Amino acids and their derivatives are a class of compounds that contain both amino and carboxyl groups in their molecules. Appearing in a free or bound state within an organism. Free amino acids are distributed in all animal cells and body fluids, while bound amino acids are mainly the basic components of proteins and peptides.

Natural amino acids are colorless crystalline substances with a high melting point, most of which are above 200 ℃. Usually soluble in water, but difficult to dissolve in non-polar organic solvents. But tyrosine and cysteine are insoluble in water, while proline and carboxyproline are soluble in ethanol and ether. All amino acids are soluble in strong acid and strong alkaline solutions.

According to α- The polarity properties of the side chain R group in amino acids, and the 20 common amino acids that make up proteins can be divided into 4 groups:

① The R group is a non-polar amino acid. There are a total of 8 types, 5 of which have aliphatic side chains, namely alanine, leucine, isoleucine, valine, and proline, 2 are aromatic amino acids, namely phenylalanine and tryptophan, and 1 is sulfur-containing amino acid, namely methionine; The solubility of this group of amino acids in water is lower than that of polar R-group amino acids; Proline and general α- Amino acids are different, they are α- The side chain on an amino acid replaces a hydrogen atom on the amino acid, which is actually a subamino acid.

② Amino acids with polarity but no charge on the R-group. There are a total of 7 types, namely serine, threonine, and tyrosine with hydroxyl groups in the R group, cysteine with thiol groups in the R group, glutamine and asparagine with amide groups in the R group, and glycine as the other amino acid; Glycine molecules do not have an R group, but have a certain polarity, so they belong to this group; The side chains of this group of amino acids contain insoluble polar groups that can form hydrogen bonds with water and are more soluble in water.

③ Amino acids with a positive charge on the R base. There are three types, namely lysine, arginine, and histidine; Carrying a positive charge at pH 7.0, also known as alkaline amino acids.

④ The R group is a negatively charged amino acid. There are two types, namely glutamic acid and aspartic acid; At pH 7.0, molecules carry a negative charge, also known as acidic amino acids. The structural formulas, abbreviated symbols, and related constants of 20 amino acids are shown in the table. In addition to the 20 common amino acids mentioned above, there are also some proteins such as diiodotyrosine, thyroxine, hydroxyproline, and hydroxylysine. In addition to the amino acids involved in protein composition, over 200 other amino acids have been found in various tissues and cells, most of which are the ones that make up proteins α- Derivatives of amino acids. However, some are β-、γ- or δ- Amino acids, and some are D-type amino acids, such as β- Alanine γ- Aminobutyric acid, as well as phenylalanine in antibiotic short peptide-S, D-alanine and D-glutamic acid in Gram positive bacterial cell walls. Some non protein amino acids act as important precursors or intermediates in metabolism, among which β- Alanine is a precursor of vitamin pantothenic acid, while citrulline and ornithine are precursors for the synthesis of arginine, γ- Aminobutyric acid is a chemical substance involved in nerve conduction. Plants contain a significant amount of non protein amino acids, which belong to plant secondary substances such as theanine, aspartic acid, coumarine, and lysine β- Cyanoalanine, etc.

In addition to the 20 amino acids that make up animal bodies and their product proteins, nearly 200 have been discovered in nature so far, most of which occur in the plant kingdom and have complex molecular structures unrelated to protein metabolism. However, there are relatively few amino acids that appear in the animal kingdom. Some of them are formed by chemical modification of amino acids that have already been bound to special proteins. For example, in collagen, some of the proline and lysine are often rehydroxylated into hydroxyproline and hydroxylysine; For example, actin and myosin often have small amounts of lysine and histidine that are methylated, respectively ε N-methyllysine and 3N-methylhistidine; Furthermore, acetylation of the ∈ amino group of lysine and phosphorylation of the OH group of serine were found in histones; Thyroid globulin contains iodothyrosine and iodothyronine; τ The N-terminus of globulin heavy chains and some proteins have pyroglutamic acid formed from glutamine; Cysteine, which is formed by two molecules of cysteine, also exists in general proteins.