734 



SCIENCE 



[Vol. LVI, No. 1461 



The titration curves prove anotlier fact, 

 namely, tiiat the salts of proteins are strongly 

 hydix)lyzed. When we add aoid, e. g., HCl, to 

 isoelectric protein, pant of the acid combines 

 wi'tih the protein giving- rise to protein chloride, 

 wtele the rest of tlhe acid remains free. There 

 is then an equilite-ium between free HCl, pro- 

 Itein chloride, and non-ionogenic (or isoelec- 

 tric) protein. The more acid is added to 

 originally isoelectric protein, the more protein 

 chloride is formed untU finally all the protein 

 exisits in the form of pi-oitein chloride. It is 

 possible to find on)t from the Pn measurements 

 how much of the acid added is free and by de- 

 ducting this value we know how much is in 

 oombination wiith 'tihe protein. By saturating 

 the protein with acid the combining weight of 

 a protelin with acid can be found. Hiitohcock 

 found in this way tlhat tlhe combining weight of 

 gelaitin is about 1090. 



Ill 



The colloidal 'behajvior of proteins shows 

 itself in a peculiar effect of eleotrolytes — acids, 

 alkalies or salts — on such properties as the 

 swelling of gels or the osmotic pressure or vis- 

 cosity of protein solutions. All ithese proper- 

 ties, swelling, osmotic pressure, viscosity, are 

 affected by eleotrolytes in a very simUar way; 

 suggesting itbat all are due to the same cause. 

 We shall see that by giving the explanation for 

 one of these properties, osmoltie pressure, we 

 shall "by implication give 'the explanation for 

 all of tchem. 



Measuremenlts of the osmotic pressure of 

 solutions of a protein — ^gelatin, crystalline egg 

 albumin, casein and edestin— were made with 

 solutions containing 1 gm. dry weight of onig- 

 inally isoelectric protein in 100 cc. of solution; 

 and the 100 ec. of solution included also vary- 

 ing concentrations of 0.1 N acid. These solu- 

 itiions were put into collodion bags suspended in 

 waiter free from protein. The outside water 

 was at 'the ibeginning of Ithe experiment ibrought 

 to the same Ph 'as that of tlie protein solution, 

 using always the same acid as that added to 

 the protein. The m:easurements of the osmotic ■ 

 pressure were read after 18 hours when osmotic 

 equilibrium was eBtablished. It was found ithat 



the osmotic pressure varied in a characterisitic 

 ■way wiith the pjj of the protein solution and 

 Ithe valency of Ithe anion of the acid used. This 

 effect is sihown in the curves in Fig. 2 which 

 were obtained from gelatin solutions. But the 

 curves are similar in the ease of otlier pro- 

 teins sudh as crysltalline egg albumin, casein or 

 edestin. These cui'ves show that the osmotic 

 pressure of a protein solution is a minimum at 

 the isoelectric poinjt, that it increases when 

 little acid is added until a maximum is reached, 

 and that on the further addition of acid the 

 osmotic pressure is again diminished. They 

 show, moreover, that only the valency and not 

 the nature of the anion of the acid influences 

 the osmotic pressure of a protein solution. We 

 know from the titration curves that in the case 

 of H3PO4 the anion in combination with itihe 

 protein is not the trivalent PO4 but the mono- 

 valent H2PO4; and the curves in Fig. 2 show 

 fchait the influence of phosphoric aoid and 

 'hydrochloric acid on the osmotic pressure is 

 the same if measured for ithe same pu of ibhe 

 protein solution. Oxalic acid is a monobasic 

 acid ibelow Pn 3.0 and we noitice that the de- 

 scending branoh of the oxalic acid curve below 

 Pjj 3.0 practically coincides "with the descend- 

 ing ibranch of the HCl curve. The curve for 

 the influence of H2SO4 is only about half as 

 high as that for HCl 'and we know from the 

 titration curves that the anion of protein sul- 

 fate is bivalent. lit was found that all mono- 

 'basiic acids, e. g., HBr, HNO37 acetic acid, etc., 

 •and all weak dibasic or tribasic acids, e. g., 



pH 2.0 22 2A 26 2fl 30 12 04 36 38 4jO 42 4,4 4fl 



Fig. 2 



