Amino Acid Analysis

The amino acid composition of proteins is analyzed after hydrolysis of the proteins. Acids or proteases are used for the hydrolysis. Because of the lability of some amino acids (eg, tryptophan, especially in the presence of carbohydrates, and sulfur-containing amino acids), enzymatic hydrolysis is preferable. However, because of the difficulty in achieving complete hydrolysis and high costs of the required enzyme mixtures, acid hydrolysis is more popular. To avoid the destruction of those amino acids during hydrolysis, methanesulfonic acid (7), mercaptoethanesulfonic acid (8), or thioglycolic acid (9) may be used for hydrolysis instead of the otherwise commonly used 6 N hydrochloric acid.

High-performance liquid chromatography (HPLC), especially using a cation exchange column, has been the most popular method for amino acid analysis of the resultant protein hydrolysate. However, reverse-phase (RP) chromatography with a C18 or C8 column before or after deriv-atization of amino acids with o-phthaldialdehyde is convenient for detection using a fiuorometric detector (10). This is because of simpler solvent systems required for

Table 1. Properties of the Amino Acid Residues of Proteins

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