Protein Chemistry - Part 1 - General Protein Chemistry - Ashmarin I. P. 1968

Chemical composition of proteins
Chemical reactions of amino acids
Reactions for individual amino acids

Virtually every amino acid has one or more specific qualitative or quantitative reactions. However, many of these Procedures are rather complex and rarely used in routine laboratory practice. Here, we will focus only on the most commonly employed reactions for Certain Amino Acids. For instance, Arginine can be quantified—both in its free state and within Proteins—using the Sakaguchi reaction with a-naphthol and hypobromite. This reaction produces a red coloration; the method is highly sensitive and capable of detecting less than 1 mcg/ml of arginine.

Tryptophan can be identified via Ehrlich's color reaction using p-dimethylaminobenzaldehyde and sodium nitrite in concentrated sulfuric acid. The sensitivity of this test is approximately 1 mcg/ml, yielding a purple color, and it detects both free and peptide-bound tryptophan. Tyrosine is readily detected by the Folin-Denis reaction with a-nitroso-β-naphthol, with a sensitivity of up to 10 mcg/ml. It can also be identified using the specific Millon's test with mercurous and mercuric salts in nitrous acid. However, this method is less sensitive, requiring up to 50 mcg of tyrosine per milliliter of sample.

For the detection of Histidine, the Pauli reaction is employed using diazotized sulfanilic acid in a weakly alkaline medium, which results in a red color development. This is a qualitative or semi-quantitative test; besides histidine, it also yields a positive reaction with tyrosine, albeit with lower intensity.

Finally, for the determination of Cysteine and cystine, the nitroprusside reaction with sodium nitroprusside and sodium cyanide in an alkaline alcohol medium is utilized, producing a cherry-red coloration. This reaction is not strictly specific.