Amino Acids, Peptides and Proteins - Dévényi T., Gergely J. 1976

Gas chromatographic analysis of amino acid derivatives. Current status. Capabilities and limitations of the method.
Properties of amino acid derivatives
Formation of derivatives during amino acid degradation

In all the Methods considered so far, despite the transformations, The amino acid Structure was essentially preserved. To some extent, this also applies to the amines obtained by Bier and Teitelbaum [8] by the Decarboxylation of amino Acids (a method, however, that was not subsequently used). For the identification of substituted Amino Acids, the decisive factor is that the degradation products differ in their properties. Oxidative degradation in an alkaline hypochlorite solution [55] leads to The formation of an aldehyde containing one less carbon atom than the parent compound. However, studies on the application of this reaction in GC [4] revealed certain limitations to its use for these purposes.

It was found that simple amino acids yield the corresponding aldehydes quantitatively, whereas acidic and Sulfur-Containing Amino Acids produce several different products, and different Amino acids can give identical decomposition products; consequently, this technique was not used further. Similarly, The oxidation of free amino acids with N-Bromosuccinimide in aqueous solution [86], yielding nitriles with one less carbon atom, is unsuitable for GC because Side Reactions also occur here, the main one being the Formation of the corresponding aldehydes.

The first method of amino acid conversion for GC analysis was the reaction with ninhydrin. As is well known, this reaction, along with colored substances and CO₂, produces the aforementioned aldehydes containing one less carbon atom than the parent molecule. Based on the quantitative determination method for amino acids developed from this reaction [92], these volatile aldehydes were successfully separated and identified using GC [37]. Obviously, this method is suitable only for those amino acids that yield volatile aldehydes in the reaction with ninhydrin; thus, Pro and related Amino acids are naturally excluded from this group [61]. Side reactions during GC, such as polymerization, make the identification of Certain amino acids difficult or even impossible [130]. To overcome these difficulties, the aldehydes were oxidized [3] to carboxylic acids and chromatographed as methyl esters. Despite the noted drawbacks, Zlatkis et al. [130] point out that this amino acid modification process is of technical interest. According to the reaction principles used in GC, The conversion of amino acids, followed by the Separation and Quantitative determination of aldehydes (converted to methane via catalytic hydrocracking), can occur as a continuous process, with the reactions taking place in an aqueous solution containing ninhydrin. However, quantitative data could only be obtained for 7 simple amino acids by this method; thus, considering that several simpler methods exist for detecting such amino acids, its application seems hardly justified.