Features, roles and chiral analyses of proteinogenic amino acids

Amino acids (AAs) are important biomolecules responsible for plethora of functions in both prokaryotic and eukaryotic systems. There are 22 naturally occurring amino acids, among which 20 common amino acids appear in the genetic code and known as proteinogenic amino acids, which are the building blocks of proteins. Proteinogenic amino acids exist in two isomeric forms (except glycine) and the biological activity is often attributed to a specific stereoisomer. Most of the amino acids found in protein is predominantly L-AAs. A few D-amino acids can be found in bacterial cell wall, some marine invertebrates and higher organisms including frog, snail, spider, rat, chicken and human. Besides their role in protein synthesis, different proteinogenic amino acid stereoisomers have found to perform important biological roles either by functioning as precursors of a myriad of other bioactive molecules or by indicating several biological phenomena i.e. acting as markers for different physiological conditions. Given the biological importance of L- and D-amino acids (AAs), improved analytical methods for their resolution and accurate quantification remain of keen interest. Chiral analysis of amino acids in complex biological matrices poses numerous analytical challenges that are exacerbated by broad differences in polarity and ionization efficiencies of the proteinogenic amino acid stereoisomers. To date, various analytical methods are reported for chiral amino acid analysis including chromatographic, spectrometric, enzymatic, fluidic, electrophoretic and microbiological techniques. Advances in stationary phase, derivatization chemistry and instrumentation contributed to achieving baseline separation and accurate detection of trace levels of proteinogenic amino acids stereoisomers in different matrices. However, liquid and gas chromatography coupled to mass spectrometry based analytical methods remain the most popular platform in chiral amino acid analysis in terms of feasibility and performance. This review discusses different physicochemical and biological features and functions of proteinogenic amino acid stereoisomers and lists a variety of established chromatography and mass spectrometry based analytical methods for their analysis reported to date.