Natural amino acids and their derivatives

A constituent of a protein, a carboxylic acid whose molecule contains an amino group. There are more than 200 kinds of amino acids in nature, but there are only 20 kinds of amino acids that make up the animal body and its products. The structure of amino acids and the structure of classified amino acids The amino acids obtained by protein hydrolysis are mostly α-amino acids, which are bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom and a side chain group near the α-carbon atom of the carboxyl group. group. The side chain is represented by the letter R, and R is different to constitute a different amino acid. The basic structural formula of the amino acid is as follows: Figure 1 shows the basic structural formula of the amino acid. Classification of Amino Acids Amino acids are classified into non-polar R-based amino acids, uncharged polar R-based amino acids, positively charged R-based amino acids, and negatively charged R-based amino acids according to the polar nature of R. The first category includes alanine, leucine, isoleucine, valine, proline, phenylalanine, and methionine; the second class includes glycine, serine, threonine, cysteine, and tyrosine. The third group includes lysine, arginine and threonine; the fourth category includes aspartic acid and glutamic acid. In livestock production, amino acids can be divided into essential and non-essential amino acids, limiting amino acids and non-limiting amino acids according to different classification methods. In addition to glycine (whose R group is a hydrogen atom), the α-carbon atoms of all amino acids are asymmetric carbon atoms, and thus have stereoisomers and optical rotation. The amino acids are classified into L-form and D-form according to the configuration of the α-carbon atom. The amino acids produced by protein hydrolysis of plants and animals are L-form, and the amino acids produced by chemical synthesis and fermentation include L-form amino acids, D-type amino acids, and L-form amino acids in the same amount of L-form and D-form. The animal enzyme system can only directly synthesize tissue proteins using L-form amino acids. Except for methionine, the D-type and DL-type amino acid utilization rates are very low, and even can not be used by animals at all. General Properties of Amino Acids Physical Properties 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. The acid-basic amino acid of an amino acid is an ampholyte which reacts with an acid or a base to form a salt. Carboxyl groups and amino groups in the same molecule can also form internal salts. In solution, amino acids are mainly present in ionic form. For example, when the pH of an amino acid solution is adjusted to be non-conductive, the ions move neither to the cathode nor to the anode. The pH of the solution is the amino acid. Electrical point (PI). Chemical reactions related to the determination of amino acids (1) Reaction with nitrous acid Amino acids can react with nitrous acid to form hydroxy acids and water, and release a certain amount of nitrogen. (2) Reaction with 2, 4-dinitrofluorobenzene Under the condition of near neutral or slightly alkaline room temperature, the amino group in the amino acid molecule can react with 2,4-dinitrofluorobenzene to form amino acid 2, A derivative of 4-dinitrofluorobenzene, a yellow DNP-amino acid. (3) Reaction with dimethylaminonaphthalenesulfonyl chloride (DNS-CL) The fluorescent reagent DNS-CL binds to the amino group of the amino acid to form a fluorescent DNS amino acid. Amino acids such as lysine, histidine, aspartic acid, and tyrosine can form a stable double DNS-amino acid derivative. (4) Reaction with hydrazine ketone The α-amino acid is oxidatively decomposed into aldehyde, ammonia and CO2 by hydrazine ninhydrin, and ninhydrin is reduced. The evolved ammonia is condensed with a further reduction product of the other molecule ninhydrin and one molecule ninhydrin to form a blue-violet compound. The separation of amino acids and the determination of amino acids are mostly in a bound state. The first step in separating amino acids is to hydrolyze proteins with acids or bases, prepare amino acid mixtures, and then further separate and determine amino acids by partition chromatography and ion exchange chromatography.