Amino acids refer to carboxylic acids containing amino groups. The various proteins in the body are made up of 20 basic amino acids. Except for glycine, all are L-α-amino acids. (Proline is an L-α-imino acid), and its structural formula is shown in Fig. (R group is a variable group):
In addition to glycine, the α-carbon atoms of other protein amino acids are all asymmetric carbon atoms (ie, the four substituents bonded to the α-carbon atom are different from each other), so amino acids may have stereoisomers, that is, they may have different The configuration (D-type and L-type configuration).
Colorless crystals, melting point is extremely high, generally above 200 °C. Different amino acids have different tastes, some are odorless, some are sweet, some are bitter, and the monosodium salt of glutamic acid has an umami taste, which is the main component of MSG. The solubility of various amino acids in water varies greatly and can be dissolved in dilute acid or dilute alkali, but not in organic solvents. Normally alcohol can precipitate amino acids from its solution.
One of the important optical properties of amino acids is absorption of light.
20 kinds of Pr--AA have no light absorption in the visible light region and absorb light in the far ultraviolet region (<220 nm). In the ultraviolet region (near-UV region) (220 nm to 300 nm), only three kinds of AA have light absorption capability. The three amino acids are phenylalanine, tyrosine, and tryptophan because their R groups contain a benzene ring conjugated double bond system. The maximum light absorption of styrene-propylene AA is 259 nm, that of AA is 278 nm, and that of AA is 279 nm. Proteins generally contain these three AA residues, so the maximum light absorption is at a wavelength of about 280 nm. Therefore, it is convenient to use spectrophotometry. Determine the protein content. Spectrophotometric determination of protein content is based on Lambert-Beer law. The absorbance of the protein solution at 280 nm is directly proportional to its concentration.