The determination of the tyrosine content of gelatins

Abstract

A colorimetric method for determining tyrosine has been developed for use with gelatins. The reliability of the [alpha]-nitroso-[beta]-naphthol reaction has been improved by using a "promoter" solution, which made possible the use of more reproducible reaction conditions. The determination of tyrosine in gelatins by spectrophotometric and chromatographic methods has been discussed. The former has been considered with special reference to the elimination of interference by certain constituents of gelatins. The tyrosine contents of a number of gelatins have been determined by the three methods. Results are comparable; in addition similar values are obtained from intact and hydrolysed samples of the same gelatins. The gelatins studied contained 0.2-0.8% of tyrosine. Fractionation of gelatin by alcohol coacervation concentrates impurities into the first and last fractions, which have a considerably enhanced tyrosine content. The intermediate fractions have tyrosine contents that do not exceed that of the original gelatin, no fraction containing less than 0.2% of tyrosine. The results support the view that a proportion of the tyrosine in gelatins is combined in the protein structure, together with a tyrosine-contain-ing impurity.