Selective photodestruction of α-amino acids

Abstract

A problem typically encountered in the analysis of amino acids in chemical evolution experiments and in extracts of meteorites is the large number present. For example, α-, β-, and γ-amino acids, N-mono substituted α-amino acids, and dicarboxylic α-amino acids have been found in extracts of the Murchison meteorite, and many more amino acids are present than have been positively identified by computerized gas chromatographic mass spectrometry. This paper reports an analytical method to selectively destroy the α-amino acids, with only the β- and γ-amino acids remaining in the solution. It is based on the ability of Cu 2+ to complex with amino acids, the order of stability of these complexes being α > β > γ, = δ, = ϵ = 0. Aqueous solutions of α-amino acid-Cu 2+ chelates are known to be decomposed by 254 nm light as well as by nonmonochromatic uv light, yielding a precipitate of Cu 2O. This paper shows that at 254 nm (ligand-metal charge transfer band) the rate of destruction of amino acids in Cu 2+ aqueous solutions is in the following order, dicarboxylic α-amino acids > α-amino acids > N-monosubstituted α-amino acids ⪢ β-amino acids ≈ γ-amino acids. Thus by irradiation with 254 nm light in the presence of Cu 2+ all the amino acids can be destroyed except the β- and γ-amino acids. When almost 100% of the α-amino acids are destroyed, 80% of the β- and γ-amino acids still exist in solution. With this procedure, complex mixtures of amino acids can be simplified to make identification by gas chromatographic mass spectrometry casier.