-tocopherol is 12.2, while that for ascorbic acid is −1.85,30 showing their lipophilic and hydrophilic characters, respectively.
Chemical and physical properties of vitamin E

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