New Naturally Occurring Amino Acids

Abstract

Dedicated to Professor Hans Brockman on the occasion of his 80th birthdayThe majority of the 500 or so naturally occurring amino acids known today were discovered during the last 30 years, for example during the search for new antibiotics in the culture media of microorganisms, or as components of the antibiotics in fungi, seeds, in numerous plants and fruits, and in the body fluids of animals. Some 240 of these amino acids occur free in nature, some only as intermediates in metabolism. This article provides an overview of the developments that have taken place in this area since 1956 when the last review appeared. Summary accounts are presented, dealing with new unsaturated amino acids, cyclopropane-and cyclobutane-amino acids, heterocyclic amino acids, halogen-containing amino acids, sulfur-, selenium-and phosphorus-containing amino acids as well as aliphatic amino acids. In a few cases, e.g. betalamic acid and muscaflavin, the biosynthesis is described in detail. Historical IntroductionIn 1820 glycine was isolated from gelatin hydrolyzates. In the following 100 years the remaining twenty amino acids, which are normally found in proteins, were identified. Only four more were found between 1920 and 1940. With the development of chromatographic methods the number of known naturally occurring amino acids increased rapidly. In 1956 alone over 60 new ones were reported [']. At the moment 500 are known, of which ca. 240 occur free in nature. Still further naturally occurring amino acids are being found eachThe methods of detection and determination of structure have improved so much in recent years that traces of a new fluorescent spot can be discerned in the two dimensional chromatogram of the dansyl derivatives (dansyl = 5-dime- thylamino-l-naphthyl~ulfonyl)[~-~~. Combined GC-MS ofthe N-trifluoroacetyl-methyl ester derivative^'^' confirms that the sample is homogeneous and provides an empirical formula and often a hint as to the structure, which can be corroborated by 'H-and I3C-NMR on a few mg of the amino acid. In particularly difficult cases, such as for example streptolidinis"], an X-ray structure analysis is also necessaryisb1. Nowadays one can determine the enantiomeric purity, usually to better than 0.1%, by gas chromatography of volatile amino acid derivatives on a peptide-ester stationary phase. Equally good results are obtained by chromatography of the free amino acids as diastereoisomeric copper complexes with L-proline on octadecylated silica-gel with aqueous eluents [']. The absolute configuration can be easily determined from CD spectra if a characteristic longwave transition can be produced by making a complextIoal or a dinitrophenyl derivative"ob1. If there are a number of chiral centers the unambiguous assignment of all the isomers is possible through stereospecific synthesis, Unsaturated Amino AcidsThermocymocidin 1 from a thermophilic Eumyces spec i e~~~~,~' ,~~~ and myriocin from the thermophilic fungus Myriococcum albomyce~['~~ are identical, both in their constitution, with a trans double-bon...

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