Ascorbic acid vitamin forms

Copper is a metallic element brass is an alloy or mixture of the metallic elements copper and zinc. The surfaces of copper and brass items tarnish with prolonged exposure to air, particularly in moist environments with high carbon dioxide (CO2) or sulfur dioxide (SO2) concentrations (see color Fig. 5.2.1). The compounds that form on the surface, ranging in color from black to blue to dark green, dissolve readily in acidic solutions. Vinegar contains acetic acid, ketchup contains tomatoes rich in ascorbic acid (Vitamin C), and onions contain malic acid and citric acid. All of these foods provide variable amounts of acid to dissolve the tarnish on copper surfaces. 

Ascorbic acid (vitamin C), the antiscurvy vitamin, is used by the body to form and maintain intercellular and skeletal material such as the collagen of fibrous tissue and the matrix of bone, dentin, and cartilage. Milk and milk products are not considered a significant source of this vitamin and should not be relied upon as such. Freshly drawn cow s milk contains about 2 mg ascorbic acid per 100 g milk, but as vitamin C is heat labile and easily destroyed by oxidation, the vitamin C content of pasteurized milk is reduced to about 0.94 mg/100 g. 

Ascorbic acid (vitamin C fig. 10.16) is the reducing agent required to maintain the activity of a number of enzymes, most notably proline hydroxylase, which forms 4-hydroxyproline residues in collagen. Hydroxyproline (see fig. 10.16c) is not synthesized biologically as a free amino acid but rather is created by modification of proline residues already incorporated into collagen. The hydroxylation reaction occurs as the protein is synthesized in the endoplasmic reticulum. At least a third of the numerous proline residues in collagen are modified in this way, substantially increasing the resistance of the protein to thermal denaturation. 

One of the best examples of this type of process is seen in the copper(u)-catalysed oxidation of ascorbic acid (vitamin C). Co-ordination of ascorbic acid (in the enediolate form) to copper(n) is a very favourable process and results in the formation of complex 9.2 containing a five-membered chelate ring (Fig. 9-2). 

Figure 17 Pseudo-first-order plot of thermal degradation of ascorbic acid (vitamin C) in a solid dosage form.

One-third of the amino acid residues in collagen are Gly, while another quarter are Pro. The hydroxylated amino acids 4-hydroxyproline (Hyp) and 5-hydroxylysine (Hyl) are formed post-translationally by the action of proline hydroxylase and lysine hydroxylase. These Fe2+-containing enzymes require ascorbic acid (vitamin C) for activity. In the vitamin C deficiency disease scurvy, collagen does not form correctly due to the inability to hydroxylate Pro and Lys. Hyl residues are often post-translationally modified with carbohydrate. 

Other redox reagents include dinucleotides such as FAD (flavine adenine dinucleotide), lipoic acid, which we will meet when we discuss the chemistry of thiamine, and ascorbic acid (vitamin C)> which you met in Chapter 49. Ascorbic acid can form a stable enolate anion that can transfer a hydride ion to a suitable oxidant. 

Table 8.9 lists the water-soluble vitamins —ascorbic acid (vitamin C) and a series known as the vitamin B complex (Figure 8.32). Ascorbate, the ionized form of ascorbic acid, serves as a reducing agent (an antioxidant), as will be discussed shortly. The vitamin B series comprises components of coenzymes. Note that, in all cases except vitamin C, the vitamin must be modified before it can serve its function. 

Figure 8.34. Forms of Ascorbic Acid (Vitamin C). Ascorbate is the ionized form of vitamin C, and dehydroascorbic acid is the oxidized form of ascorbate.

Ascorbic acid, vitamin C, has one very acidic H (p fal =4.1), and all the others are not very acidic (p Ta2 =11.8). Use your knowledge of anion stability and resonance to find the acidic H and explain why it is so acidic. Draw all the resonance forms of the conjugate base. 

The answer is e. (Murray, pp 627-661. Sciiver, pp 3897-3964. Sack, pp 121-138. Wilson, pp 287-320.) Ascorbic acid (vitamin C) is found in fresh fruits and vegetables. Deficiency of ascorbic acid produces scurvy, the sailor s disease. Ascorbic acid is necessary for the hydroxylation of proline to hydroxyproline in collagen, a process required in the formation and maintenance of connective tissue. The failure of mesenchymal cells to form collagen causes the skeletal, dental, and connective tissue deterioration seen in scurvy. Thiamine, niacin, cobalamin, and pantothenic acid can all be obtained from fish or meat products. The nomenclature of vitamins began by classifying fat-soluble vitamins as A (followed by subsequent letters of the alphabet such as D, E, and K) and water-soluble vitamins as B. Components of the B vitamin fraction were then given subscripts, e.g., thiamine (Bi), riboflavin (B2), niacin [nicotinic acid (B3)], panthothenic acid (B5), pyridoxine (Bg), and cobalamin (B ). The water-soluble vitamins C, biotin, and folic acid do not follow the B nomenclature. 

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