Chemistry of Ascorbic Acid
Pure vitamin C is a white crystalline odorless substance with a sour (acid) taste. It melts 90°-192°C, and in the crystalline form it is stable for years. It is insoluble in most oragnic elements, although a 2 per cent solution can be made in alcohol. In water the vitamin is soluble :he extent of 1 gin 3 ml. The strong reducing property of vitamin C depends on the loss of • drogen atoms from the hydroxyls on the double-bonded (endio!) carbons. At pH 4 and at 35 °C the E' 0 +0.166 volt.
A dilute solution of vitamin C has a pH of about 3. The acidity is due to ionization of the 1 group on carbon atom 3. The first pKa is 4.17; this indicates that ascorbic acid is iderably more dissociated than acetic, for instance. The second pK9 is 11.57. Vitamin C, L-ascorbic acid, in water has a specific rotation [a]~o = +23°. The ultraviolet absorption 'mum of ascorbic acid is at a wavelength of 265 mJl. Ascorbic acid readily forms salts of veral metals, It takes up iodine at the double bond and can be reduced here by hydrogenation. Oxidation of ascorbic acid yields dehydroascorbic acid. This is a freelY reversible action. H2S, among other things, may be used to reduce the oxidized form in the laboratory.
The dehydro form, except in rather acid solution, undergoes hydrolysis at the lactone ring .th the formation of diketogulonic acid. The reverse of this Reaction does not proceed in e body but can be brought about in the laboratory. The greater stability of ascorbic acid in . d solution depends on the decreased tendency toward hydrolysis of the J actone ring with
easing pH. In alkaline solution the hydrolysis is fairly rapid, and such solutions lose vitamin 'vity in a short period of time. The oxidation of ascorbic acid in vitro is catalyzed by various substances.
The copper ion is quite active and, of course, the plant ascorbic acid oxidase (a copperprotein enzyme) is highly active. The rate of destructive oxidation is greater with increasing pH. This type of oxidation involves molecular oxygen, and, consequently, in processing vitamin Cntaining foods, such as orange juice, the removal of oxygen by nitrogen or CO2 results in reased losses of the vitamin during canning or other processing. Low-temperature storage ·vegetables before processing, though usually impractical, and a quick preheating (blanching) just previ6ils to canning or freezing also aid in decreasing ascorbic acid destruction. --nfortunately, the vitamin C loss during blanching of many foods may be considerable owing part to oxidation at the elevated temperature, and perhaps to a greater extent through
solubility of the vitamin in the blanching water:
Several synthetic compounds have some vitamin-C activity. Thus, 6-desoxy-L-ascorbic acid has about one-third, L-rhamnoascorbic about one-fifth, D-arabo-ascorbic about one-twentieth, and L-glucoascorbic around one-fortieth the activity ofL-ascorbic acid. The activity of D-ascorbic acid is nil.
Pure vitamin C is a white crystalline odorless substance with a sour (acid) taste. It melts 90°-192°C, and in the crystalline form it is stable for years. It is insoluble in most oragnic elements, although a 2 per cent solution can be made in alcohol. In water the vitamin is soluble :he extent of 1 gin 3 ml. The strong reducing property of vitamin C depends on the loss of • drogen atoms from the hydroxyls on the double-bonded (endio!) carbons. At pH 4 and at 35 °C the E' 0 +0.166 volt.
A dilute solution of vitamin C has a pH of about 3. The acidity is due to ionization of the 1 group on carbon atom 3. The first pKa is 4.17; this indicates that ascorbic acid is iderably more dissociated than acetic, for instance. The second pK9 is 11.57. Vitamin C, L-ascorbic acid, in water has a specific rotation [a]~o = +23°. The ultraviolet absorption 'mum of ascorbic acid is at a wavelength of 265 mJl. Ascorbic acid readily forms salts of veral metals, It takes up iodine at the double bond and can be reduced here by hydrogenation. Oxidation of ascorbic acid yields dehydroascorbic acid. This is a freelY reversible action. H2S, among other things, may be used to reduce the oxidized form in the laboratory.
The dehydro form, except in rather acid solution, undergoes hydrolysis at the lactone ring .th the formation of diketogulonic acid. The reverse of this Reaction does not proceed in e body but can be brought about in the laboratory. The greater stability of ascorbic acid in . d solution depends on the decreased tendency toward hydrolysis of the J actone ring with
easing pH. In alkaline solution the hydrolysis is fairly rapid, and such solutions lose vitamin 'vity in a short period of time. The oxidation of ascorbic acid in vitro is catalyzed by various substances.
The copper ion is quite active and, of course, the plant ascorbic acid oxidase (a copperprotein enzyme) is highly active. The rate of destructive oxidation is greater with increasing pH. This type of oxidation involves molecular oxygen, and, consequently, in processing vitamin Cntaining foods, such as orange juice, the removal of oxygen by nitrogen or CO2 results in reased losses of the vitamin during canning or other processing. Low-temperature storage ·vegetables before processing, though usually impractical, and a quick preheating (blanching) just previ6ils to canning or freezing also aid in decreasing ascorbic acid destruction. --nfortunately, the vitamin C loss during blanching of many foods may be considerable owing part to oxidation at the elevated temperature, and perhaps to a greater extent through
solubility of the vitamin in the blanching water:
Several synthetic compounds have some vitamin-C activity. Thus, 6-desoxy-L-ascorbic acid has about one-third, L-rhamnoascorbic about one-fifth, D-arabo-ascorbic about one-twentieth, and L-glucoascorbic around one-fortieth the activity ofL-ascorbic acid. The activity of D-ascorbic acid is nil.
