Some evidence confirming the specificity of Barrnett and Seligman's technique for demonstrating side-chain carboxyl groups in proteins.

Abstract

There is some confusion in the literature as to whether the protein carboxyl groups demonstrable with Barrnett and Seligman's histochemical technique are C-terminal or side-chain. It seems that in the first step of this technique, hot mixtures of acetic anhydride and pyridine convert the commonly-occurring side-chain carboxyls into mixed acid anhydrides. That they do has now been confirmed; proteins in tissue sections treated with such acetic anhydride mixtures do not react with hydroxynaphthoic acid hydrazide after being left for a long time in either cold methanol or a cold solution of aniline in xylene. In this they resemble the acid anhydrides used for synthezising peptides in vitro. Thus all the lilac-reddish colours observable in tissue proteins on which the Barrnett and Seligman technique proper has been carried out can be ascribed to side-chain carboxyl groups.