Irregular series in protein precipitation.

Abstract

The “irregular series” or “tolerance zone” in the precipitation of albumin with heavy metals has been observed by many investigators of the properties of proteins. Many theories have been advanced to account for the phenomenon but none of them have, up to the present time, given a satisfactory explanation. The present work being conducted by the authors has linked the irregular series with the isoelectric point of the protein, thus accounting for the phenomenon by a known physical property of albumin rather than assuming the formation of a series of hypothetical compounds of varying composition and solubility. In the experiments a solution of dried egg albumin was dialyzed against distilled water until practically ash free, filtered and kept under toluene. Series were then set up in fifteen cubic-centimeter graduated centrifuge tubes, such that the concentration of albumin remained constant in the final dilution, while the concentration of heavy metal salt varied. The tubes were allowed to stand thirty minutes, centrifuged twenty minutes and the volume of precipitate measured. The hydrogen ion concentration of the supernatant liquid was determined by means of the platinum electrode. The aqueous solutions of heavy metal salts are decidedly acid due to the hydrolysis of the salt. The hydrogen ion concentration increases with increasing concentration of salt. In albumin solutions containing a low concentration of heavy metal salt there is an increasing precipitation with increasing salt concentration until the hydrogen ion concentration of the solution passes the isoelectric point of the protein (pH 4.8), when there is a marked falling off and disappearance of the precipitate.