Physical properties of amino acids

- May 03, 2018-

Amino acids are colorless crystals with a melting point of more than 200 ℃, much higher than the melting point of ordinary organic compounds. α one amino acid has sour, sweet, bitter, fresh 4 kinds of different taste feeling. Monosodium glutamate and glycine are the most used flavor seasonings. Amino acids are generally soluble in water, acid solution and alkali solution, insoluble or slightly soluble in ethanol or ether and other organic solvents. The solubility of amino acids in water is very different, for example, the solubility of tyrosine is the smallest, and in 25 ℃, the tyrosine in the G-water only dissolves 0.045 g, but the solubility of tyrosine in the hot water towel is large.

Lysine and arginine are often present in the form of hydrochloric acid, because they are easily soluble in water and are difficult to crystallize because they are hygroscopic. (1) color and color of various common amino acids easily become colorless crystallization, crystalline shape because of the structure of amino acids vary.

For example, L-glutamic acid is a quadrangular crystal, and D-glutamic acid is a diamond-shaped crystal.

(2) Melting point amino acid crystallization of a higher melting point, generally in 200~300℃, many amino acids in the reach or close to the melting point will be decomposed into amine and CO2. 

(3) Solubility most amino acids can dissolve in water. The solubility of amino acids in water was different, such as lysine, arginine and proline, the solubility of tyrosine, cysteine and histidine was very small. All kinds of amino acids can dissolve in strong alkali and strong acid.

But amino acids are insoluble or slightly soluble in ethanol. 

(4) Taste sense of amino acids and their derivatives have a certain taste, such as acid, sweet, bitter, and so on. The type of flavour is related to the type of amino acid and the stereoscopic structure.

From the stereo structure, generally speaking, D-type amino acids are sweet, and its sweet intensity is higher than the corresponding L-type amino acid. 

(5) Ultraviolet absorption characteristic all kinds of common amino acids have no absorption ability to visible light. However, tyrosine, tryptophan and phenylalanine have obvious light absorption in the ultraviolet light region. And most proteins contain these 3 kinds of amino acids, especially tyrosine. Therefore, the content of protein can be quantitatively measured by ultraviolet absorption at 280hm wavelength.