Acid Hydrolysis of Proteins for Chromatographic Analysis of Amino Acids

The conditions used to hydrolyze proteins are vital in determining amino acid compositions because they necessarily represent a compromise aimed at yielding the best estimate of amino acid composition. Variations in ease of peptide bond cleavage, differences in amino acid stabilities, and matrix effects from nonproteinaceous components all militate against a single set of hydrolysis conditions that quantitatively hydrolyze every peptide bond and concurrently cause no destruction of any amino acid. This presentation summarizes and reviews an extensive study which evaluated a number of variations in the techniques and procedures of the classical 6N HC1, 110°C, 24 h hydrolysis of protein. The objectives of the recent investigation were: (/) to compare hydrolysis at 145°C, 4 h with 110°C, 24 h for proteins in a wide range of different sample matrixes; (2) to compare protein hydrolysis at 110°C, 24 h conducted in sealed glass ampoules after vacuum removal of air with hydrolysis in glass tubes with Teflon-lined screw caps after removal of air by vacuum, nitrogen purge, and sonication; (3) to evaluate a performic acid oxidation procedure before hydrolysis for the analysis of cystine and methionine in the different sample matrixes; (4) to evaluate multiple hydrolysis times at 145°C; (5) to evaluate the variation of interlaboratory hydrolysates prepared at 145°C, 4 h in 2 different laboratories on the amino acid analysis of an array of protein-containing matrixes. The major sources of inaccuracy and lack of precision arising from the application of ion-exchange or gas chromatography, both of which provide excellent accuracy and precision, are prechromatographic sample handling and the method used for hydrolysis of the protein sample itself. Hydrolysate preparation is the area that requires the most attention to solve problems of variability of amino acid analyses.