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According to a rigid definition, an amino acid is an organic acid that possesses at least one amino group. Almost a limitless number of molecules with various functional groups fall under this definition, but in more general terms, the definition limits the number of choices to the small group of natural L-a-amino acids that make up proteins and some other naturally occurring compounds.
Most proteins are composed of approximately 20 types of amino acids in varying proportions. Some additional amino acids are only found in special proteins; for example, hydroxyproline occurs in collagen and gelatin. All amino acid constituents of proteins are a-amino acids, referring to their molecular structure wherein the amino group is attached to the same carbon atom as the carboxyl group. Amino acids with a b-, g- and d- structure, or even with a sulfonate acid group instead of a carboxyl group, are found in living organisms in forms of small peptides or as free amino acids. The term free here is used to describe the amino acids that are not embedded in proteins.
For most consumers, the term amino acid is still not as familiar as protein or vitamin, yet amino acids have been used in cosmetics for a long time. Among the various applications, the most significant is the use of natural moisturizing factor (NMF) amino acids and hydrolyzed proteins, and the latter has been used in hair care applications for nearly 50 years. Several moisturizers for cosmetic applications containing amino acids were reported as early as 1983.
This is only an excerpt of the full article that appeared in Cosmetics & Toiletries, but you can purchase the full-text version.