설명(영) |
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Amino acids are compounds carrying both amino groups (see Amines) and Carboxylic Acid groups on the same molecule. In modern usage, the term "amino acid" is sometimes (erroneously) restricted to the principal products of the hydrolysis of Proteins. In this sense, amino acids are the monomeric building blocks of peptides and proteins. Most of the amino acids from natural sources, the so-called alpha-amino acids, may be represented by the general structure RCH(NH2)COOH, in which R represents an aliphatic, aromatic, or heterocyclic grouping. In general, the natural amino acids, with the exception of Glycine, possess one or more asymmetric centers and, therefore, may exist in at least two optically active forms. Most of the natural amino acids are of the L-configuration; however, a number of D-amino acids have been found in bacterial cell wall hydrolysates or in some fungal antibiotics.
Since amino acids carry two types of salt-forming functions, their behavior in aqueous solutions is dependent on pH. At low pH, the amino group is protonated, and amino acids act as cations. At high pH, the carboxyl groups are ionized, and these acids behave as anions. At intermediate pHs, i.e., near neutrality, the amino group may partially neutralize the carboxyl group. In this case, the amino acid exists as a zwitterionic molecule possessing both cationic and anionic charges on the same molecule .
The carboxylic acid groups of amino acids can be esterified, as, e.g., in Ethyl Aspartate (see Esters). Alternately, the amino group can be acylated with a Fatty Acid, as, e.g., in Sodium Cocoyl Glutamate (see Amides).
Amino acids derived from the hydrolysis of various proteins are widely used as conditioning and specialty ingredients in cosmetics. Other commonly used amphoteric amino acids include the Alkylamido Alkylamines, a variety of Alkyl-Substituted Amino Acids, and Betaines, all of which are found in shampoos, hair coloring products, skin or hair conditioners, and skin cleansers. |