Amino acids and their derivatives

Amino acids and derivatives thereof are a class of compounds that contain both an amino group and a carboxyl group in the molecule. Appears in a free or bound state in a living body. Free amino acids are distributed in all animal cells and body fluids, and the bound amino acids are mainly the basic components of proteins and peptides. Natural amino acids are colorless crystalline materials with a high melting point, most of which are above 200 °C. Usually soluble in water, difficult to dissolve in non-polar organic solvents. However, tyrosine and cystine are hardly soluble in water, and proline and carboxyproline are soluble in ethanol and ether. All amino acids are soluble in strong acid and strong base solutions. According to the polar nature of the side chain R group in the α-amino acid, the 20 common amino acids constituting the protein can be divided into four groups: The 1R group is a non-polar amino acid. There are 8 species, 5 with aliphatic side chains, namely alanine, leucine, isoleucine, valine and valine, and 2 are aromatic amino acids, namely phenylalanine and color ammonia. Acid, one is a sulfur-containing amino acid, namely methionine; the solubility of this group of amino acids in water is smaller than that of polar R-based amino acids; proline is different from general α-amino acids, which is a side chain substitution on α-amino acids. The formation of a hydrogen atom on an amino acid is actually an amino acid. The 2R group has a polar but non-charged amino acid. There are 7 species, ie, R-based hydroxyl-containing serine, threonine and tyrosine, R-based thiol-cysteine, R-based amide-based glutamine and asparagine, and the other amino acid is glycine. The glycine molecule has no R group, but has a certain polarity, so it belongs to this group; the amino acid side chain of this group contains non-dissociated polar groups, which can form hydrogen bonds with water and are more soluble in water. 3R baseband positively charged amino acid. There are three kinds, namely lysine, arginine and histidine; at pH 7.0, it carries a positive charge, also called a basic amino acid. The 4R group is a negatively charged amino acid. There are two kinds, namely glutamic acid and aspartic acid; at pH 7.0, the molecule has a negative charge, also called an acidic amino acid. The structural formulas, abbreviations and related constants of the 20 amino acids are shown in the table. In addition to the above 20 common amino acids, diiodotyrosine, thyroxine, hydroxyproline, hydroxylysine, etc. are found in certain proteins. In addition to the amino acids involved in protein composition, more than 200 other amino acids are found in various tissues and cells, most of which are derivatives of those alpha-amino acids that make up proteins. However, some are β-, γ- or δ-amino acids, and some are D-type amino acids, such as β-alanine, γ-aminobutyric acid, and phenylalanine in the antibiotic gramicidin-S, D-alanine and D-glutamic acid in the cell wall of Gram-positive bacteria. Some non-protein amino acids are important precursors or intermediates in metabolism, of which β-alanine is a precursor of vitamin pantothenic acid, citrulline and ornithine are precursors of synthetic arginine, γ-aminobutyric acid It is a chemical that is transmitted by nerves. Plants contain a large number of non-protein amino acids, belonging to plant sub-biomass, such as theanine, alanine, canavanine, lysine and β-cyanoalanine. In addition to the 20 amino acids that make up the animal's body and its protein, nearly 200 species have been found in nature, most of which occur in the plant kingdom, with complex molecular structures that are not related to protein metabolism. Less, some of which are formed by chemical modification of amino acids that have been incorporated into specific proteins, such as proline and lysine in collagen proteins, which are often partially rehydroxylated to hydroxyproline and hydroxylysine. Another example is that actin and myosin have a small amount of lysine and histidine that are methylated to εN methyl lysine and 3N methyl histidine, respectively; The amide amino group is acetylated and the OH group of serine is phosphorylated; the thyroglobulin contains iodinated tyrosine and iodinated thymidine; the tau globulin heavy chain and the N-terminus of certain proteins There is pyroglutamic acid formed by glutamine; there is also a cystine formed by a dimeric cysteine in a general protein.