Hill, supra

The principal disadvantages of acid hydrolysis are the destruction of some amino acids, notably serine, threonine, cystine, and tryptophan, and the slow release of amino acids from some dipeptide combinations, notably those involving isoleucine and valine (Hill, 1965). For most of these, timed analyses allow the extrapolations that give excellent estimations of the original amino acid content of the sample. Cystine can be readily estimated as one of several derivatives vide supra), and tryptophan can be either analyzed after alternative hydrolysis procedures or determined directly on the intact protein or peptide by spectrophotometric techniques (Edelhoch, 1967). Two amino acids, glutamine and asparagine, are quantitatively destroyed and can be determined only on enzymatic hydrolysates. 

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