The density increments of proteins

Abstract

The density increment, symbolized k d , of a protein is defined as the difference between the density of the protein solution and that of its dialysate, divided by the protein concentration in g./ml. The density increment of the protein has been shown to be a function of the protein concen­tration and of the hydrogen ion and the salts present in the dialysate. Under the conditions investigated, the influence of protein concentration is so small that density increments of haemoglobin are constant within 0⋅2 % over a range of protein con­centration up to 46 %. A fall in pH value from 7⋅8 to 6⋅35 increases the density increment from 0⋅248 to 0⋅253. Detailed calculations (table 9) show that the increase is mainly due to the combination of haemoglobin with anions. Salts, if present in high concentration, may cause a considerable reduction in the density increment of the protein. Theoretical formulae, (11) and (17), have been given which relate this reduction to the specific volume of the protein and the net excess of water bound by the protein. The apparent density of protein is determined when the density of a protein solution is equal to that of its dialysate. The apparent density of human-serum albumin dialyzed against an equimolecular mixture of KH 2 PO 4 and K 2 HPO 4 was found to be 1⋅219 at 1⋅0° C.