Determination of carboxyl groups in wheat and barley flour proteins

Abstract

The method of Hoare and Koshland for the estimation of free carboxyl groups in proteins has been scaled down and applied to wheat proteins. With standard proteins, provided the carboxyl groups were not inaccessible (as in lysozyme), the error was as large as 19%. This level of error is acceptable for estimating by difference amide contents when these are high, as in gluten proteins, because the error is greatly reduced by the molar ratio of side-chain CONH2:COOH-a 'leverage' effect. About 95 76 of the side-chain COOH groups appear to be amidated in the prolamins and about 50% in the albumins. The proportion of Glx+Asx in the amide form seems to decrease slightly as the electrophoretic mobility of a prolamin increases. The errors in calculating the numbers of positive and negative charges that a protein bears at pH 8.9 become serious when superimposed on the net charge, which is the small difference between these numbers. The net charge at pH 8.9, therefore, could not be obtained accurately by calculation. The polarity of the net charge can be determined by electrophoresis using a marker for electroendosmosis.