Amino-acid analyser combined, hydrolysis

A similar problem associated with synthetic peptides of chain length > 5 amino acid residues is the identification of side reactions. Those involve substitutions on imidazole-, phenol- and indole moieties of histidine, tyrosine, and tryptophane as well as conversions, transamidation, and cyclizations of aspartic and glutamic side chains. As long as structural variations are stable under the reaction conditions of an Edman degradation, they can be detected from the proper phenylthiohydantoines in combination with H-NMR- and mass spectrometry. Quantitative amino acid analyses of impure peptides after acidic total hydrolysis do not indicate those structural deviations between main product and contaminations. 

There are, unfortunately, no studies to date of the dissolved protein content of microlayer samples. With the recent development of many sensitive techniques for the analysis of amino-acid mixtures in seawater using liquid chromatography and fluorescence detectors (e.g., Dawson and Pritchard, 1978), it should be relatively simple to analyse for combined amino acids after hydrolysis of the microlayer samples. Analyses of free amino acids in the microlayer seem not to have been performed to date either, but since considerable degradation of surface-adsorbed proteins may take place as a result of UV irradiation, this may be a fruitful area for future research. 

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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