The course of tryptic digestion of proteins. On hydrolysing a protein, both NH 2 groups and COOH groups are set free in equal numbers by the splitting of - CONH - groups, and therefore the reaction cannot be followed by simple titration. One method of estimating the breakdown products (peptones, peptides, amino-acids, etc.) is Sorensen's formaldehyde titration method, the theory of which has been fully discussed under the Estimation of Glycine (p. 463). From the description given there it is obvious that the quantitative value of the formaldehyde titration will increase as the digestion proceeds, until finally a value is obtained which corresponds (theo-retically at least) to the constituent amino-acids of the original protein. Another important method of following protein hydrolysis is that due to Van Slyke, and consists in estimating the free amino groups liberated by treatment with nitrous acid, whereby gaseous nitrogen is evolved and meas-ured in a special apparatus. NH
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