The effect of hydrogen ion concentration on the precipitating and protective actions of mixtures of two proteins on colloidal gold and gum benzoin

IN a recent communication [Hopkins, Kermack and Spragg, 1931] the results were given of a series of experiments on the precipitation of colloidal gold and gum benzoin by the proteins, gelatin, ovalbumin, serum-albumin, the serumglobulins and oxyhaemoglobin at different concentrations of protein and of hydrogen ions.Observations were also made on the protective actions of these proteins against precipitation of the colloids by 1*25 % sodium chloride.Except in the case of oxyhaemoglobin, the protective power was minimal in the neighbourhood of the isoelectric point of the protein.It seemed desirable to extend the observations to mixtures of two proteins in order to find out, for example, what occurs when conditions are such that one protein alone will precipitate and the other protect.It was decided for this purpose to limit the experiments to the three proteins, gelatin, ovalbumin and oxyhaemoglobin.Of these three, the first two have approximately the same isoelectric point, PH 4.7, whilst that of the third is considerably higher, PH 6'8.Further, gelatin