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Physical Properties Of Amino Acids

- Sep 15, 2018 -

Amino acids are colorless crystals with a melting point exceeding 200 degrees, much higher than the melting point of ordinary organic compounds. Alpha amino acid has 4 different tastes: sour, sweet, bitter and fresh.

Monosodium glutamate and glycine are the most delicious flavor condiments. Amino acids are generally soluble in water, acid and alkali solutions, insoluble or slightly soluble in ethanol or ether and other organic solvents. The solubility of amino acids in water varies greatly, for example, the solubility of tyrosine is the smallest. At 25 C, the solubility of tyrosine in 100 g water is only 0.045 g, but the solubility of tyrosine in hot water towel is larger. Lysine and arginine often exist in the form of hydrochloride, because they are very soluble in water, and difficult to obtain crystalline because of the hydrolysis 

(1) Various common amino acids in color and color are easy to become colorless crystals, and the crystalline shape varies with the structure of amino acids. For example, L glutamic acid is four corner shaped crystal, while D glutamic acid is rhombohedral crystalline.

(2) The melting point of amino acid crystallization is higher, generally in 200-300 C, when many amino acids reach or near the melting point will be decomposed into amine and CO2.

(3) most of the amino acids are soluble in water. The solubility of different amino acids in water is different, such as lysine, arginine, proline solubility is larger, tyrosine, cysteine, histidine solubility is very small. All kinds of amino acids can be dissolved in strong base and strong acid. But amino acids are insoluble or slightly soluble in ethanol.

(4) flavour amino acids and their derivatives have a certain taste, such as acid, sweet, bitter, and so on. The type of taste is related to the type and structure of amino acids. Generally speaking, D-type amino acids have sweet taste, and their sweetness intensity is higher than the corresponding L-type amino acids.

(5) UV absorption characteristics. All kinds of common amino acids have no absorbability to visible light. However, tyrosine, tryptophan and phenylalanine have obvious light absorption in UV light. Most of the proteins contain these 3 amino acids, especially tyrosine. Therefore, the ultraviolet absorption characteristic of 280hm wavelength can be used to quantitatively detect the protein content. 

Amino acid

An important optical property of amino acids is the absorption of light. The 20 kinds of Pr-AA have no light absorption in the visible region, all have light absorption in the far ultraviolet region (< 220 nm), and only three kinds of AA have light absorption ability in the ultraviolet region (near ultraviolet region) (220 nm ~ 300 nm). These three kinds of amino acids are phenylalanine, tyrosine and tryptophan, because their R groups contain phenylcyclic conjugated double bond system. The maximum light absorption of phenylpropylene AA is 259 nm, casein AA is 278 nm, and color AA is 279 nm. Proteins generally contain these three AA residues. Therefore, the maximum light absorption of phenylpropylene AA is about 280 nm. Therefore, spectrophotometry can be used to determine the content of protein conveniently. The determination of protein content by spectrophotometric method is based on Lambert Bill law. At 280nm, the absorbance value of protein solution is directly proportional to its concentration. 


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