Structure and physicochemical properties of vitamin A

Vitamin A refers to all compounds that have the biological activity of retinol. There are two broad classes of substances that provide retinol biological activity. One is retinol, its metabolites, and synthetic analogs with similar structures. This class is also known as retinoids, also known as pre-formed vitamin A. The main dietary source is contained in animal foods. Retinol and retinyl ester. Another type of substance is vitamin A protocarotenoids, which are carotenoids that can be converted into retinol in the body from plant foods. They are precursors of dietary retinol, mainly including beta-carotene. , α-carotene and β-cryptoxanthin.
Vitamin A is a family of 20 carbon structures with a β-ionone ring, a side chain composed of four end-to-end isoprenoid units, and a hydroxyl group at the carbon-15 position. Molecular collection of alcohols, or aldehyde groups (retinal), or carboxylic acid groups (retinoic acid), or ester groups (retinyl esters). Carotenoids are polyisoprene compounds or terpenoids. It has been found that there are more than 600 forms of carotenoids in nature, only some of which have the original nutritional activity of vitamin A, but only β-carrots with the meaning of dietary vitamin A. Three kinds of hormones, α-carotene and β-cryptoxanthin. The all-trans isomer is the most common and stable form of each carotenoid, however, many cis isomers are also present. Carotenoids typically contain 40 carbon atoms and have a broad conjugated double bond system with one or two cyclic structures at the end of their conjugated carbon chain. An exception is lycopene, which has no cyclic structure and no vitamin A activity.
Vitamin A is a fat-soluble vitamin that dissolves in most organic solvents to varying degrees but is insoluble in water. Vitamin A and its derivatives are easily oxidized and isomerized, especially when exposed to light (especially ultraviolet light), oxygen, reactive metals, and high temperature environments. But the general cooking process does not cause too much damage to vitamin A in food. Under ideal conditions, such as cryopreservation, serum, tissue or crystalline retinoids can remain stable for long periods of time. Under anaerobic conditions, retinal is relatively stable to alkali, but unstable in acid, and dehydrogenation or double bond rearrangement can occur. In the process of rancidity, the vitamin A and carotene contained in the oil will be seriously damaged. Phospholipids, vitamin E or other antioxidants in foods have the effect of increasing the stability of vitamin A. Among the derivatives of vitamin A, retinoic acid and retinyl ester have the best stability.
Both retinol and other retinoids have continuous conjugated double bonds that produce unique ultraviolet or visible light absorption spectra. The maximum absorption wavelength in ethanol is 325 nm for all-trans retinol, 381 nm for all-trans retinal, and 350 nm for all-trans retinoic acid. Retinol can produce 470 nm fluorescence when exposed to ultraviolet light at 325 nm. At present, the most common method for detecting retinoids is to use the above characteristics, using reversed-phase high performance liquid chromatography with an ultraviolet/fluorescence detector. Vitamin A is mainly stored in the liver in the body, accounting for about 90%-95% of the total amount, and a small amount is present in the adipose tissue.
Beta-carotene is one of the most prominent components of carotenoids because it is the first known carotenoid component; it is almost a high carotenoid component in the human body; it is most distributed in our food. Wide and rich in content, especially in vegetables and fruits, almost all vegetables and fruits have more or less traces; in addition, it is also the most active vitamin A in the carotenoid component.
The β-carotene has a molecular formula of C40H56 and a molecular weight of 536.87. Its molecular structure has many conjugated double bonds. These double bonds can absorb certain spectra in visible light, giving them a special color and making them extremely strong. The ability to quench the active oxygen free radicals can reduce the body's antioxidant damage and thus play a disease prevention role. The β-carotene molecule is actually two tail-linked retinol molecules that can be converted into two or one vitamin A by central cleavage or eccentric cleavage. Beta-carotene is further divided into all-trans and cis isomers. All-trans-beta-carotene is cleaved centrally to produce two molecules of all-trans retinol (vitamin A), and the conversion of cis-beta-carotene to vitamin A is lower.
Α-carotene is similar to β-carotene in molecular structure and is an isomer. The difference is that the 5', 6' double bond in the β-ionone ring at one end changes, and the β-ionone ring is The structure necessary for vitamin A activity. Therefore, the conversion of alpha-carotene to vitamin A is only half that of beta-carotene. In addition to vitamin A activity. The nature and efficacy of alpha-carotene is similar to that of beta-carotene.
Beta-stabilized yellow pigment, also known as β-cryptoxanthin, β-carotene-3-ol, is an oxygenated lutein-like carotenoid with a molecular formula of C40H56O and a molecular weight of 552.87. Compared with β-carotene, the β-cryptoxanthin molecular structure is replaced by a light base at the 3 position, and its molecule has one more oxygen atom than β-carotene, thereby causing β-ionone Changes in the structure of the ring make this half molecule lose the activity of vitamin A. Therefore, like β cryptoxanthin and α-carotene, the yield of conversion to vitamin A is only half of that of β-carotene. In addition to vitamin A activity, β cryptoxanthin also has strong antioxidant activity.
The carotenoids in the diet are relatively stable, less damage during cooking, and the processing and heat treatment of foods can increase the release of carotene in plant cells and increase their absorption rate. However, the long-term high temperature, especially under the conditions of aerobic and ultraviolet radiation, will increase significantly. In China's cooking method, the preservation rate of carotene is 70% to 90%.