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Physical properties of amino acids
Amino acids are colorless crystals with a melting point of more than 200 ℃, which is much higher than that of ordinary organic compounds. α- Amino acids have four different flavors: sour, sweet, bitter and fresh. Monosodium glutamate and glycine are the most delicious condiments. Amino acids are generally soluble in water, acid solution and alkali solution, insoluble or slightly soluble in organic solvents such as ethanol or ether. The solubility of amino acids in water varies greatly. For example, the solubility of tyrosine is the smallest. At 25 ℃, only 0.045g of tyrosine is dissolved in 100g of water, but the solubility of tyrosine in hot water is large. Lysine and arginine often exist in the form of hydrochloride, because they are very soluble in water and difficult to crystallize due to deliquescence.
(1) Color and luster: various common amino acids are easy to become colorless crystals, and the crystal shape varies with the structure of amino acids. For example, L-glutamic acid is a tetragonal column crystal, while D-glutamic acid is a rhombic sheet crystal.
(2) Melting point: the melting point of amino acid crystal is high, generally at 200 ~ 300 ℃, and many amino acids will decompose into amines and CO2 when reaching or approaching the melting point.
(3) Solubility: most amino acids are soluble in water. There are differences in the solubility of different amino acids in water, such as lysine, arginine and proline, while tyrosine, cysteine and histidine have little solubility. All kinds of amino acids can be dissolved in strong bases and acids. But amino acids are insoluble or slightly soluble in ethanol.
(4) Taste: amino acids and their derivatives have a certain taste, such as acid, sweet, bitter, salty, etc. The type of taste is related to the type and three-dimensional structure of amino acids. In terms of three-dimensional structure, generally speaking, D-type amino acids have sweet taste, and their sweet intensity is higher than the corresponding L-type amino acids.
UV absorption of aromatic amino acids
(5) UV absorption characteristics: 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 region. Most proteins contain these three amino acids, especially tyrosine. Therefore, the UV absorption characteristics at the wavelength of 280nm can be used to quantitatively detect the content of protein.
An important optical property of amino acids is their absorption of light. 20 kinds of PR AA have no light absorption in the visible light region, and have light absorption in the far ultraviolet region (<220nm). In the ultraviolet region (near ultraviolet region) (220nm ~ 300nm), only three kinds of AA have light absorption ability. These three amino acids are phenylalanine, tyrosine and tryptophan, because their R groups contain benzene ring conjugated double bond system.
The maximum light absorption of phenylpropanoic AA is 259nm, casein AA is 278nm, and color AA is 279nm. Proteins generally contain these three aa residues, so the maximum light absorption is at about 280nm wavelength. Therefore, it is very convenient to determine the content of proteins by spectrophotometry. The basis of determining protein content by spectrophotometry is Lambert Beer law. At 280nm, the absorbance of protein solution is proportional to its concentration.
Amino
What foods contain amino acids?
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